Comparative efficacy of enrofloxacin to that of tulathromycin for the control of bovine respiratory disease and prevalence of antimicrobial resistance in Mannheimia haemolytica in calves at high risk of developing bovine respiratory disease

Development of a detection chip for major pathogenic drug-resistant genes and drug targets in bovine respiratory system diseases

Bovine respiratory disease (BRD) is a significant veterinary challenge, often exacerbated by pathogen resistance, hindering effective treatment. Traditional testing methods for primary pathogens - Mycoplasma bovis, Pasteurella multocida, and Mannheimia haemolytica - are notably time-consuming and lack the rapidity required for effective clinical decision-making. This study introduces a TaqMan MGB probe detection chip, utilizing fluorescent quantitative PCR, targeting key BRD pathogens and associated drug-resistant genes and sites. We developed 94 specific probes and primers, embedded into a detection chip, demonstrating notable specificity, repeatability, and sensitivity, reducing testing time to under 1 h. Additionally, we formulated probes to detect mutations in the quinolone resistance-determining region, associated with fluoroquinolone resistance in BRD pathogens. The chip exhibited robust sensitivity and specificity, enabling rapid detection of drug-resistant mutations in clinical samples. This methodology significantly expedites the diagnostic process for BRD and sensitive drug screening, presenting a practical advancement in the field.

Antimicrobial Metaphylaxis and its Impact on Health, Performance, Antimicrobial Resistance, and Contextual Antimicrobial Use in High-Risk Beef Stocker Calves

The objective of this blinded, cluster-randomized, complete block trial was to evaluate the impact of metaphylaxis on health, performance, antimicrobial resistance, and contextual antimicrobial use (AMU) in high-risk beef stocker calves. Calves (n = 155) were randomly assigned to receive either saline or tulathromycin at the time of arrival processing. Deep nasopharyngeal swabs were collected from each calf at arrival and 14 d later. Calves were monitored for bovine respiratory disease (BRD) for 42 d. Body weights were obtained at arrival, days 14, 28, and 42. Contextual antimicrobial use (AMU) was calculated using dose and mass-based metrics. Calves given tulathromycin had a greater average daily gain (0.96 ± 0.07 kg vs. 0.82 ± 0.07 kg; P = 0.034) and lower prevalence of BRD than controls (17% vs. 40%; P = 0.008). Proportions of calves with BRD pathogens identified at arrival were similar between treatment groups [17%; P = 0.94]. Proportions of calves with BRD pathogens identified at day 14 were lower for calves receiving tulathromycin compared to controls (15% vs. 60%, P < 0.001). Overall, 81% of Pastuerella multocida isolates and 47% of Mannheimia haemolytica isolates were pansusceptible. When measured as regimens per head in, AMU in calves receiving tulathromycin was higher than calves receiving saline (P = 0.01). Under the conditions of this study, metaphylaxis had positive impacts on the health and performance of high-risk beef stocker calves, did not contribute to the selection of resistant bacterial isolates in the nasopharynx of treated cattle, and increased AMU.

Enhanced phagocytosis and complement-mediated killing of Mannheimia haemolytica serotype 1 following in-frame CMP-sialic acid synthetase (neuA) gene deletion

IMPORTANCE

The Gram-negative coccobacillus Mannheimia haemolytica is a natural inhabitant of the upper respiratory tract in ruminants and the most common bacterial agent involved in bovine respiratory disease complex development. Key virulence factors harbored by M. haemolytica are leukotoxin, lipopolysaccharide, capsule, adhesins, and neuraminidase which are involved in evading innate and adaptive immune responses. In this study, we have shown that CMP-sialic acid synthetase (neuA) is necessary for the incorporation of sialic acid onto the membrane, and inactivation of neuA results in increased phagocytosis and complement-mediated killing of M. haemolytica, thus demonstrating that sialylation contributes to the virulence of M. haemolytica.

