Salt tolerance of methicillin-resistant and methicillin-susceptible Staphylococcus aureus

First report of enterotoxigenic Staphylococcus argenteus as a foodborne pathogen

Staphylococcal enterotoxins preformed in food are the causative agents of staphylococcal food poisoning outbreaks (SFPO). In this study we characterised in depth two coagulase-positive non-pigmented staphylococci involved in two independent outbreaks that occurred in France. While indistinguishable from Staphylococcus aureus using PCR methods and growth phenotype comparisons, both isolates were identified as Staphylococcus argenteus by whole genome sequencing. The genomes were analysed for the presence of enterotoxin genes, whose expression was determined in laboratory medium and, for the first time, in artificially-contaminated milk samples by using liquid chromatography-mass spectrometry and ELISA methods. The concentration measured for the SEB toxin in milk (0.67 ng/ml) was comparable to concentrations reported for other types of enterotoxins behind SFPO. From a collection of publicly available genomes, we performed an unprecedented systematic investigation of the enterotoxin gene set of S. argenteus, including variants and pseudogenes. The most prevalent genes were sex, followed by sel26, sel27 and sey. The egc cluster was less frequent and most of the time carried a dysfunctional seg gene. Our results shed light on the enterotoxigenic properties of S. argenteus, and emphasize the importance in monitoring of S. argenteus as an emerging foodborne pathogen.

Isolation and characterization of halophilic Kocuria salsicia strains from cheese brine

Kocuria salsicia can survive in extreme environments and cause infections, including catheter-related bacteremia, in humans. Here, we investigated and evaluated the characteristics of nine K. salsicia strains (KS1–KS9) isolated from cheese brine from a farmstead cheese-manufacturing plant in Korea from June to December, 2020. Staphylococcus aureus American Type Culture Collection (ATCC) 29213 was used as a positive control in the growth curve analysis and biofilm-formation assays. All K. salsicia isolates showed growth at 15% salt concentration and temperatures of 15°C, 25°C, 30°C, 37°C, and 42°C. KS6 and KS8 showed growth at 5°C, suggesting that they are potential psychrotrophs. In the biofilm-formation analysis via crystal violet staining, KS6 exhibited the highest biofilm-forming ability at various temperatures and media [phosphate buffered saline, nutrient broth (NB), and NB containing 15% sodium chloride]. At 25°C and 30°C, KS3, KS6, and KS8 showed higher biofilm-forming ability than S. aureus ATCC 29213. The antimicrobial resistance of the isolates was evaluated using the VITEK^® 2 system; most isolates were resistant to marbofloxacin and nitrofurantoin (both 9/9, 100%), followed by enrofloxacin (7/9, 77.8%). Five of the nine isolates (5/9, 55.6%) showed multidrug resistance. Our study reports the abilities of K. salsicia to grow in the presence of high salt concentrations and at relatively low temperatures, along with its multidrug resistance and tendency to form biofilms.

Paper-based analytical devices for colorimetric detection of S. aureus and E. coli and their antibiotic resistant strains in milk

Animal derived milk which is an important part of human diet due to its high nutritional value not only supports humans but also presents a growth environment for pathogenic bacteria. Milk may become contaminated with bacteria through udder infections or through contact within the dairy farm environment. Infections are treated with antibiotics, with β-lactams most commonly used in veterinary medicine. However, their frequent use leads to the emergence of β-lactam resistant bacterial strains, which causes difficulties in the treatment of infections in both humans and animals. Detection of pathogens as well as their antibiotic sensitivity is a pre-requisite for successful treatment and this is generally achieved with laboratory-based techniques such as growth inhibition assays, enzyme-linked immunosorbent assays (ELISA) or polymerase chain reactions (PCRs), which are unavailable in resource-limited settings. Here, we investigated paper-based analytical devices (μPADs) for the presumptive detection of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) and their antibiotic resistant bacterial strains in milk samples. The μPADs were fabricated on filter paper using wax printing, and then impregnated with chromogenic substrates, which reacted with bacterial enzymes to form coloured products. Limits of detection of S. aureus and E. coli and their antibiotic resistant strains in milk samples were found to be 106 cfu mL-1. Enrichment of milk samples in a selective medium for 12 h enabled detection as low as 10 cfu mL-1. The paper devices tested on a set of 640 milk samples collected from dairy animals in Pakistan demonstrated more than 90% sensitivity and 100% selectivity compared to PCR, showing promise to provide inexpensive and portable diagnostic solutions for the detection of pathogenic bacteria in resource-limited settings.

