Antimicrobial susceptibility monitoring of mastitis pathogens isolated from acute cases of clinical mastitis in dairy cows across Europe: VetPath results

Endolysin NC5 improves early cloxacillin treatment in a mouse model of Streptococcus uberis mastitis

Streptococcus uberis frequently causes bovine mastitis, an infectious udder disease with significant economic implications for dairy cows. Conventional antibiotics, such as cloxacillin, sometimes have limited success in eliminating S. uberis as a stand-alone therapy. To address this challenge, the study objective was to investigate the VersaTile engineered endolysin NC5 as a supplemental therapy to cloxacillin in a mouse model of bovine S. uberis mastitis. NC5 was previously selected based on its intracellular killing and biofilm eradicating activity. To deliver preclinical proof-of-concept of this supplemental strategy, lactating mice were intramammarily infected with a bovine S. uberis field isolate and subsequently treated with cloxacillin (30.0 μg) combined with either a low (23.5 μg) or high (235.0 μg) dose of NC5. An antibiotic monotherapy group, as well as placebo treatment, was included as controls. Two types of responders were identified: fast (n = 17), showing response after 4-h treatment, and slow (n = 10), exhibiting no clear response at 4 h post-treatment across all groups. The high-dose combination therapy in comparison with placebo treatment impacted the hallmarks of mastitis in the fast responders by reducing (i) the bacterial load 13,000-fold (4.11 ± 0.78 Δlog10; p < 0.001), (ii) neutrophil infiltration 5.7-fold (p > 0.05), and (iii) the key pro-inflammatory chemokine IL-8 13-fold (p < 0.01). These mastitis hallmarks typically followed a dose response dependent on the amount of endolysin added. The current in vivo study complements our in vitro data and provides preclinical proof-of-concept of NC5 as an adjunct to intramammary cloxacillin treatment. KEY POINTS: • Engineered endolysin NC5 was preclinically evaluated as add-on to cloxacillin treatment. • Two types of mice (slow and fast responding) were observed. • The add-on treatment decreased bacterial load, neutrophil influx, and pro-inflammatory mediators.

In vitro antimicrobial resistance of Escherichia coli, Serratia marcescens, Klebsiella oxytoca and Klebsiella pneumoniae on Bavarian dairy farms between 2014-2022

The objective of this study was to describe the prevalence of antimicrobial resistance of E. coli, K. oxytoca, K. pneumoniae, and S. marcescens from quarter milk samples submitted to the udder health laboratory of the Bavarian Animal Health Services (TGD) in Southern Germany between 2014 and 2022. All samples were tested with the California Mastitis Test and analyzed with a standard microbroth dilution to determine the minimum inhibitory concentrations (MIC). The antimicrobials tested were amoxicillin/clavulanate, cefazoline, kanamycin/cefalexin, cefoperazone, cefquinome, and marbofloxacin. Breakpoints were chosen in accordance with CLSI. Over the study period, E. coli, K. oxytoca, and K. pneumoniae showed only few resistances to all antimicrobials tested. For those pathogens MIC 50 and MIC 90 were below breakpoint for all antimicrobials except cefoperazone over the 9 years. A decrease in MIC could be seen for E. coli and K. oxytoca for all of the antimicrobials. While the MIC for K. pneumoniae stayed more stagnant, the prevalence of resistance still decreased overall. S. marcescens isolates were proven intrinsically resistant to amoxicillin/clavulanate and cefazolin and while in vitro resistances were low for all other antimicrobials tested, S. marcescens tended toward higher MIC for most of the antimicrobials over the years. Over time, there was also an overall increase in the number of isolates for all 4 pathogens per year. Starting 2018 there was steep increase in the number of isolates particularly from clinical cases. This jump in numbers coincided with a change of the regulation for veterinary drug prescriptions in Germany in 2018 that required, among other things, antimicrobial resistance testing before a change of antibiotics in the course of treatment and the use of critically important antimicrobials. Overall, while the pathogens increased in numbers, the prevalence of their antimicrobial resistance remained low.

Estimation of epidemiological cut-off values for eight antibiotics used for treatment of bovine mastitis caused by Streptococcus uberis and Streptococcus dysgalactiae subsp. dysgalactiae

Interpretive criteria for antimicrobial susceptibility testing are lacking for most antimicrobials used for bovine streptococcal mastitis. The objectives of this study were to determine (tentative) epidemiological cut-off ((T)ECOFF) values for clinically relevant antibiotics used for treatment of bovine mastitis, and to estimate the proportion of acquired resistance (non-wild-types) in Streptococcus dysgalactiae subsp. dysgalactiae and Streptococcus uberis. A total of 255 S. uberis and 231 S. dysgalactiae subsp. dysgalactiae isolates were obtained in Denmark and Norway from bovine mastitis. The isolates were tested for susceptibility to 10 antibiotics using broth microdilution. In accordance with the European Committee on Antimicrobial Susceptibility Testing (EUCAST) standard operating procedure, additional published MIC distributions were included for the estimation of ECOFFs for cloxacillin, cephapirin, lincomycin and tylosin, and TECOFFs for amoxicillin, benzylpenicillin, cephapirin and oxytetracycline. The proportion of non-wild-type (NWT) isolates for the beta-lactams was significantly higher in the Danish S. uberis (45-55%) compared to the Norwegian isolates (10-13%). For oxytetracycline, the proportion of NWT was significantly higher in the Danish isolates, both for S. uberis (28% vs. 3%) and S. dysgalactiae (22% vs. 0%). A bridging study testing in parallel MICs in a subset of isolates (n = 83) with the CLSI-specified and the EUCAST-specified broths showed excellent correlation between the MICs obtained with the two methods. The new ECOFFs and TECOFFs proposed in this study can be used for surveillance of antimicrobial resistance, and - for antimicrobials licensed for streptococcal bovine mastitis - as surrogate clinical breakpoints for predicting their clinical efficacy for this indication.

Relationship between Microflora Changes and Mammary Lipid Metabolism in Dairy Cows with Mastitis

Dairy mastitis is an inflammatory reaction caused by mechanical injury and stress within the mammary gland, during which microbial changes and abnormal lipid metabolism occur. However, the underlying mechanism is still unclear. The present study used a combination of 16S rDNA sequencing technology and lipidomics techniques to reveal the effects of mastitis on lactic microbiota and metabolites in the milk of dairy cows. Twenty multiparous Holstein dairy cows (2-3 parities) with an average body weight of 580 ± 30 kg were selected for this study. The dairy cows were allocated to control group (<5 × 104 cells /mL)) and mastitis group (>5 × 106 cells /mL) based on the somatic cell count. The results showed that mastitis caused a decrease trend in milk production (p = 0.058). The results of the 16 s sequencing indicated a significant decrease (p < 0.05) in the number of Proteobacteria, Tenericutes colonized in mastitis milk, and the number of Firmicutes, Bacteroidetes and Actinobacteria communities increased significantly (p < 0.05). The lipidomics results revealed that the changes in lipid content in mastitis milk were correlated with arachidonic acid metabolism, α -linolenic acid metabolism and glycerol phospholipid metabolism. The results showed that mastitis may cause abnormal lipid metabolism in milk by regulating the diversity of milk microflora, and ultimately affect the milk quality.

Effect of dry cow therapy on antimicrobial resistance of mastitis pathogens post-calving

The aim of this study was to evaluate the effect of dry cow therapy (DCT) on the antimicrobial resistance (AMR) profile of mastitis pathogens post-calving. A repository of isolates based on a DCT trial was utilized for the current study. A stratified random survey sample of cows from the trial were identified within the strata of season, herd, and trial treatment resulting in 382 cows. All isolates from the 382 cows were selected for the current study, which identified 566 isolates from milk samples collected at dry off (S1), post-calving (S2), and at the first clinical mastitis event up to 150 days in milk (S3). The AMR profiles were determined using broth microdilution method. Less than 10% of the coagulase-negative Staphylococcus species (CNS) isolates (n = 421) were resistant to tetracycline, ceftiofur, penicillin/novobiocin or erythromycin, while higher proportions of resistance to sulfadimethoxine (72%) and penicillin (28%) were observed. All Staphylococcus aureus (S. aureus) isolates (n = 4) were susceptible to all tested AMD except sulfadimethoxine, to which all isolates were resistant. Similarly, all Streptococcus spp. (n = 37) were susceptible to penicillin, penicillin/novobiocin, and ampicillin while resistant to tetracycline (17%). All coliforms (n = 21) were susceptible to ceftiofur, but resistance was recorded for sulfadimethoxine (70%), cephalothin (56%), and tetracycline (43%). The increased resistance percent from S1 to S2 was observed in CNS isolates from AMD-treated cows, with the highest increase recorded for penicillin (12.2%). Parametric survival interval regression models were used to explore the association between antimicrobial drug (AMD) therapy at dry off and the AMR phenotype post-calving. The accelerated failure-time metric was adopted to minimum inhibitory concentration measurements to permit interpretation of model exponentiated coefficients. Models for cows with CNS isolated at both S1 and S2 showed increased resistance against cephalothin, oxacillin, and ceftiofur in cows that received DCT from the same drug class, or a class with a shared resistance mechanism. In contrast, resistance of CNS isolates to tetracycline were associated with any AMD therapy at dry off. Resistance of CNS isolates to Penicillin decreased in CNS isolates in cows that received any AMD therapy at dry off compared to those that didn't. The study provided evidence that dry-cow IMM AMD was associated with AMR post-calving.

Bayesian latent class models to determine diagnostic sensitivities and specificities of two point of care rapid tests (Selma plus, Dipslide) for the detection of Streptococcus uberis associated with mastitis in dairy cows

Introduction

Development and validations of accurate mastitis diagnostics are crucial to make timely and evidence-based decisions on mastitis therapy in order to reduce its impact on productivity, animal welfare and practicing the prudent use of antimicrobials on dairy farms.

Methods

The objectives of this study were to assess the agreement between test results from reference laboratory and two point of care tests (Selma plus, Dipslide) and to estimate the test accuracies with Bayesian latent class models (BLCMs). In total of 509 single quarter milk samples from cows with mastitis were included in the study.