Bovine Airway Models: Approaches for Investigating Bovine Respiratory Disease

Bovine respiratory disease (BRD) is a multifactorial condition where different genera of bacteria, such as Mannheimia haemolytica, Histophilus somni, Pasteurella multocida, and Mycoplasma bovis, and viruses, like bovine respiratory syncytial virus, bovine viral diarrhea virus, and bovine herpes virus-1, infect the lower respiratory tract of cattle. These pathogens can co-infect cells in the respiratory system, thereby making specific treatment very difficult. Currently, the most common models for studying BRD include a submerged tissue culture (STC), where monolayers of epithelial cells are typically covered either in cellular or spent biofilm culture medium. Another model is an air-liquid interface (ALI), where epithelial cells are exposed on their apical side and allowed to differentiate. However, limited work has been reported on the study of three-dimensional (3D) bovine models that incorporate multiple cell types to represent the architecture of the respiratory tract. The roles of different defense mechanisms in an infected bovine respiratory system, such as mucin production, tight junction barriers, and the production of antimicrobial peptides in in vitro cultures require further investigation in order to provide a comprehensive understanding of the disease pathogenesis. In this report, we describe the different aspects of BRD, including the most implicated pathogens and the respiratory tract, which are important to incorporate in disease models assembled in vitro. Although current advancements of bovine respiratory cultures have led to knowledge of the disease, 3D multicellular organoids that better recapitulate the in vivo environment exhibit potential for future investigations.

In vitro evaluation of the effect of transport medium, temperature, and time on the recovery of Mannheimia haemolytica and Pasteurella multocida

Appropriate sample collection, storage conditions, and time for transport to the laboratory are important for an accurate diagnostic result. We evaluated the effects of transport storage medium type, time of storage, and storage temperatures on Mannheimia haemolytica (MH) and Pasteurella multocida (PM) recovery using an in vitro model simulation. A quantitative culture method, using colony-forming units per milliliter, was used to recover MH or PM by an in vitro model with cotton swabs. Three independent trials were conducted, in which cotton swabs were inoculated with MH or PM and placed in either (1) a sterile 15-mL polypropylene tube without transport medium (dry), (2) Amies culture medium with charcoal (ACM), or (3) Cary-Blair transport agar (CBA). Swabs were evaluated for recovery of MH or PM when stored at 3 temperatures (4°C, 23°C, or 36°C) and after storage for 8 h, 24 h, or 48 h. From all study group combinations, a total of 162 individual independent swabs were evaluated. The nonparametric Dunn all-pairs approach was used to compare the proportion of culturable bacteria, between the various storage media, temperature, and time point combinations. The proportion of MH in samples stored at 4°C was significantly higher for ACM and CBA than dry storage at 24 and 48 h. The MH samples stored at 36°C had a significantly higher proportion for ACM and CBA than dry storage at 24 h. The proportion of PM in samples stored at 4°C was significantly lower for ACM compared with dry at 8 h but significantly higher at 48 h. The PM samples stored at 23°C in ACM had a significantly higher proportion than dry samples at 24 h, and, at 48 h, ACM and CBA had a significantly higher proportion than the dry group. All swabs stored at 36°C for 48 h had a proportion close to zero, indicating decreasing diagnostic efficacy. These results support the use of transport media such as ACM and CBA for increasing the detection of PM and MH from samples, especially when samples are exposed to high temperatures. The combination of longer periods from collection of samples to diagnostic evaluation (>24 h) and higher storage temperatures (>23°C) were shown to significantly impair diagnostic accuracy.

Tulathromycin – a semi-synthetic macrolidе antibiotic. II. Usage in veterinary medicine

This article presents the results of studies on the use of the new macrolide antibiotic tulathromycin in veterinary medicine. The information is presented according to the species of domestic animals, diseases, doses and shemes for therapy and metaphylaxis. The first section includes data for cattle and calves affected by respiratory diseases, caused by M. haemolytica, M. bovis, H. somni and P. multocida, and keratoconjunctivitis caused by M. bovis. The second one shows data for swine and pigs affected by the respiratory pathogens A. pleuropneumoniae, P. multocida, H. parasuis, B. bronhiseptica and M. hyopneumoniae. A third group presents information about small ruminants with manifested respiratory diseases caused by M. haemolytica, P. multocida, Mycoplasma spp., Pseudomonas spp. and hoof diseases from Dehelobacter nodosus. Fourth group includes the first test results of Rhodococcus equi, Streptococcus zooepidemicus, Theileria equi, Babesia bovis and Babesia bigemina, causing diseases in horses. The fifth section includes reports of treated rabbits most commonly suffering from P. multocida respiratory infections. Finally, data from new studies on the use of tulathromycin in exotic animals and cases of some tropical diseases are listed. The provided information gives proof for the high sensitivity of the tested pathogens to tulathromycin and high clinical and economical effect that is efficient in single dose of 2.5 mg/kg. Compared to other antibacterial agents there is no resistance according to data available so far. Local accumulation and prolonged persistence of the drug in lung tissues, that result in a treatment regime with a single low-volume dose (2.5 mg/kg), are associated with positive clinical outcome in domestic animals: large ruminants, pigs, small ruminants and rabbits with respiratory diseases caused by M. haemolytica, H. somni, P. multocida and M. bovis; A. pleuropneumoniae, B. bronchiseptica, P. multocida, H. parasuis and M. hyopneumoniae; S.aureus, C. pseudotuberculosis and Streptococcus spp. and P.multocida

High-Risk Cattle Management and Stocker Calf Health: Modulation of the Bovine Respiratory Microbiome from a Systems Perspective

Bovine respiratory disease (BRD) affects animals in all segments of the North American beef industry. The segmented nature of the beef industry results in the marketing of cattle that are considered to be at high risk of developing BRD. The microbiota is the complex microbial ecosystem that exists in and on the body of all animals. The respiratory tract has its unique microbiota that is shaped by many factors. Stress reduction, appropriate nutritional management, strategic use of vaccines, and antimicrobial administration targeted to the highest risk individuals have the potential to stabilize an inherently unstable microbial population and enhance calf health.

Enrofloxacin—The Ruthless Killer of Eukaryotic Cells or the Last Hope in the Fight against Bacterial Infections?

Enrofloxacin is a compound that originates from a group of fluoroquinolones that is widely used in veterinary medicine as an antibacterial agent (this antibiotic is not approved for use as a drug in humans). It reveals strong antibiotic activity against both Gram-positive and Gram-negative bacteria, mainly due to the inhibition of bacterial gyrase and topoisomerase IV enzymatic actions. The high efficacy of this molecule has been demonstrated in the treatment of various animals on farms and other locations. However, the use of enrofloxacin causes severe adverse effects, including skeletal, reproductive, immune, and digestive disorders. In this review article, we present in detail and discuss the advantageous and disadvantageous properties of enrofloxacin, showing the benefits and risks of the use of this compound in veterinary medicine. Animal health and the environmental effects of this stable antibiotic (with half-life as long as 3–9 years in various natural environments) are analyzed, as are the interesting properties of this molecule that are expressed when present in complexes with metals. Recommendations for further research on enrofloxacin are also proposed.

In-vitro antibiotic resistance phenotypes of respiratory and enteric bacterial isolates from weaned dairy heifers in California