Results

Among all analyzed mastitis pathogens, Streptococcus spp. was detected in up to one third of all analyzed samples and for Selma all Streptococcus samples were considered as Streptococcus uberis. The agreement (κ) when comparing two tests varied greatly depending on the bacteria, ranging from no agreement to good agreement (κ = negative to 0.86) depending on the prevalence of identified pathogens. Based on BLCMs to assess diagnostic test accuracies for the pathogen Streptococcus uberis, posterior sensitivities of 76, 71, and 64% for Selma plus, Dipslide and laboratory standard culture and specificities of 93%, 98% for Selma and Dipslide, respectively, were obtained.

Discussion

The two point of care rapid culture systems Dipslide and Selma plus plate can provide important preliminary pathogen identification for targeted mastitis therapy, especially when general information about growth and a rough classification of the bacteria into groups have an impact on treatment strategy. The two evaluated rapid culture systems, Dipslide and Selma plus plate, show good test accuracies for Streptococcus uberis at least at genus level. Therefore, using these tests may contribute to prudent use of antibiotics.

Susceptibility of caprine mastitis pathogens to tildipirosin, gamithromycin, oxytetracycline, and danofloxacin: effect of serum on the in vitro potency of current macrolides

Mastitis is a significant disease in dairy ruminants, causing economic losses to the livestock industry and severe risks to public health. Antibiotic therapy is one of the most crucial practices to treat mastitis, although the susceptibility of caprine mastitis pathogens to current antibiotics has not been tested under standard or modified incubation conditions. This work evaluated the in vitro activity of tildipirosin, gamithromycin, oxytetracycline, and danofloxacin against caprine mastitis pathogens incubated following standard conditions of Clinical and Laboratory Standards Institute (CLSI) and deviation method by 25% supplementation with goat serum. Mycoplasma agalactiae, Escherichia coli, Staphylococcus aureus, Streptococcus spp., and coagulase-negative Staphylococci (CNS) were isolated from dairy goats with mastitis in Spain. Minimum inhibitory concentrations (MICs) were determined using the broth microdilution technique. The lowest MIC₉₀ under standard conditions was obtained with danofloxacin for mastitis-causing pathogens. An exception was M. agalactiae, where danofloxacin and oxytetracycline obtained low values. However, after adding serum, gamithromycin showed the lowest MIC₅₀ for S. aureus, Streptococcus spp., and CNS. The lowest MIC₅₀ was obtained with all the antibiotics tested (< 0.125 µg/ml) against M. agalactiae. Supplementing with serum resulted in a significant variation in tildipirosin and gamithromycin MIC values for CNS, S. aureus, M. agalagtiae, and E. coli. In brief, the MIC for antibiotics used against mastitis should be determined under conditions closely resembling intramammary infections to obtain representative susceptibility patterns against mastitis pathogens. Caprine mastitis pathogens were broadly susceptible to danofloxacin under standard conditions. The potency of macrolides against caprine mastitis pathogens increases when serum is present in culture media.

The Prevalence of Klebsiella spp. Associated With Bovine Mastitis in China and Its Antimicrobial Resistance Rate: A Meta-Analysis

Understanding distribution of bovine mastitis pathogen Klebsiella spp. can contribute to the treatment decision and the control within programs of bovine mastitis, we conducted a meta-analysis to investigate the epidemiology and antimicrobial resistance rates of Klebsiella spp. associated with bovine mastitis in China. Three databases, namely, PubMed, Google scholar, and China National Knowledge Infrastructure database, were utilized to obtain relevant publications. According to PRISMA reporting standards, a total of 38 publications were included in the research, among them, 7 papers included an AMR test. The pooled prevalence of Klebsiella spp. was 5.41% (95% CI: 3.87–7.50%). Subgroup analysis revealed that the prevalence was higher in South China (8.55%, 95% CI: 3.57–19.09%) than in North China (4.22%, 95% CI: 2.46–7.14%), in 2010–2020 (7.45%, 95% CI: 5.29–110.40%) than in 2000–2010 (3.14%, 95% CI: 1.90–15.14%), and in the clinical bovine mastitis cases (7.49%, 95% CI: 3.71–14.54%) than in the subclinical cases (4.03%, 95% CI: 1.55–10.08%). The pooled AMR rate revealed that Klebsiella spp. were most resistant to sulfonamides (45.07%, 95% CI: 27.72–63.71%), followed by tetracyclines (36.18%, 95% CI: 23.36–51.34%), aminoglycosides (27.47%, 95% CI: 17.16–40.92%), β-lactams (27.35%, 95% CI: 16.90–41.05%), amphenicol (26.82%, 95% CI: 14.17–44.87%), lincosamides (21.24%, 95% CI: 7.65–46.75%), macrolides (20.98%, 95% CI: 7.20–47.58%), polypeptides (15.51%, 95% CI: 6.46–32.78%), and quinolones (7.8%, 95% CI: 3.25–17.56%). The climate difference between South and North China and the natural pathogenicity of Klebsiella spp. may be the primary reasons for its distribution, and the prevalence of Klebsiella spp. indicated that the genus is an increasing hazard to the dairy industry. The prevalence of AMR in China is commonly higher than in the European countries and Canada, this is a very important concern for strategy programs to control bovine mastitis caused by Klebsiella spp. in China.

Antimicrobial Resistance in Escherichia coli Isolates from Healthy Food Animals in South Korea, 2010-2020

Antimicrobial-resistant bacteria in food animals pose a major public health threat worldwide. In this study, we aimed to assess the antimicrobial resistance profiles and resistance trends of commensal Escherichia coli isolated from the feces of healthy cattle, pigs, and chickens in South Korea during 2010 and 2020. A total of 7237 E. coli isolates (2733 cattle, 2542 pig, and 1962 chicken isolates) were tested for susceptibility towards 12 antimicrobials. About 48%, 90%, and 97% of cattle, pig, and chicken isolates, respectively, were resistant to one or more antimicrobial agents. Cattle isolates presented low resistance (<15%) to most of the tested antimicrobials. In contrast, chicken and pig isolates demonstrated a relatively high (>45%) resistance rate to ampicillin, chloramphenicol, streptomycin, and tetracycline. We observed high ciprofloxacin and nalidixic acid resistance rates in chicken (76.1% and 88.6%, respectively), isolates in pig (12.7% and 26.7%, respectively) and cattle (2.7% and 8.2%, respectively) isolates. Notably, a very small proportion of isolates (<5%) from cattle, chickens, and pigs demonstrated resistance to amoxicillin/clavulanic acid, cefoxitin, and colistin. We identified ceftiofur resistance in a small proportion of chicken (8.8%), pig (3.7%), and cattle (0.7%) isolates. We noted an increasing but fluctuating trend of ampicillin, amoxicillin/clavulanic acid, ceftiofur, cefoxitin, chloramphenicol, ciprofloxacin, and streptomycin resistance in pig isolates. Similarly, the ampicillin, ceftiofur, and chloramphenicol resistance rates were increased but fluctuated through time in chicken isolates. Overall, 56% of the isolates showed multidrug-resistant (MDR). The proportion of MDR isolates was low in cattle (17.1%); however, this proportion was high in chickens (87.1%) and pigs (73.7%). Most of the resistance patterns included streptomycin and tetracycline in pigs and cattle, and ciprofloxacin and nalidixic acid in chickens. In conclusion, this study showed high resistance of commensal E. coli isolated from major food animals in Korea to commonly used antimicrobials including critically important antimicrobials. These bacteria could not only be a resistance reservoir but also could have potential to spread this resistance through gene transfer to pathogenic bacteria. Thus, the high prevalence of antimicrobial resistance in food animals highlights the urgent need for measures to restrict and ensure the prudent use of antimicrobials in Korea.

Rapid determination of pathogens in mastitic milk of dairy cows using Gram staining

This study aimed to determine whether causative pathogens in mastitic milk can be determined by Gram staining after the centrifugation of milk. Gram staining was performed using unconcentrated and concentrated milk cells. Using this method, we found that the background of microscopic image of unconcentrated milk cells was complex and bacteria were difficult to detect. In contrast, the background of the smears in the concentrated milk cells was translucent, and bacterial and somatic cells were clearly visible. The sensitivity and specificity of the Gram staining of concentrated milk cells were 84.4% and 86.0% and 50.0% and 94.5% for the detection of gram-positive and gram-negative bacteria, respectively. The presented method provides a simple and inexpensive means of determining mastitis-causing pathogens.

Antimicrobial Resistance in Coagulase-Positive Staphylococci Isolated from Various Animals in Bosnia and Herzegovina

The objective of this study was to determine and compare the antimicrobial resistance profiles of clinical Staphylococcus pseudintermedius (n = 90) and Staphylococcus aureus (n = 61) isolates from different animal species in Bosnia and Herzegovina. Among S. pseudintermedius isolates from dogs (n = 86), resistance to penicillin (87.2%) was most common followed by amoxicillin (76.7%) and amoxicillin/clavulanic acid (67.4%). A total of 25 isolates were found to be resistant to oxacillin of which 21 were mecA-positive and multidrug resistant (MDR). The most frequent MDR pattern was penicillins-lincosamides-cephalosporins-macrolides-fluoroquinolones. Among the 61 methicillin-susceptible S. pseudintermedius isolates, 18 were MDR (penicillins-lincosamides-macrolides). Most of S. aureus isolates were resistant to penicillin (50.8%), followed by amoxicillin (41%) and amoxicillin/clavulanic acid (31%). Resistance to cefoxitin was detected in only two isolates. All S. aureus isolates were mecA and mecC-negative. MDR was observed in six S. aureus isolates of which five were from cattle (penicillins-lincosamides-fluoroquinolones). Isolates from red foxes (Vulpes vulpes) were sensitive to most of the antimicrobials tested. The high number of methicillin-resistant S. pseudintermedius and MDR isolates in dogs exposed in this study underlines the urgent need for establishment of national antimicrobial resistance surveillance program in animals in the country, as well as for the surveillance of veterinary antimicrobial consumption.