Antimicrobial drug (AMD) use for bovine respiratory disease (BRD) continues to be concerning for development of antimicrobial resistance (AMR) in respiratory and enteric bacteria of cattle. This study aimed to provide data regarding AMR in respiratory isolates, and identify relationships between respiratory and enteric AMD susceptibility, in weaned dairy heifers. A cross-sectional study was performed between June of 2019 and February 2020, on 6 calf rearing facilities in California. Deep nasopharyngeal and rectal swabs were collected from 341 weaned heifers and submitted for selective bacterial culture and AMR testing. Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni were selectively isolated from respiratory samples; Escherichia coli and Enterococcus spp. were selectively isolated from rectal swabs. Minimum inhibitory concentrations (MIC) were determined for selected isolates against 19 AMD. The proportion of resistant isolates was calculated using Clinical Laboratory Standards Institute (respiratory) or USDA NARMS (enteric) breakpoints; when no applicable breakpoint was available, the distribution of MIC was described and compared. Association between AMR in a calf’s respiratory isolate and a higher or lower MIC of the matched enteric isolates was determined. More than 50% of P. multocida isolates were resistant to each of 7 AMD commonly used to treat BRD (florfenicol, gamithromycin, tildipirosin, tilmicosin, danofloxacin, enrofloxacin and tetracycline). Resistance in respiratory isolates was only associated with higher matched enteric MIC for gamithromycin and tulathromycin. Multidrug resistance was reported in >70% of P. multocida and M. haemolytica isolates. Antimicrobial resistance, including multidrug resistance, in respiratory isolates appears to be widespread in weaned dairy heifers; this finding has not previously been reported and raises concern for the future efficacy of AMD used to treat respiratory diseases in weaned dairy heifers. Enteric bacterial MIC appear to have limited direct association with respiratory isolate AMR classification.

Antimicrobial resistance in Mannheimia haemolytica: prevalence and impact

Bovine respiratory disease (BRD) is the most common cause of morbidity and mortality in North American beef cattle. In recent years, isolation of strains of Mannheimia haemolytica that are resistant to multiple different classes of antimicrobials has become commonplace. New research would suggest that the routine use of antimicrobials by some cattle operations might be driving emerging resistance patterns, with the majority of the spread observed due to propagation of strains of M. haemolytica that have acquired integrative conjugative elements. To date, there is little information evaluating the impact of antimicrobial resistance on clinical outcome in cattle with BRD.

Concordance of disk diffusion, broth microdilution, and whole-genome sequencing for determination of in vitro antimicrobial susceptibility of Mannheimia haemolytica

Background

Extensive drug resistance (XDR) is an emerging concern with Mannheimia haemolytica, and a variety of testing methods are available for characterizing in vitro antimicrobial susceptibility.

Objectives

To compare the concordance among disk diffusion, broth microdilution, and whole genome sequencing (WGS) for susceptibility testing of M. haemolytica before and after mass treatment using tulathromycin.

Animals

Forty‐eight M. haemolytica isolates collected from high‐risk beef stocker calves before and after mass treatment (metaphylaxis) using tulathromycin (Draxxin, Zoetis, Parsippany, NJ) given at the label dosage of 2.5 mg/kg body weight SC in the neck.

Methods

In vitro antimicrobial susceptibility was determined for all 48 isolates using disk diffusion, broth microdilution, and WGS. Concordance was calculated between pairs of susceptibility testing methods as follows: number of isolates classified identically by the 2 testing methods for each timepoint, divided by the number of isolates tested at that timepoint. Discordance was calculated as follows: number of isolates classified differently by the 2 testing methods for each timepoint, divided by the number of isolates tested at that timepoint.

Results

Concordance between testing methods ranged from 42.3% to 100%, depending on antimicrobial evaluated, timing of sample collection, and testing method used. Very major errors were identified in up to 7.7% of classifications whereas minor errors were seen in up to 50% of classifications depending on antimicrobial evaluated, timing of sample collection, and testing method used.

Conclusions and Clinical Importance

Our results show that discrepancies in the results of different susceptibility testing methods occur and suggest a need for greater harmonization of susceptibility testing methods.

Longitudinal effects of enrofloxacin or tulathromycin use in preweaned calves at high risk of bovine respiratory disease on the shedding of antimicrobial-resistant fecal Escherichia coli