Bacteria Isolated From Milk of Dairy Cows With and Without Clinical Mastitis in Different Regions of Australia and Their AMR Profiles

Mastitis is the most common disease in dairy cattle worldwide. The objectives of this study were to estimate the prevalence of different bacterial species associated with mastitis from dairy herds located in geographically and climatically distinct zones in Australia, and to evaluate the antimicrobial susceptibility of the isolated bacteria. Quarter-level milk samples (n = 419) were collected from 151 mastitis cases and 268 healthy controls originating from 18 dairy herds located in tropical (Northern Queensland), subtropical (Southeast Queensland) and temperate zones (Victoria) between March and June 2019. Milk samples were cultured, and the isolated bacteria were grouped into six groups: Enterobacteriaceae spp.; Streptococcus spp.; Staphylococcus aureus, non-aureus staphylococci (NAS); Bacillus spp.; and Others. Mixed effects conditional logistic regression models were applied to quantify the association between the prevalence of each bacterial group and the herd zone and bulk milk tank somatic cell counts (BMTSCC). Of the 205 isolates, 102 (50%) originated from mastitis cases, and 103 (50%) from controls. Staphylococci were the most prevalent (NAS 32% and S. aureus 11%). Contagious mastitis bacteria were more prevalent in Victoria compared to Queensland dairy herds. NAS species (P < 0.001) were less prevalent in herds with BMTSCC >300,000 cells/mL compared with herds with low BMTSCC ≤150,000 cells/mL. Enterobacteriaceae and Streptococcus spp. groups showed high resistance rates to 1 (51 and 47%, respectively), and 2 (11 and 23%, respectively), antimicrobials. More than one third of the Enterobacteriaceae (48%) and Others (43%) groups spp. were resistant to at least three antimicrobials. This study provided a unique opportunity to investigate the prevalence of mastitis-associated bacteria in clinical cases and in apparently healthy controls. The findings of this study help inform mastitis control and antimicrobial stewardship programs aimed to reduce the prevalence of mastitis and antimicrobial resistance in dairy herds.

Association between antimicrobial use and antimicrobial resistance of Streptococcus uberis causing clinical mastitis

It is unknown whether overuse of antimicrobials against clinical mastitis (CM) from Streptococcus uberis is associated with increased antimicrobial resistance (AMR). Therefore, the aim of this study was to evaluate the association between antimicrobial use (AMU) and AMR in relation to the Strep. uberis causing CM in dairy herds. A total of 83 Strep. uberis isolates were selected from a collection created during a previous study evaluating the epidemiology of CM in dairy herds (n = 17) of southeastern Brazil. For each case of CM identified on farm, the following information was recorded: cow's identification number, affected mammary quarter, date of CM diagnosis, antimicrobial commercial names, number of administrations, and descriptions of protocol changes during the treatment. Streptococcus uberis isolates were confirmed by conventional culture, MALDI-TOF mass spectrometry, and quantitative multiplex PCR analyses. Thus, a total of 8 antimicrobials commonly used for CM treatment were evaluated for antimicrobial activity against Strep. uberis isolates. The minimum inhibitory levels of antimicrobials were determined at the lowest concentrations able to inhibit 50 and 90%, respectively, of Strep. uberis isolates. Data related to the antibiotics used for treatment of CM was used to calculate the frequency of administered antimicrobials as the number of defined daily doses (DDD). The highest frequencies of resistant Strep. uberis were observed for erythromycin (80.7% resistant, R), tetracycline (R = 59%), and penicillin G (R = 57.8%), whereas against ceftiofur only 10.8% of Strep. uberis isolates were resistant, and only 1.2% of the Strep. uberis isolates were resistant to enrofloxacin. Regarding the evaluation of resistance for antimicrobial classes, the highest frequency was observed for macrolides (R = 80.7%; 19.3% susceptible, S). Additionally, a frequency of 18.7% of Strep. uberis isolates were resistant to cephalosporins (S = 81.3%), respectively. Further, 94% of Strep. uberis isolates were multiresistant; all these isolates presented resistance to at least 3 different antimicrobial classes. The overall monthly average of antimicrobial treatment incidence (ATI) among the 17 herds enrolled in the study was 23.7 DDD per 1,000 lactating dairy cows [standard deviation (SD) = 13.9], ranging from 5.0 to 55.4 DDD per 1,000 cows in lactation-day. Cephalosporins and penicillins were the most commonly used antimicrobial classes among the evaluated herds (n = 16; 94.1%), followed by tetracyclines (n = 15 herds; 88.2%), fluoroquinolones (n = 14; 82.3%), and sulfonamides (n = 14; 82.3%). The tetracycline class had the highest ATI mean (5.0 DDD per 1,000 lactating cow-days, SD = 5.8), followed by fluoroquinolones (4.7 DDD per 1,000 lactating cow-days, SD = 6.0) and cephalosporins (3.8 DDD per 1,000 lactating cow-days, SD = 6.0). The overall use of antimicrobials was associated with the resistance of Strep. uberis to the antimicrobial tetracycline.

Microbial Aetiology, Antibiotic Susceptibility and Pathogen-Specific Risk Factors for Udder Pathogens from Clinical Mastitis in Dairy Cows

Simple Summary

Mastitis is among the diseases in dairy cows that most often require antibiotic treatment. In order to maintain optimal treatment, it is important to have updated knowledge about the causative agents and their antibiotic resistance patterns. This investigation aimed to reveal the most important bacterial pathogens and their resistance patterns in Sweden, and we also identified some risk factors for infection with certain pathogens. The bacteria that were the most common causes of mastitis were, in descending order, Staphylococcus aureus, Streptococcus dysgalactiae, Escherichia coli, and Streptococcus uberis. Only a few Gram-positive bacteria were resistant to penicillin, and in general, the occurrence of antibiotic resistance was low. Therefore, the potential for antibiotic treatment of bovine mastitis in Sweden is good.

Abstract

Mastitis is one of the most important infectious diseases and one of the diseases that causes the greatest use of antibiotics in dairy cows. Therefore, updated information on the bacteria that cause mastitis and their antibiotic susceptibility properties is important. Here, for the first time in over 10 years, we updated the bacterial findings in clinical mastitis in Swedish dairy cows together with their antibiotic resistance patterns and risk factors for each bacterial species. During the period 2013–2018, samples from clinical mastitis were collected, together with information on the cows and herds of origin. The samples were cultured, and a total of 664 recovered bacterial isolates were subjected to susceptibility testing. Staphylococcus aureus (S. aureus) was the most common pathogen and accounted for 27.8% of diagnoses, followed by Streptococcus dysgalactiae (S. dysgalactiae) (15.8%), Escherichia coli (E. coli) (15.1%), Streptococcus uberis (S. uberis) (11.4%), Trueperella pyogenes (T. pyogenes) (7.7%), non-aureus staphylococci (NAS) (2.8%), Klebsiella spp. (2.7%), Enterococcus spp. (1.3%), and Streptococcus agalactiae (S. agalactiae) (1.2%). Various other bacteria accounted for 2.6%. Staphylococci were, in general, susceptible to most antibiotics, but 2.6% of S. aureus and 30.4% of NAS were resistant to penicillin. No methicillin-resistant staphylococci were found. All S. agalactiae were susceptible to penicillin. Bimodal and trimodal MIC distributions for penicillin in S. dysgalactiae and S. uberis, respectively, indicate acquired reduced susceptibility in some isolates. The mostly unimodal MIC distributions of T. pyogenes indicate that acquired resistance does usually not occur in this species. Among E. coli, 14.7% were resistant to at least one antibiotic, most often ampicillin (8.7%), streptomycin (7.8%), or sulphamethoxazole (6.9%). Klebsiella spp. had low resistance to tetracycline (9.1%) but is considered intrinsically resistant to ampicillin. Pathogen-specific risk factors were investigated using multivariable models. Staphylococcus aureus, S. dysgalactiae, and T. pyogenes were more common, while E. coli was less common in quarters with more than one pathogen. S. aureus and T. pyogenes were mostly seen in early lactation, while E. coli was more common in peak to mid lactation and S. dysgalactiae in early to peak lactation. Trueperella pyogenes and Klebsiella spp. were associated with a previous case of clinical mastitis in the current lactation. Staphylococcus aureus was associated with tie stalls and T. pyogenes with loose housing. All pathogens except E. coli and S. dysgalactiae had a seasonal distribution. In conclusion, the aetiological agents for clinical bovine mastitis have remained relatively stable over the last 10–15 years, S. aureus, S. dysgalactiae, E. coli and S. uberis being the most important. Resistance to penicillin among Gram-positive agents was low, and in general, antibiotic resistance to other compounds was low among both Gram-positive and Gram-negative agents.

The Role of Streptococcus spp. in Bovine Mastitis

The Streptococcus genus belongs to one of the major pathogen groups inducing bovine mastitis. In the dairy industry, mastitis is the most common and costly disease. It not only negatively impacts economic profit due to milk losses and therapy costs, but it is an important animal health and welfare issue as well. This review describes a classification, reservoirs, and frequencies of the most relevant Streptococcus species inducing bovine mastitis (S. agalactiae, S. dysgalactiae and S. uberis). Host and environmental factors influencing mastitis susceptibility and infection rates will be discussed, because it has been indicated that Streptococcus herd prevalence is much higher than mastitis rates. After infection, we report the sequence of cow immune reactions and differences in virulence factors of the main Streptococcus species. Different mastitis detection techniques together with possible conventional and alternative therapies are described. The standard approach treating streptococcal mastitis is the application of ß-lactam antibiotics. In streptococci, increased antimicrobial resistance rates were identified against enrofloxacin, tetracycline, and erythromycin. At the end, control and prevention measures will be considered, including vaccination, hygiene plan, and further interventions. It is the aim of this review to estimate the contribution and to provide detailed knowledge about the role of the Streptococcus genus in bovine mastitis.

Genotyping and Antimicrobial Susceptibility Profiling of Streptococcus uberis Isolated from a Clinical Bovine Mastitis Outbreak in a Dairy Farm

Streptococcus uberis, an environmental pathogen responsible also for contagious transmission, has been increasingly implicated in clinical mastitis (CM) cases in Europe. We described a 4-month epidemiological investigation of Strep. uberis CM cases in an Italian dairy farm. We determined molecular characteristics and phenotypic antimicrobial resistance of 71 Strep. uberis isolates from dairy cows with CM. Genotypic variability was investigated via multiplex PCR of housekeeping and virulence genes, and by RAPD-PCR typing. Antimicrobial susceptibility was assessed for 14 antimicrobials by MIC assay. All the isolates carried the 11 genes investigated. At 90% similarity, two distinct clusters, grouping 69 of the 71 isolates, were detected in the dendrogram derived from the primer ERIC1. The predominant cluster I could be separated into two subclusters, containing 38 and 14 isolates, respectively. Strep. uberis strains belonging to the same RAPD pattern differed in their resistance profiles. Most (97.2%) of them were resistant to at least one of the drugs tested, but only 25.4% showed a multidrug resistance phenotype. The highest resistance rate was observed for lincomycin (93%), followed by tetracycline (85.9%). This study confirmed a low prevalence of β-lactam resistance in Strep. uberis, with only one isolate showing resistance to six antimicrobial classes, including cephalosporins.

Molecular Characterization of Antimicrobial Resistance and Virulence Genes of Bacterial Pathogens from Bovine and Caprine Mastitis in Northern Lebanon

Mastitis is an infectious disease encountered in dairy animals worldwide that is currently a growing concern in Lebanon. This study aimed at investigating the etiology of the main mastitis-causing pathogens in Northern Lebanon, determining their antimicrobial susceptibility profiles, and identifying their antimicrobial resistance (AMR) genes. A total of 101 quarter milk samples were collected from 77 cows and 11 goats presenting symptoms of mastitis on 45 dairy farms. Bacterial identification was carried out through matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Antimicrobial susceptibility was tested by disc diffusion and broth microdilution methods. Molecular characterization included polymerase chain reaction (PCR) screening for genes encoding extended-spectrum beta-lactamases (ESBLs) and plasmid-mediated AmpC among Enterobacterales isolates, and virulence factors among Staphylococcus isolates. Escherichia coli isolates were subjected to phylogenetic typing by a quadruplex PCR method. The most frequently identified species were Streptococcus uberis (19.2%), Streptococcus agalactiae (15.1%), E. coli (12.3%), and Staphylococcus aureus (10.96%). Gram-positive bacteria were resistant to macrolides and tetracycline, whereas gram-negative bacteria displayed resistance to ampicillin and tetracycline. Two ESBL genes, blaTEM (83.3%) and blaOXA (16.7%), and one AmpC beta-lactamase gene, blaCMY-II (16.7%), were detected among six E. coli isolates, which mainly belonged to phylogenetic group B1. Among Staphylococcus spp., the mecA gene was present in three isolates. Furthermore, four isolates contained at least one toxin gene, and all S. aureus isolates carried the ica operon. These findings revealed the alarming risk of AMR in the Lebanese dairy chain and the importance of monitoring antimicrobial usage.

Multidrug Resistance and Molecular Characterization of Streptococcus agalactiae Isolates From Dairy Cattle With Mastitis

Streptococcus agalactiae is a pathogen-associated to bovine mastitis, a health disorder responsible for significant economic losses in the dairy industry. Antimicrobial therapy remains the main strategy for the control of this bacterium in dairy herds and human In order to get insight on molecular characteristics of S. agalactiae strains circulating among Argentinean cattle with mastitis, we received 1500 samples from 56 dairy farms between 2016 and 2019. We recovered 56 S. agalactiae isolates and characterized them in relation to serotypes, virulence genes, and antimicrobial susceptibility. Serotypes III and II were the most prevalent ones (46% and 41%, respectively), followed by Ia (7%). In relation to the 13 virulence genes screened in this study, the genes spb1, hylB, cylE, and PI-2b were present in all the isolates, meanwhile, bca, cpsA, and rib were detected in different frequencies, 36%, 96%, and 59%, respectively. On the other hand, bac, hvgA, lmb, PI-1, PI-2a, and scpB genes could not be detected in any of the isolates. Disk diffusion method against a panel of eight antimicrobial agents showed an important number of strains resistant simultaneously to five antibiotics. We also detected several resistance-encoding genes, tet(M), tet(O), ermB, aphA3, and lnu(B) (9%, 50%, 32%, 32%, and 5%, respectively). The results here presented are the first molecular data on S. agalactiae isolates causing bovine mastitis in Argentina and provide a foundation for the development of diagnostic, prophylactic, and therapeutic methods, including the perspective of a vaccine.

Efficacy Prediction of Four Pharmaceutical Formulations for Intramammary Administration Containing Aloe vera (L.) Burm. f. Combined With Ceftiofur or Cloxacillin in Lactating Cows as an Alternative Therapy to Treat Mastitis Caused by Staphylococcus aureus

Synergy or additive effect between Aloe vera (L.) Burm.f. and beta-lactam (β-lactam) antibiotics has been reported against Staphylococcus aureus, one of the most important etiological agents of cow mastitis. The goal of the present study was to predict the efficacy of intramammary formulations containing the Aloe vera gel extract in the combination with cloxacillin or ceftiofur at low concentrations in lactating cows as an alternative therapy. Each quarter of 20 healthy Holstein Friesian lactating cows were treated with a single dose of one of the following formulations, corresponding to one of these treatment groups: A1, A2, A3, and A4. A1 and A2 contained cloxacillin at 0.25 and 0.5 mg/ml, whereas A3 and A4 contained ceftiofur 0.25 and 0.5 mg/ml, respectively. In addition, all formulations contained 600 mg/ml of an alcoholic extract of Aloe vera. Milk samples were taken at predefined time points. Antibiotics and aloin (active compound of Aloe vera) concentrations were assessed by liquid chromatography mass spectrometry system (LC-MS/MS). Pharmacokinetic profiles were obtained, and the efficacy index, the fraction of dosing interval in which the antimicrobial concentration remains above the minimum inhibitory concentrations (MICs) (T > MIC) for each formulation, was calculated considering MIC values against Staphylococcus aureus ATCC 29213 as obtained for the combination Aloe vera + antibiotic and aloin concentration in the extract. Mammary gland safety assessment was performed for each combination. Values of the main efficacy index for this study, T > MIC (h) for Aloe vera were 23.29, 10.50, 27.50, and 13.89, whereas for cloxacillin or ceftiofur were 19.20, 10.9, 19.74, and 15.63, for A1, A2, A3, and A4, respectively. Only A1 and A3 reached aloin and antibiotic recommended values as predictors of clinical efficacy for cloxacillin, ceftiofur, and aloin (50, 70, and 60%, respectively), assuming a dose interval of 24 h. The efficacy index values obtained suggest that A1 and A3 might be an effective therapy to treat bovine mastitis caused by S. aureus after a single dose. Nevertheless, further trials in S. aureus mastitis clinical cases are mandatory to confirm the efficacy of Aloe vera formulations.

In vitro antimicrobial resistance profiles of Streptococcus uberis, Lactococcus spp., and Enterococcus spp. from quarter milk samples of cows between 2015 and 2019 in Southern Germany

The objective was to describe and compare antimicrobial resistance patters of esculin-hydrolyzing streptococci and streptococcal-like organisms (Streptococcus uberis, Enterococcus faecium, Enterococcus faecalis, Lactococcus garvieae, Lactococcus lactis) from routine diagnostic samples of the udder health laboratory of the Bavarian Animal Health Services between 2015 and 2019. All routine diagnostic samples of the udder health laboratory of the Bavarian Animal Health Services, that were tested with a standard microbroth dilution, were eligible to be included in this retrospective case series. A California Mastitis Test result was available for all samples. Most Strep. uberis and L. lactis were susceptible to all antibiotics tested. Enterococcus faecium had consistently the highest minimum inhibitory concentration required to inhibit the growth of 90% of tested isolates. The resistance patterns of Lactococcus garvieae were positioned between enterococci and L. lactis. The minimum inhibitory concentration for various antibiotics and pathogens tended to decrease over the 5-yr period. Regardless of the pathogen, isolates of clinical cases were less likely to express in vitro resistance than isolates of healthy or subclinical cases. Streptococcus uberis or L. lactis showed hardly any in vitro resistance to tested antibiotic groups. Penicillin should remain the first-choice antimicrobial for the therapy of Strep. uberis and Lactococcus spp. However, a success of any antimicrobial treatment of enterococcal infections seems questionable.

Antimicrobial Resistance of Major Bacterial Pathogens from Dairy Cows with High Somatic Cell Count and Clinical Mastitis

Simple Summary

Mastitis is the most prevalent disease of dairy cattle that causes significant economic losses. Different agents cause mastitis which leads to increased somatic cell count (SCC) and low milk quality. Treating mastitis with antimicrobials is essential to reduce SCC and improve milk quality. Excessive use or misuse of antimicrobials in dairy farms leads to the development of antimicrobial resistant bacteria. The objectives of this study were (1) to isolate and identify the causative agent of mastitis and (2) determine antimicrobial resistance profiles of bacterial isolates. A total of 174 quarter milk samples from 151 cows with high SCC and clinical mastitis from 34 dairy farms in Tennessee, Kentucky, and Mississippi were collected. Bacterial causative agents were determined by bacteriological and biochemical tests. Antimicrobial resistance of bacterial isolates against 10 commonly used antimicrobials was tested. A total of 193 bacteria consisting of six bacterial species, which include Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, Escherichia coli, Klebsiella oxytoca and Klebsiella pneumoniae were isolated. Staphylococcus aureus was the predominant isolate. The proportion of resistant isolates was relatively higher in Gram-negatives than Gram-positives. Continuous antimicrobial resistance testing and identification of reservoirs of resistance traits in dairy farms are essential to implement proper mitigation measures.

Abstract

Mastitis is the most prevalent and economically important disease caused by different etiological agents, which leads to increased somatic cell count (SCC) and low milk quality. Treating mastitis cases with antimicrobials is essential to reduce SCC and improve milk quality. Non-prudent use of antimicrobials in dairy farms increased the development of antimicrobial resistant bacteria. This study’s objectives were (1) to isolate and identify etiological agents of mastitis and (2) to determine antimicrobial resistance profiles of bacterial isolates. A total of 174 quarter milk samples from 151 cows with high SCC and clinical mastitis from 34 dairy farms in Tennessee, Kentucky, and Mississippi were collected. Bacterial causative agents were determined by bacteriological and biochemical tests. The antimicrobial resistance of bacterial isolates against 10 commonly used antimicrobials was tested. A total of 193 bacteria consisting of six bacterial species, which include Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, Escherichia coli, Klebsiella oxytoca and Klebsiella pneumoniae were isolated. Staphylococcus aureus was the predominant isolate followed by Strep. spp., E. coli, and Klebsiella spp. Results of this study showed that Gram-negatives (E. coli and Klebsiella spp.) were more resistant than Gram-positives (Staph. aureus and Streptococcus spp.). Continuous antimicrobial resistance testing and identification of reservoirs of resistance traits in dairy farms are essential to implement proper mitigation measures.