The objective of this study was to longitudinally quantify Escherichia coli resistant to ciprofloxacin and ceftriaxone in calves treated with enrofloxacin or tulathromycin for the control of bovine respiratory disease (BRD). Dairy calves 2 to 3 wk of age not presenting clinical signs of pneumonia and at high risk of developing BRD were randomly enrolled in 1 of 3 groups receiving the following treatments: (1) single label dose of enrofloxacin (ENR); (2) single label dose of tulathromycin (TUL); or (3) no antimicrobial treatment (control, CTL). Fecal samples were collected immediately before administration of treatment and at d 2, 4, 7, 14, 21, 28, 56, and 112 d after beginning treatment. Samples were used for qualification of E. coli using a selective hydrophobic grid membrane filter (HGMF) master grid. The ENR group had a significantly higher proportion of E. coli resistant to ciprofloxacin compared with CTL and TUL at time points 2, 4, and 7. At time point 28, a significantly higher proportion of E. coli resistant to ciprofloxacin was observed only compared with CTL. The TUL group had a significantly higher proportion of E. coli resistant to ciprofloxacin compared with CTL at time points 2, 4, and 7. None of the treatment groups resulted in a significantly higher proportion of E. coli isolates resistant to ceftriaxone. Our study identified that treatment of calves at high risk of developing BRB with either enrofloxacin or tulathromycin resulted in a consistently higher proportion of ciprofloxacin-resistant E. coli in fecal samples.

Antimicrobial resistance genes in bacteria from animal-based foods

Antimicrobial resistance is a worldwide public health threat. Farm animals are important sources of bacteria containing antimicrobial resistance genes (ARGs). Although the use of antimicrobials in aquaculture and livestock has been reduced in several countries, these compounds are still routinely applied in animal production, and contribute to ARGs emergence and spread among bacteria. ARGs are transmitted to humans mainly through the consumption of products of animal origin (PAO). Bacteria can present intrinsic resistance, and once antimicrobials are administered, this resistance may be selected and multiply. The exchange of genetic material is another mechanism used by bacteria to acquire resistance. Some of the main ARGs found in bacteria present in PAO are the bla, mcr-1, cfr and tet genes, which are directly associated to antibiotic resistance in the human clinic.

Mannheimia haemolytica and Pasteurella multocida in Bovine Respiratory Disease: How Are They Changing in Response to Efforts to Control Them?

The bacteria Mannheimia haemolytica and Pasteurella multocida contribute to bovine respiratory disease (BRD), which is often managed with antimicrobials. Antimicrobial resistance in these bacteria has been rare, but extensively drug-resistant strains have recently become common. Routine antimicrobial use may be driving this resistance. Resistance spread is caused in part by propagation of strains harboring integrative conjugative elements. The impact of antimicrobial resistance on treatment outcomes is not clear, but clinical observations suggest that response to first treatment has decreased over time, possibly because of resistance. Clinicians should consider antimicrobial resistance when designing BRD treatment and control programs.

Antimicrobial Resistance in Members of the Bacterial Bovine Respiratory Disease Complex Isolated from Lung Tissue of Cattle Mortalities Managed with or without the Use of Antimicrobials

Over a two-year period, Mannheimia haemolytica (MH; n = 113), Pasteurella multocida (PM; n = 47), Histophilus somni (HS; n = 41) and Mycoplasma bovis (MB; n = 227) were isolated from bovine lung tissue at necropsy from cattle raised conventionally (CON, n = 29 feedlots) or without antimicrobials [natural (NAT), n = 2 feedlots]. Excluding MB, isolates were assayed by PCR to detect the presence of 13 antimicrobial resistance (AMR) genes and five core genes associated with integrative and conjugative elements (ICEs). Antimicrobial susceptibility phenotypes and minimum inhibitory concentrations (MICs, µg/mL) were determined for a subset of isolates (MH, n = 104; PM, n = 45; HS, n = 23; and MB, n = 61) using Sensititre analyses. A subset of isolates (n = 21) was also evaluated by whole-genome sequencing (WGS) based on variation in AMR phenotype. All five ICE core genes were detected in PM and HS by PCR, but only 3/5 were present in MH. Presence of mco and tnpA ICE core genes in MH was associated with higher MICs (p < 0.05) for all tetracyclines, and 2/3 of all macrolides, aminoglycosides and fluoroquinolones evaluated. In contrast, association of ICE core genes with MICs was largely restricted to macrolides for PM and to individual tetracyclines and macrolides for HS. For MH, the average number of AMR genes markedly increased (p < 0.05) in year 2 of the study due to the emergence of a strain that was PCR positive for all 13 PCR-tested AMR genes as well as two additional AMR genes (aadA31 and bla_ROB-1) detected by WGS. Conventional management of cattle increased (p < 0.05) MICs of tilmicosin and tulathromycin for MH; neomycin and spectinomycin for PM; and gamithromycin and tulathromycin for MB. The average number of PCR-detected AMR genes in PM was also increased (p < 0.05) in CON mortalities. This study demonstrates increased AMR especially to macrolides by bovine respiratory disease organisms in CON as compared to NAT feedlots and a rapid increase in AMR following dissemination of strain(s) carrying ICE-associated multidrug resistance.