Molecular Characterization of Multidrug-Resistant Escherichia coli Isolated from Milk of Dairy Cows with Clinical Mastitis in Algeria

The objective of this study was to investigate the occurrence of multidrug-resistant Escherichia coli in cows with clinical mastitis in 42 different dairy farms located in the Bordj Bou Arreridj region of Algeria. Milk samples were cultured on Columbia blood agar, and isolates were then identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. In total, 200 samples were screened and 52 E. coli strains confirmed as causative agents were obtained. The antimicrobial susceptibility testing was performed by disk diffusion method. Antibiotic resistance genes, including those conferring resistance to extended-spectrum β-lactamases (i.e., blaTEM, blaSHV, and blaCTX-M), tetracyclines (tetA, tetB, tetC, and tetJ), aminoglycosides [aph(3'), aac(3'), aac(6'), ant, aad, and armA], and quinolones (qnrA and qnrB) were amplified by standard PCR and sequenced when positive. Transferability of resistance genes has been investigated by conjugation experiments and multilocus sequence typing. The most frequently observed resistance was to amoxicillin (86.5%), followed by tetracycline (75%), amoxicillin-clavulanic acid (59.6%), trimethoprim-sulfamethoxazole (36.5%), doxycycline (13.5%), and ciprofloxacin (13.5%). Multidrug resistance was observed in 38.4% of isolates. Genotypic characterization showed that tetA (44.2%) and blaTEM-1 (30.7%) genes were the most prevalent. Screening for plasmid-mediated quinolone resistance genes demonstrated that seven isolates (13.5%) expressed qnrB and one isolate (1.9%) harbored qnrA. In addition, aminoglycoside resistance determinants including aadA1 and aac(3)-Id were detected in seven and two isolates, respectively. Moreover, blaTEM, tetA, tetB, qnrB, and aadA1 were successfully transferred horizontally to transconjugant strains. The multilocus sequence typing revealed the presence of three different sequence types (ST162, ST371, and ST 949).

Outcome prediction from the first examination in clinical mastitis using ultrasonography in dairy cows

The aim of this study was to examine whether ultrasonography can be used to predict the outcome of clinical mastitis in dairy cows. Forty-seven mastitic quarters of Holstein-Friesian cows were examined using ultrasonography at the time of the first examination. In mastitic mammary tissue, three sonographic signs indicating tissue abnormality were found: a hyperechoic spot in the parenchyma area, structural changes to the milk duct, and non-homogeneous parenchyma. Logistic regression was used to evaluate whether the abnormal findings in the sonographic images can be used to predict the outcome of clinical mastitis. The outcomes of clinical mastitis were defined by the return, or failure to return, to marketable milk production. The sonogram finding of non-homogeneous parenchyma in the first examination did predict the outcome of clinical mastitis, whereas the type of systemic symptoms (severe or moderate) was not a predictor in this regression model. Therefore, ultrasound examinations of mammary glands in the first examination could be a useful method for predicting outcome of clinical mastitis. There is an economic benefit if ultrasound examination in first examination helps in the decision of whether or not to treat the mastitic cows.

Enterobactin Deficiency in a Coliform Mastitis Isolate Decreases Its Fitness in a Murine Model: A Preliminary Host-Pathogen Interaction Study

Iron is an essential nutrient for bacterial growth. Therefore, bacteria have evolved chelation mechanisms to acquire iron for their survival. Enterobactin, a chelator with high affinity for ferric iron, is secreted by Escherichia coli and contributes to its improved bacterial fitness. In this preliminary study, we evaluated enterobactin deficiency both in vitro and in vivo in the context of E. coli mastitis. Firstly, we showed that expression of lipocalin 2, a protein produced by the host that is able to both bind and deplete enterobactin, is increased upon E. coli infection in the cow's mastitic mammary gland. Secondly, we demonstrated in vitro that enterobactin deficiency does not alter interleukin (IL)-8 expression in bovine mammary epithelial cells and its associated neutrophil recruitment. However, a significantly increased reactive oxygen species production of these neutrophils was observed. Thirdly, we showed there was no significant difference in bacterial in vitro growth between the enterobactin-deficient mutant and its wild-type counterpart. However, when further explored in a murine model for bovine mastitis, the enterobactin-deficient mutant vs. the wild-type strain revealed a significant reduction of the bacterial load and, consequently, a decrease in pro-inflammatory cytokines (IL-1α,-1β,-4,-6, and-8). A reduced neutrophilic influx was also observed immunohistochemically. These findings therefore identify interference of the enterobactin iron-scavenging mechanism as a potential measure to decrease the fitness of E. coli in the mastitic mammary gland.

Antimicrobial & antiparasitic use and resistance in British sheep and cattle: a systematic review

A variety of antimicrobials and antiparasitics are used to treat British cattle and sheep to ensure animal welfare, a safe food supply, and maintain farm incomes. However, with increasing global concern about antimicrobial resistance in human and animal populations, there is increased scrutiny of the use of antimicrobials in food-producing animals. This systematic review sought to identify and describe peer and non-peer reviewed sources, published over the last ten years, detailing the usage of, and resistance to, antimicrobials and antiparasitics in sheep and cattle farming systems in Britain as well as identify knowledge gaps. Applying the PRISMA review protocol and guidelines for including grey literature; Scopus, Web of Science, Medline, and government repositories were searched for relevant articles and reports. Seven hundred and seventy titles and abstracts and 126 full-text records were assessed, of which 40 scholarly articles and five government reports were included for data extraction. Antibiotic usage in sheep and cattle in Britain appear to be below the UK average for all livestock and tetracyclines and beta-lactam antibiotics were found to be the most commonly used. However, the poor level of coverage afforded to these species compared to other livestock reduced the certainty of these findings. Although resistance to some antibiotics (using Escherichia coli as a marker) appeared to have decreased in sheep and cattle in England and Wales over a five-year period (2013-2018), levels of resistance remain high to commonly used antibiotics. The small number and fragmented nature of studies identified by this review describing anthelmintic usage, and the lack of available national sales data, prevented the identification of trends in either sheep or cattle. We recommend that additional efforts are taken to collect farm or veterinary level data and argue that extraction of this data is imperative to the development of antimicrobial and antiparasitic resistance strategies in Britain, both of which are needed to reduce usage of these anti-infective agents, curb the development of resistance, and safeguard national agricultural production. Finally, metrics produced by this data should be generated in a way to allow for maximum comparability across species, sectors, and countries.

Seasonal variations in the concentration of antimicrobial components in milk of dairy cows

The incidence of bovine mastitis and the bulk milk somatic cell count (BMSCC) are influenced by season, which may be associated with innate immune functions, including antimicrobial components in mammary glands. Therefore, the present study was conducted to examine the effect of season on antimicrobial components in milk. Rectal temperature and plasma cortisol, thyroxine, and derivatives of reactive oxygen metabolites (d-ROMs) were measured as stress parameters. Concentrations of lactoferrin (LF), lingual antimicrobial peptide (LAP), psoriasin (S100A7), and Immunoglobulin A (IgA) in milk were measured as indicators of innate immune function. LF and LAP concentrations were significantly lower in summer than in winter and spring, respectively, whereas the concentration of S100A7 was significantly lower in winter than in spring and autumn. The rectal temperature was significantly higher in summer than in other seasons, whereas plasma cortisol, thyroxine, and d-ROMs did not exhibit any seasonal variation. In conclusion, even though stress parameters were not changed, the concentration of antimicrobial components, such as LF and LAP, decreased in summer, which may explain the frequent occurrence of mastitis during this season.

Antimicrobial susceptibility of nine udder pathogens recovered from bovine clinical mastitis milk in Europe 2015-2016: VetPath results

VetPath is an ongoing pan-European antimicrobial susceptibility monitoring programme collecting pathogens from diseased cattle, pigs and poultry not recently treated with antibiotics. Non-duplicate isolates (n = 1244) were obtained from cows with acute clinical mastitis in eight countries during 2015-2016 for centrally antimicrobial susceptibility testing according CLSI standards. Among Escherichia coli (n = 225), resistance was high to ampicillin and tetracycline, moderate to kanamycin and low to amoxicillin/clavulanic acid and cefazolin. The MIC_50/90 of danofloxacin, enrofloxacin and marbofloxacin were 0.03 and 0.06 μg/mL. For Klebsiella spp. (n = 70), similar results were noted, except for ampicillin and kanamycin. We detected 3.7 % (11/295) Enterobacteriaceae isolates carrying an ESBL/AmpC gene. Staphylococcus aureus (n = 247) and coagulase-negative staphylococci (CoNS; n = 189) isolates were susceptible to most antimicrobials tested except to penicillin (25.1 and 29.1 % resistance). Two S. aureus and thirteen CoNS isolates harboured mecA gene. Streptococcus uberis isolates (n = 208) were susceptible to β-lactam antibiotics (87.1-94.7 % susceptibility), 23.9 % were resistant to erythromycin and 37.5 % to tetracycline. Resistance to pirlimycin was moderate. For Streptococcus dysgalactiae (n = 132) the latter figures were 10.6 and 43.2 %; pirlimycin resistance was low. MIC values for Streptococcus agalactiae, Trueperella pyogenes and Corynebacterium spp. were generally low. This current VetPath study shows that mastitis pathogens were susceptible to most antimicrobials with exceptions of staphylococci against penicillin and streptococci against erythromycin or tetracycline. For most antimicrobials, the percentage resistance and MIC_50/90 values among the major pathogens were comparable to that of the preceeding VetPath surveys. This work highlights the high need to set additional clinical breakpoints for antimicrobials frequently used to treat mastitis.

Comparison of phenotypic and genotypic antimicrobial resistance patterns associated with Staphylococcus aureus mastitis in German and Danish dairy cows

Staphylococcus aureus is one of the most common pathogens associated with bovine mastitis in Germany and Denmark. Successful therapy is strongly linked to the susceptibility of the pathogen to the administered antimicrobial. An increase in resistant pathogens in human and veterinary medicine has become a concern worldwide and hampers therapy due to reduced susceptibility. In the present study, susceptibility testing was performed for 85 and 93 S. aureus isolates originating from mastitis cases on 12 German and 8 Danish dairy farms, respectively. Phenotypic examination was performed by detection of minimal inhibitory concentration (MIC) values using the broth microdilution method, followed by genotypic investigations of the blaZ and mecA resistance genes via PCR. The tested antimicrobials were the most frequently used β-lactams in German and Danish dairy farms, including cefquinome, cefoperazone, cephapirin, penicillin, oxacillin, cloxacillin, amoxicillin-clavulanic acid, and cephalexin-kanamycin. Special attention was paid to varying therapy concepts because, in Germany, third- and fourth-generation cephalosporins have been predominantly used in mastitis therapy, whereas in Denmark, restrictive use of penicillin is followed by a general avoidance of cephalosporins. Differences in MIC values were mainly based on determined MIC₉₀ values (MIC at which 90% of isolates are inhibited). In general, Danish S. aureus isolates were inhibited at comparatively lower MIC₉₀ values than S. aureus isolated from German dairy farms for most β-lactams. No differences were observed regarding cefquinome, because both German and Danish isolates exhibited MIC₅₀ and MIC₉₀ values of 0.5 and 1 µg/mL, respectively. In contrast, the MIC₉₀ for penicillin against German and Danish S. aureus were 0.5 and ≤0.06 µg/mL, respectively. Resistance genes (blaZ, mecA) were only detected in German S. aureus isolates on 3 dairy farms in Germany. A total of 5 isolates tested positive for both blaZ and mecA, whereas 1 isolate carried the blaZ resistance gene only. A direct correlation between frequently used antimicrobials and reduced susceptibility could not be determined based on results of the present study. In addition to further research to determine factors associated with resistance development, we emphasize the urgent need for internationally standardized clinical breakpoints to assess resistance situations more accurately.

Antimicrobial Effects Caused by Aloe barbadensis Miller on Bacteria Associated with Mastitis in Dairy Cattle

It is known that the primary etiological agents associated with bovine mastitis show high levels of antimicrobial resistance. In this paper, we studied a possible alternative to antimicrobial treatment, Aloe barbadensis Miller ( A. vera). Our goal was to determine the viability of bacteria upon treatment with a methanolic extract of A. vera gel, rich in anthraquinones such as aloin A, aloin B, and aloe emodin. To this purpose, we used fluorescence spectrometry to study the following bacteria: Staphylococcus aureus, Escherichia coli, Streptococcus uberis, and Methicillin-resistant Staphylococcus aureus (MRSA). The results show that treatment with A. vera gel extract disrupted the cell membrane causing lysis in 75% of Staphylococcus aureus, in 88% of E. coli, in 97% of Streptococcus uberis, and in 88% of MRSA cells. Cell membrane disruption is attributed to the presence of anthraquinones. Further study is needed to determine whether other phenolic compounds present in the extract, influencing antimicrobial activity, could be used to develop pharmaceutical formulations to treat bovine mastitis.

Phenotypic antimicrobial susceptibility of environmental bacteria from mastitic milk of pastured dairy cows of S. Miguel (Azores)

The aim of this study was to evaluate the phenotypic (in vitro) antimicrobial susceptibility of milk pathogens isolated from subclinical and clinical mastitis in outdoor dairy herds of S. Miguel, Azores. Between January and March 2018, a total of 144 isolates was obtained from dairy cows with mastitis. Escherichia coli (38.9%; n = 56), Streptococcus uberis (20.1%; n = 29), and coagulase-negative staphylococci (17.4%; n = 25) were the major milk pathogens isolated. An in vitro average susceptibility of 52.0% was observed for 13 different antimicrobials (n = 725). According to an analysis of the mean for proportions, the proportions of bacterial isolates presenting in vitro susceptibility to danofloxacin (75.3%; P < 0.001) and to trimethoprim-sulfamethoxazole (25.0%; P < 0.05) were outside of the upper (65.8%) and lower (25.6%) decision lines, respectively. This profile was related to mainly with E. coli and Strep. uberis isolates. Multidrug resistance was observed in 2.1% isolates, namely, in two Strep. uberis strains from the same farm and one Enterococcus sp. strain. In conclusion, varying degrees of in vitro susceptibility of milk pathogens to the tested antimicrobials were observed, suggesting that these environmental bacteria probably play an important role in the spread of antimicrobial resistances in pastures. The use of fluoroquinolones to treat mastitis of dairy cows should be carefully evaluated in order to maintain their suitability for human medicine.

Antimicrobial Susceptibility of Streptococci Most Frequently Isolated from Czech Dairy Cows with Mastitis

The aim was to investigate the antimicrobial susceptibility of most frequently isolated streptococci from Czech dairy herds. A total of 3,719 quarter milk samples were collected and cultivated between January 2017 and June 2018 from cows with clinical or subclinical mastitis from 112 farms. Only one isolate of each species, collected from the same farm per six-month period, was included in the susceptibility testing. The susceptibilities of Streptococcus uberis (163 isolates) and S. dysgalactiae (25 isolates) to 10 antimicrobials (penicillin – PEN, amoxicillin/clavulanic acid – AMC, ceftiofur – EFT, clindamycin – CLI, gentamicin – GEN, streptomycin – STR, trimethoprim/sulfamethoxazole – SXT, enrofloxacin – ENR, tetracycline – TET, rifampicin – RIF) from 9 groups were determined by measuring their minimum inhibitory concentrations. The percentages of resistant S. uberis isolates to the antimicrobials were as follows: TET (63.2%), STR (52.1%), CLI (30.1%), and RIF (2.5%). Intermediate susceptibility was found to RIF (63.2%), PEN (35%), ENR (2.5%), EFT (1.8%), and AMC (1.2%). All the S. uberis isolates were susceptible to GEN and SXT (100%). However, only 6.7% of S. uberis isolates were susceptible to all tested antimicrobials, and 38.7% of isolates were multidrug resistant (≥ 3 groups of antimicrobials). All the S. dysgalactiae isolates were susceptible to PEN, AMC, EFT, GEN, SXT, and ENR (100%). Resistant S. dysgalactiae isolates were found to TET (60%), STR (28%), CLI (12%), and intermediate to TET (24%) and RIF (20%). Sixteen percent of S. dysgalactiae isolates were multidrug resistant. The relatively high occurrence of (multiple) resistance, relative to mastitis pathogens, highlights the importance of monitoring this condition in dairy herds.

Identification of Multidrug-Resistant Escherichia coli from Bovine Clinical Mastitis Using a Ceftiofur-Supplemented Medium

Escherichia coli causes a significant number of clinical mastitis cases in dairy cattle worldwide. The antimicrobial susceptibility of E. coli is important for both human and animal health. Surveillance reports recorded that the efficacy of most antibiotics is substantially preserved but detection of E. coli from clinical mastitis cases producing extended-spectrum beta-lactamases and plasmid-encoded AmpC beta-lactamases has been reported. These resistance determinants have frequently been associated with multidrug resistance. The aim of this study was to determine if a MacConkey agar medium supplemented with 8 mg/L of ceftiofur (MC-CEF) could be a useful tool to identify cephalosporin-resistant and multidrug-resistant (MDR) E. coli among bovine mastitis isolates. During the period 2010-2011, 773 E. coli were isolated from bovine clinical mastitis milk samples collected in 80 dairy farms in Northern Italy. A total of 105 E. coli were selected and assigned either to group randomly selected E. coli (RSEC; n = 53), based on a random selection among the whole collection of 773 E. coli, or to group ceftiofur-resistant E. coli (CEFREC; n = 52). CEFREC isolates were identified by spreading the 773 E. coli isolates on MC-CEF. Minimum inhibitory concentration (MIC) was used to test the phenotypic antimicrobial susceptibility to 16 antibiotics. The MIC results confirmed the ceftiofur resistance in 73.1% (38/52) of CEFREC isolates, whereas all RSEC isolates were susceptible to ceftiofur. The comparison of MIC values for each antibiotic tested between the two groups revealed significantly higher frequencies of resistance to antimicrobials other than ceftiofur in the CEFREC group. Resistance profiles highlighted a significantly higher frequency of MDR isolates among CEFREC (73.1%) than RSEC (17%) E. coli. The results showed that MC-CEF may be a useful selective medium to identify cephalosporin-resistant and MDR E. coli on dairy farms, without performing MIC on all the isolates.

Antimicrobial resistance profiles of 5 common bovine mastitis pathogens in large Chinese dairy herds

The prevalence of antimicrobial resistance (AMR) is increasing in human and animal pathogens, becoming a concern worldwide. However, prevalence and characteristics of AMR of bovine mastitis pathogens in large Chinese dairy herds are still unclear. Therefore, our objective was to determine the AMR profile of bacteria isolated from clinical mastitis in large (>500 cows) Chinese dairy herds. A total of 541 isolates of the 5 most common species, Staphylococcus aureus (n = 103), non-aureus staphylococci (NAS; n = 107), Streptococcus species (n = 101), Klebsiella species (n = 130), and Escherichia coli (n = 100), isolated from bovine clinical mastitis on 45 dairy farms located in 10 provinces of China were included. Presence of AMR was determined by minimum inhibitory concentrations using the microdilution method. Prevalence of multidrug resistance (resistance to >2 antimicrobials) was 27% (148/541). A very wide distribution of minimum inhibitory concentrations was screened in all isolates, including Staph. aureus isolates, which were resistant to penicillin (66%). In addition, NAS (30%) were more resistant than Staph. aureus to oxacillin (84%), penicillin (62%), tetracycline (34%), and clindamycin (33%). Prevalence of resistance to tetracycline was high (59%) in Streptococcus spp. Additionally, prevalence of resistance of both E. coli and Klebsiella spp. was high to amoxicillin/clavulanate potassium (81 and 38%, respectively), followed by tetracycline (only Klebsiella spp. 32%). A high proportion (27%) of isolates were multidrug resistant; the most frequent combinations were clindamycin-cefalexin-tetracycline or enrofloxacin-cefalexin-penicillin patterns for Staph. aureus; enrofloxacin-oxacillin-penicillin-tetracycline patterns for NAS; clindamycin-enrofloxacin-tetracycline patterns for Streptococcus spp.; amoxicillin/clavulanate potassium-ceftiofur-polymyxin B patterns for Klebsiella spp.; and amoxicillin/clavulanate potassium-ceftiofur-polymyxin B patterns for E. coli. Resistance for 4 kinds of antimicrobials highly critical for human medicine, including daptomycin, vancomycin, imipenem, and polymyxin B, ranged from 0 to 24%. In conclusion, prevalence of AMR in mastitis pathogens was high on large Chinese dairy farms, potentially jeopardizing both antimicrobial efficacy and public health. Results of this study highlighted the need for improvements in antimicrobial stewardship and infection control programs in large Chinese dairy farms to reduce emergence of AMR.