Association between antimicrobial drug class for treatment and retreatment of bovine respiratory disease (BRD) and frequency of resistant BRD pathogen isolation from veterinary diagnostic laboratory samples

Although 90% of BRD relapses are reported to receive retreatment with a different class of antimicrobial, studies examining the impact of antimicrobial selection (i.e. bactericidal or bacteriostatic) on retreatment outcomes and the emergence of antimicrobial resistance (AMR) are deficient in the published literature. This survey was conducted to determine the association between antimicrobial class selection for treatment and retreatment of BRD relapses on antimicrobial susceptibility of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. Pathogens were isolated from samples submitted to the Iowa State University Veterinary Diagnostic Laboratory from January 2013 to December 2015. A total of 781 isolates with corresponding animal case histories, including treatment protocols, were included in the analysis. Original susceptibility testing of these isolates for ceftiofur, danofloxacin, enrofloxacin, florfenicol, oxytetracycline, spectinomycin, tilmicosin, and tulathromycin was performed using Clinical and Laboratory Standards Institute guidelines. Data were analyzed using a Bayesian approach to evaluate whether retreatment with antimicrobials of different mechanistic classes (bactericidal or bacteriostatic) increased the probability of resistant BRD pathogen isolation in calves. The posterior distribution we calculated suggests that an increased number of treatments is associated with a greater probability of isolates resistant to at least one antimicrobial. Furthermore, the frequency of resistant BRD bacterial isolates was greater with retreatment using antimicrobials of different mechanistic classes than retreatment with the same class. Specifically, treatment protocols using a bacteriostatic drug first followed by retreatment with a bactericidal drug were associated with a higher frequency of resistant BRD pathogen isolation. In particular, first treatment with tulathromycin (bacteriostatic) followed by ceftiofur (bactericidal) was associated with the highest probability of resistant M. haemolytica among all antimicrobial combinations. These observations suggest that consideration should be given to antimicrobial pharmacodynamics when selecting drugs for retreatment of BRD. However, prospective studies are needed to determine the clinical relevance to antimicrobial stewardship programs in livestock production systems.

Characterization and comparison of cell-mediated immune responses following ex vivo stimulation with viral and bacterial respiratory pathogens in stressed and unstressed beef calves1

The goal of this study was to compare the cell-mediated immune responses of highly commingled, sale-barn origin calves (STR; n = 10) to those of single source calves that had been weaned for 60 d (UNS; n = 10). Peripheral blood mononuclear cells and neutrophils (PMNs) were isolated from jugular venous blood of each calf. Peripheral blood mononuclear cells were stimulated with Concanavalin A (ConA), BVDV-1, BVDV-2, BHV-1, Mannheimia haemolytica, and Pasteurella multocida and evaluated for clonal proliferation and secretion of IL-8 into cell culture supernatants. The native functional capacities of PMNs were evaluated in response to stimulation with heat-killed Escherichia coli and Staphylococcus aureus. Complete blood counts and serum biochemical profiles were performed for each animal at the time of sample collection. Compared with STR calves, UNS calves had greater lymphocyte proliferative responses following stimulation BVDV1 (P = 0.041), BVDV2 (P = 0.002), BHV-1 (P = 0.001), M. haemolytica (P = 0.016), and P. multocida (P = 0.049). In addition, PMNs isolated from UNS calves had a greater ability to phagocytose E. coli (P = 0.001) and S. aureus (P = 0.003) when compared with STR calves. Serum nonesterified fatty acids were higher in STR calves (P < 0.001). Serum β-hydroxybutyrate was lower in STR calves (P < 0.003). These data suggest that immunologic and physiologic differences exist between STR and UNS calves. Although the underlying mechanisms for these differences are not clear, it is possible that combinations of energy imbalances, stress-induced immunosuppression, and general immune naiveté may predispose STR calves to an increased risk of morbidity and mortality due to bovine respiratory disease.