Antimicrobial Resistance in Streptococcus spp

The genus Streptococcus includes Gram-positive organisms shaped in cocci and organized in chains. They are commensals, pathogens, and opportunistic pathogens for humans and animals. Most Streptococcus species of veterinary relevance have a specific ecological niche, such as S. uberis, which is almost exclusively an environmental pathogen causing bovine mastitis. In contrast, S. suis can be considered as a true zoonotic pathogen, causing specific diseases in humans after contact with infected animals or derived food products. Finally, Streptococcus species such as S. agalactiae can be sporadically zoonotic, even though they are pathogens of both humans and animals independently. For clarification, a short taxonomical overview will be given here to highlight the diversity of streptococci that infect animals. Several families of antibiotics are used to treat animals for streptococcal infections. First-line treatments are penicillins (alone or in combination with aminoglycosides), macrolides and lincosamides, fluoroquinolones, and tetracyclines. Because of the selecting role of antibiotics, resistance phenotypes have been reported in streptococci isolated from animals worldwide. Globally, the dynamic of resistance acquisition in streptococci is slower than what is experienced in Enterobacteriaceae, probably due to the much more limited horizontal spread of resistance genes. Nonetheless, transposons or integrative and conjugative elements can disseminate resistance determinants among streptococci. Besides providing key elements on the prevalence of resistance in streptococci from animals, this article will also largely consider the mechanisms and molecular epidemiology of the major types of resistance to antimicrobials encountered in the most important streptococcal species in veterinary medicine.

Antimicrobial resistance in bacteria isolated from mastitis in dairy cattle in France, 2006-2016

In dairy cattle, mastitis is the most frequent bacterial disease, and the routine use of antibiotics for treatment and prevention can drive antimicrobial resistance (AMR). The aim of our study was to estimate the levels of AMR of the 3 main bacteria isolated from dairy cattle with mastitis in France (Streptococcus uberis, Escherichia coli, and coagulase-positive staphylococci) and to investigate their changes over time. Data collected between 2006 and 2016 by the French surveillance network for AMR in pathogenic bacteria of animal origin (called RESAPATH) were analyzed. The proportions of mono- and multidrug resistance were calculated and the trends were investigated using nonlinear analyses applied to time series. Over the whole period, the lowest proportions of resistance in S. uberis isolates were observed for oxacillin (2.2%) and gentamicin (2.4%) and most resistance levels were below 20%. The trends in resistance showed some significant variation, mainly for S. uberis, but without a common pattern across the various antibiotics examined. For only 2 combinations of bacteria-antibiotic the trend in resistance showed a continuous increase from 2006 to 2016: tetracycline resistance in S. uberis isolates and third-generation cephalosporin resistance in E. coli isolates. In E. coli, the highest proportions of resistance were observed for amoxicillin (28.1%) and tetracycline (23.1%). Resistance to third-generation cephalosporins in E. coli from dairy cattle was almost nil in 2006, but reached 2.4% in December 2016. This increase is particularly concerning because these antibiotics constitute one of the latest therapeutic alternatives to fight severe infectious diseases in humans. Except for penicillin (33.9%), the proportions of resistance in coagulase-positive staphylococci were below 11% during the whole study period. Multidrug resistance (isolates with acquired resistance to at least one antibiotic in 3 or more antibiotic classes) ranged from 2.4% for coagulase-positive staphylococci to 9.9% for S. uberis. These findings can serve as guidelines for practitioners in the choice of the most appropriate antibiotic according to the prevailing epidemiological context. Ultimately, our results contribute to risk assessment of AMR and provide a baseline for setting up and evaluating control measures and designing strategies to limit AMR.

Antimicrobial susceptibility patterns of Escherichia coli phylogenetic groups isolated from bovine clinical mastitis

Determination of antimicrobial susceptibility (AMS) of Escherichia coli causing clinical mastitis (CM) according to the phylogenetic groups and its association with descriptors at the cow and herd level may help improve specific strategies for treatment and control of this pathogen in dairy herds. The aims of the present study were to (a) determine the frequency of phylogenetic groups of E. coli isolated from CM in dairy cows, and its association with cow-level descriptors (parity, lactation stage, CM severity, and affected quarter position), housing system, and season; and (b) determine and compare AMS among E. coli phylogenetic groups. A quadruplex PCR method was used to classify E. coli isolates into 1 of the 7 phylogenetic groups. Minimal inhibitory concentrations were determined for 10 antimicrobials, and survival analysis was performed to evaluate the AMS differences among E. coli phylogroups. Most E. coli isolates belonged to phylogroups A (52%) and B1 (38%). None of the cow- and herd-level descriptors were associated with the E. coli phylogenetic groups. Overall, E. coli isolates were mostly susceptible to ceftiofur (96.8%), sulfadimethoxine (75.5%), and cephalothin (74.5%). Based on the survival analysis, differences in AMS between phylogenetic groups of E. coli was observed only for cephalothin, in which strains of phylogroup A were inhibited at lower minimum inhibitory concentration than strains of phylogroup B1. Results of this study indicated low susceptibility of E. coli isolates identified from CM to most antimicrobials. In addition, differences in AMS can occur among E. coli phylogenetic groups, although they may be uncommon as they were limited to only one antimicrobial (i.e., cephalothin).

Repurposing Ionophores as novel antimicrobial agents for the treatment of bovine mastitis caused by Gram-positive pathogens

Increasing reports of multidrug-resistant bacterial infections in animals has created a need for novel antimicrobial agents that do not promote cross-resistance to critically important antimicrobial classes used in human medicine. In response to the recent emergence of antimicrobial resistance in several bovine mastitis pathogens, in vitro antimicrobial susceptibility was determined for four polyether ionophores (lasalocid, monensin, narasin and salinomycin) against Staphylococcus spp. and Streptococcus spp. isolated from clinical cases. In addition, erythrocyte haemolysis and WST-1 cell proliferation assays were used to assess in vitro mammalian cell cytotoxicity and biofilm susceptibility testing was performed using the minimum biofilm eradication concentration (MBEC™) biofilm assay. Lasalocid, monensin, narasin and salinomycin exhibited bacteriostatic antimicrobial activity against all pathogens tested, including methicillin-resistant staphylococci, with MIC₉₀ values <16 μg/ml. Narasin and monensin displayed the least toxicity against mammalian cell lines and all compounds significantly reduced viable cell numbers in a Staphylococcus aureus biofilm. Based on in vitro characterization, all four ionophores offer potentially novel treatments against bovine mastitis but in vivo studies will be essential to determine whether acceptable safety and efficacy is present following intramammary administration.

Determination of the intramammary dose of benzylpenicillin required to maintain an adequate concentration in the milk to inhibit Gram-positive bacteria in the clinically normal udder for 24 hr

The aim of this study was to determine the intramammary dose of benzylpenicillin required to maintain a concentration in the milk above the MIC for the Gram-positive bacteria that cause mastitis. The product used in this study was a commercially available procaine benzylpenicillin in an oily suspension with micronized particles. Three dose levels were used: 200,000, 300,000, and 600,000 IU. Concentrations of benzylpenicillin in cow milk and plasma were determined after a single intramammary dose was administered into one quarter of each of the five cows in each treatment group. Samples were analyzed using an HPLC-MS/MS method, which was validated during the study. Concentrations in the milk were well above the MIC for the target pathogens for all doses tested. There was a linear dose-dependent increase in the mean AUCs of benzylpenicillin concentrations in plasma and milk. At the first milking, 12 hr after dosing, there was a significant difference between the mean milk benzylpenicillin concentrations in cows treated with a dose of 600,000 IU, and those treated with 200,000 or 300,000 IU. Although this study shows a linear relationship between the dose of procaine benzylpenicillin administered and the concentration in the milk in the healthy udder, it would be useful to conduct studies on cows with mastitis to define the optimum dose and duration of intramammary treatment with benzylpenicillin.

Genomic analysis and immune response in a murine mastitis model of vB_EcoM-UFV13, a potential biocontrol agent for use in dairy cows

Bovine mastitis remains the main cause of economic losses for dairy farmers. Mammary pathogenic Escherichia coli (MPEC) is related to an acute mastitis and its treatment is still based on the use of antibiotics. In the era of antimicrobial resistance (AMR), bacterial viruses (bacteriophages) present as an efficient treatment or prophylactic option. However, this makes it essential that its genetic structure, stability and interaction with the host immune system be thoroughly characterized. The present study analyzed a novel, broad host-range anti-mastitis agent, the T4virus vB_EcoM-UFV13 in genomic terms, and its activity against a MPEC strain in an experimental E. coli-induced mastitis mouse model. 4,975 Single Nucleotide Polymorphisms (SNPs) were assigned between vB_EcoM-UFV13 and E. coli phage T4 genomes with high impact on coding sequences (CDS) (37.60%) for virion proteins. Phylogenetic trees and genome analysis supported a recent infection mix between vB_EcoM-UFV13 and Shigella phage Shfl2. After a viral stability evaluation (e.g pH and temperature), intramammary administration (MOI 10) resulted in a 10-fold reduction in bacterial load. Furthermore, pro-inflammatory cytokines, such as IL-6 and TNF-α, were observed after viral treatment. This work brings the whole characterization and immune response to vB_EcoM-UFV13, a biocontrol candidate for bovine mastitis.