Genetic characterization of susceptible and multi-drug resistant Mannheimia haemolytica isolated from high-risk stocker calves prior to and after antimicrobial metaphylaxis

Bovine Respiratory Disease (BRD) is a major threat to animal health and welfare in the cattle industry. Strains of Mannheimia haemolytica (Mh) that are resistant to multiple classes of antimicrobials are becoming a major concern in the beef industry, as the frequency of isolation of these strains has been increasing. Mobile genetic elements, such as integrative conjugative elements (ICE), are frequently implicated in this rapid increase in multi-drug resistance. The objectives of the current study were to determine the genetic relationship between the isolates collected at arrival before metaphylaxis and at revaccination after metaphylaxis, to identify which resistance genes might be present in these isolates, and to determine if they were carried on an ICE. Twenty calves culture positive for Mh at arrival and revaccination were identified, and a total of 48 isolates with unique susceptibility profiles (26 from arrival, and 22 from revaccination) were submitted for whole-genome sequencing (WGS). A phylogenetic tree was constructed, showing the arrival isolates falling into four clades, and all revaccination isolates within one clade. All revaccination isolates, and one arrival isolate, were positive for the presence of an ICE. Three different ICEs with resistance gene modules were identified. The resistance genes aphA1, strA, strB, sul2, floR, erm42, tetH/R, aadB, aadA25, blaOXA-2, msrE, mphE were all located within an ICE. The gene bla-ROB1 was also present in the isolates, but was not located within an ICE.

Pathogenic Mannheimia haemolytica Invades Differentiated Bovine Airway Epithelial Cells

The Gram-negative bacterium Mannheimia haemolytica is the primary bacterial species associated with bovine respiratory disease (BRD) and is responsible for significant economic losses to livestock industries worldwide. Healthy cattle are frequently colonized by commensal serotype A2 strains, but disease is usually caused by pathogenic strains of serotype A1. For reasons that are poorly understood, a transition occurs within the respiratory tract and a sudden explosive proliferation of serotype A1 bacteria leads to the onset of pneumonic disease. Very little is known about the interactions of M. haemolytica with airway epithelial cells of the respiratory mucosa which might explain the different abilities of serotype A1 and A2 strains to cause disease. In the present study, host-pathogen interactions in the bovine respiratory tract were mimicked using a novel differentiated bovine bronchial epithelial cell (BBEC) infection model. In this model, differentiated BBECs were inoculated with serotype A1 or A2 strains of M. haemolytica and the course of infection followed over a 5-day period by microscopic assessment and measurement of key proinflammatory mediators. We have demonstrated that serotype A1, but not A2, M. haemolytica invades differentiated BBECs by transcytosis and subsequently undergoes rapid intracellular replication before spreading to adjacent cells and causing extensive cellular damage. Our findings suggest that the explosive proliferation of serotype A1 M. haemolytica that occurs within the bovine respiratory tract prior to the onset of pneumonic disease is potentially due to bacterial invasion of, and rapid proliferation within, the mucosal epithelium. The discovery of this previously unrecognized mechanism of pathogenesis is important because it will allow the serotype A1-specific virulence determinants responsible for invasion to be identified and thereby provide opportunities for the development of new strategies for combatting BRD aimed at preventing early colonization and infection of the bovine respiratory tract.