Bacteriophage Isolated from Sewage Eliminates and Prevents the Establishment of Escherichia Coli Biofilm

Purpose: Biofilm growth exerts a negative impact on industry and health, necessitating the development of strategies to control. The objective of this work was study the lytic activity of the phage isolated from the sewage network in the formation and degradation of Escherichia coli biofilms.

Methods: E. coli cultures were incubated in 96-well polystyrene microplates under controlled conditions to evaluate the biofilm formation. The E. coli cultures and established biofilms were treated with the suspensions of the vB_EcoM-UFV017 (EcoM017) bacteriophage obtained from sewage for 24 hours. The E. coli bacterial density was measured using absorbance at 600 nm and the biofilms were measured by crystal violet staining. Polystyrene coupons were used as support for Scanning Electron Microscopy and Confocal Microscopy to evaluate biofilm formation.

Results: The E. coli strains formed biofilms in polystyrene microplates after 48 hours’ incubation. The highest EcoM017 phage titer, in the prevention and degradation experiments, reduced the bacterial growth and the quantity of biofilm formed by E. coli in 90.0% and 87.5%, respectively. The minimum dose capable of reducing the biofilms of this bacterium was 10¹ PFU/mL after 24 hours. The preformed E. coli biofilm mass was reduced 79% post exposure to the phage in the degradation assay. Microscopic analysis confirmed the results obtained in the plates assays.

Conclusion: The EcoM017 phage prevented biofilm formation and degraded the E. coli-established ones. The EcoM017 phage isolated from sewage can reduce bacterial attachment and lyse the E. coli associated biofilm cells, offering biotechnological potential applicability for this phage.

Biofilm production and other virulence factors in Streptococcus spp. isolated from clinical cases of bovine mastitis in Poland

Background

Mastitis is a common disease in dairy cattle throughout the world and causes considerable economic losses each year. An important aetiological agent of this disease is bacteria of the genus Streptococcus; hence, exploring the mechanisms of virulence in these bacteria is an extremely important step for the development of effective prevention programmes. The purpose of our study was to determine the ability to produce biofilm and the occurrence of selected invasiveness factors among bacteria of the genus Streptococcus isolated from cattle with the clinical form of mastitis in northeastern Poland.

Results

Most of the isolates analysed demonstrated an ability to produce biofilm (over 70%). Virulence genes were searched for in the three most common streptococci in our experiment: S. agalactiae, S. uberis and S. dysgalactiae. For S. agalactiae, only four genes were confirmed: rib (33%), cylE (78%), bca (37%), and cfb (100%). The genes pavA, scpB, bac and lmb were not present in any of the tested strains. The dominant serotypes of the species were Ia (n = 8) and II (n = 8), in addition to some strains that were not classified in any of the groups (n = 6). Out of the eight selected genes for S. uberis (sua, pauA/skc, gapC, cfu, lbp, hasA, hasB, hasC), only one was not found (lbp). Finally, two genes were chosen for S. dysgalactiae (eno and napr), and their presence was confirmed in 76% and 86% of the strains, respectively.

Conclusions

The experiment showed that strains of Streptococcus spp. isolated from dairy cattle with clinical cases of mastitis in the northeastern part of Poland possess several invasiveness factors that can substantially affect the course of the disease, and this should be considered when developing targeted prevention programmes.

Monitoring of antimicrobial susceptibility of udder pathogens recovered from cases of clinical mastitis in dairy cows across Europe: VetPath results

VetPath is an ongoing pan-European antimicrobial susceptibility monitoring programme collecting pathogens from diseased cattle, pigs and poultry not recently treated with antibiotics. Non-duplicate milk samples were collected from cows with acute clinical mastitis in nine countries and 934 isolates were obtained during 2009-2012 for subsequent antimicrobial susceptibility testing in a central laboratory. CLSI broth microdilution methodology was used, and where available, MICs were interpreted using CLSI approved veterinary-specific (ceftiofur) otherwise human clinical breakpoints. Among Escherichia coli (n=207) and Klebsiella spp., (n=87), resistance was moderate to tetracycline and high to cephapirin (E. coli only) whereas resistance to other β-lactam antibiotics was very low (ceftiofur) to low (amoxicillin/clavulanic acid, cephalexin, cephalonium). The MIC₉₀ of enrofloxacin and marbofloxacin was 0.03 and 0.06μg/mL respectively (E. coli) with 0.5% strains displaying higher MICs. Staphylococcus aureus (n=192) and coagulase-negative staphylococci (CNS; n=165) strains were susceptible to most antibiotics tested except to penicillin (25.0 and 29.1% resistance), respectively. Three S. aureus and seven CNS strains were oxacillin-resistant and harboured mecA. Streptococcus uberis strains (n=188) were susceptible to the β-lactam antibiotics although 35.6% were penicillin intermediately susceptible, and 20.2% were resistant to erythromycin, 36.7% to tetracycline. For Streptococcus dysgalactiae (n=95) the latter figures were 13.7 and 56.8%, respectively. For most antibiotics, the percentage resistance among E. coli, S. aureus and S. uberis was comparable to that of the VetPath 2002-2006 survey. This current, expanded VetPath study shows that mastitis pathogens were susceptible to most antibiotics with exceptions of staphylococci tested against penicillin and streptococci against erythromycin or tetracycline. This work highlights the high need to set additional clinical breakpoints for antibiotics frequently used to treat mastitis.

Prudent use and regulatory guidelines for veterinary antibiotics-politics or science?

Regulatory guidelines are in place across the world to ensure that approval of antibiotics is consistent with current scientific understanding of quality, efficacy and safety including minimizing the risk of the development of antibiotic resistance. We suggest the regulatory process is fit for purpose and does indeed approve products that are safe for use with regard to development of antibiotic resistance. However, we maintain that in order to preserve the longevity of antibiotics, treatment should be based on an established diagnosis and normally only antibiotics authorized for the diagnosed indication and indicated bacteria are used. Furthermore, susceptibility testing should be carried out whenever possible. Despite a general acceptance that antibiotic resistance is a significant issue, antibiotics can still receive a marketing authorization without a sponsor having to generate a clinical breakpoint. The consequence of this is that for many antibiotics we have no measure of what is resistant and what is susceptible at the approved dose. We argue that the time is right for all approvals of new or existing antibiotics to have independently agreed clinical breakpoints, as part of the regulatory process, without which talk of resistance is somewhat meaningless. This is relevant not only for novel antibiotics but also for generic compounds.

Phenotypic and genotypic antimicrobial susceptibility pattern of Streptococcus spp. isolated from cases of clinical mastitis in dairy cattle in Poland

Mastitis of dairy cattle is one of the most frequently diagnosed diseases worldwide. The main etiological agents of mastitis are bacteria of the genus Streptococcus spp., in which several antibiotic resistance mechanisms have been identified. However, detailed studies addressing this problem have not been conducted in northeastern Poland. Therefore, the aim of our study was to analyze, on phenotypic and genotypic levels, the antibiotic resistance pattern of Streptococcus spp. isolated from clinical cases of mastitis from dairy cattle in this region of Poland. The research was conducted using 135 strains of Streptococcus (Streptococcus uberis, n = 53; Streptococcus dysgalactiae, n = 41; Streptococcus agalactiae, n = 27; other streptococci, n = 14). The investigation of the antimicrobial susceptibility to 8 active substances applied in therapy in the analyzed region, as well as a selected bacteriocin (nisin), was performed using the minimum inhibitory concentration method. The presence of selected resistance genes (n = 14) was determined via PCR. We also investigated the correlation between the presence of resistance genes and the antimicrobial susceptibility of the examined strains in vitro. The highest observed resistance of Streptococcus spp. was toward gentamicin, kanamycin, and tetracycline, whereas the highest susceptibility occurred toward penicillin, enrofloxacin, and marbofloxacin. Additionally, the tested bacteriocin showed high efficacy. The presence of 13 analyzed resistance genes was observed in the examined strains [gene mef(A) was not detected]. In most strains, at least one resistance gene, mainly responsible for resistance to tetracyclines [tet(M), tet(K), tet(L)], was observed. However, a relationship between the presence of a given resistance gene and antimicrobial susceptibility on the phenotypic level was not always observed.

Pharmacokinetics, milk penetration and PK/PD analysis by Monte Carlo simulation of marbofloxacin, after intravenous and intramuscular administration to lactating goats

The main objectives of this study were (i) to evaluate the serum pharmacokinetic behaviour and milk penetration of marbofloxacin (MFX; 5 mg/kg), after intravenous (IV) and intramuscular (IM) administration in lactating goats and simulate a multidose regimen on steady-state conditions, (ii) to determine the minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of coagulase negative staphylococci (CNS) isolated from caprine mastitis in Córdoba, Argentina and (iii) to make a PK/PD analysis by Monte Carlo simulation from steady-state pharmacokinetic parameters of MFX by IV and IM routes to evaluate the efficacy and risk of the emergence of resistance. The study was carried out with six healthy, female, adult Anglo Nubian lactating goats. Marbofloxacin was administered at 5 mg/kg bw by IV and IM route. Serum and milk concentrations of MFX were determined with HPLC/uv. From 106 regional strains of CNS isolated from caprine mastitis in herds from Córdoba, Argentina, MICs and MPCs were determined. MIC₉₀ and MPC₉₀ were 0.4 and 6.4 μg/ml, respectively. MIC and MPC-based PK/PD analysis by Monte Carlo simulation indicates that IV and IM administration of MFX in lactating goats may not be adequate to recommend it as an empirical therapy against CNS, because the most exigent endpoints were not reached. Moreover, this dose regimen could increase the probability of selecting mutants and resulting in emergence of resistance. Based on the results of Monte Carlo simulation, the optimal dose of MFX to achieve an adequate antimicrobial efficacy should be 10 mg/kg, but it is important take into account that fluoroquinolones are substrates of efflux pumps, and this fact may determine that assumption of linear pharmacokinetics at high doses of MFX may be incorrect.