Staphylococcus aureus-An Additional Parameter of Bathing Water Quality for Crowded Urban Beaches

During the last years, the report of the occurrence of waterborne disease symptoms related to non-enteric pathogens has increased, without any record of higher levels of indicator bacteria (Escherichia coli and intestinal enterococci). Therefore, the use of current indicators is not always adequate when assessing the overall potential health risk and the inclusion of additional parameters needs to be examined. This paper reports on the incidence and levels of Staphylococcus aureus at 258 locations in Primorje-Gorski Kotar County (Croatia) recorded by official bathing water quality monitoring, as well as supplemental monitoring carried out at the two most frequented beaches in the City of Rijeka. The number of bathers was found to be the main factor affecting S. aureus levels (r = 0.321, p < 0.05). The share of S. aureus positive samples from the official monitoring was significantly lower, when compared to the share of samples from supplemental monitoring (2.2% and 36.3%, respectively; p < 0.01). Besides the number of bathers, one of the main reasons is likely the higher sampling frequency. No correlation was found between S. aureus levels and the indicator bacteria. The results indicate that the determination of S. aureus and increased sampling frequency is recommended for overcrowded beaches.

Heat and Chemical Treatments Affect the Viability, Morphology, and Physiology of Staphylococcus aureus and Its Subsequent Antibody Labeling for Flow Cytometric Analysis

The effects of heat and chemical treatments on Staphylococcus aureus viability and physiology and their subsequent effects on antibody binding ability and cell morphology were measured. Treatments included lethal and sublethal heat; exposure to organic acids, salt, and sodium hydroxide; and freeze-thawing. Strain-related differences in viability were noted depending on treatment and were reflected in changes in physiology as monitored by flow cytometry (FCM) using three different staining protocols: SYTO 9/propidium iodide (PI), DiOC₂(3), or calcein acetoxymethyl ester (calcein-AM)/PI. Treatments that resulted in significant losses in viability as measured by plate counting were reflected better by the first two staining combinations, as intracellular calcein-AM uptake may have been impaired by certain treatments. FCM analysis using labeling by commercial anti-S. aureus antibodies indicated that differences in cell physiology as a result of treatments influenced immunofluorescence detection. The ratio of the mean fluorescence intensities of stained cells to those of unstained cells [MFI/MFI_(us)] varied with treatment, five of these treatments, including freeze-thaw, citric acid, oxalic acid, NaCl, and NaOH treatments, resulted in significantly lower fluorescence values compared to controls.IMPORTANCE FCM data indicated that cells conventionally considered to be dead and which would not give rise to CFU in a plate count assay, e.g., cells heated to 80°C, were labeled by antibody staining. This finding suggests that without the inclusion of a live/dead discriminating dye, these cells would be erroneously detected as viable within an FCM assay. Reductions in antibody staining due to physicochemical treatment were strain related, reflecting the complexity of the phenomenon under study and illustrating that substantial validation of any new antibody detection-based method, including physiological staining and cell sorting, should be undertaken. Researchers should be aware of physicochemical treatments causing false-negative results: in this study, freeze-thawing severely reduced antibody binding without affecting the viability of a substantial percentage of cells. Scanning electron microscopy carried out on treated cells revealed a range of morphological changes resulting from physicochemical treatments which may have hindered antibody binding.

Recovery method development of sodium chloride-susceptible methicillin-resistant Staphylococcus aureus isolates from ground pork samples

The growth of certain methicillin-resistant Staphylococcus aureus (MRSA) isolates could be inhibited by NaCl higher than 2.5%. The objective of this study was to develop an enrichment method to recover NaCl-susceptible MRSA isolates from meat samples. The growth of 12 MRSA and 10 non-MRSA strains was measured in Mueller-Hinton (MH) broth supplemented with 2.5%, 4%, 6.5%, and 7.5% NaCl. Selective agents, including aztreonam, polymyxin B, NaCl, nalidixic acid, and NaN3, were determined for their inhibitory effect to MRSA and non-MRSA strains in MH broth. Based on these data, a two-step enrichment method was developed to recover both NaCl-susceptible and -resistant MRSA isolates in meat products. Comparing to the enrichment method that only used MH broth supplemented with 6.5% NaCl, five additional NaCl-susceptible MRSA isolates were recovered from 92 retail ground pork samples by this newly developed two-step enrichment method. To the best of our knowledge, this is the first study that considers NaCl-susceptible MRSA recovery from ground pork samples. The application of this new enrichment method might expand the diversity of MRSA isolates recovered from various samples.

Methicillin resistant staphylococci and broad-spectrum β-lactamase producing Enterobacteriaceae in horses

The use of β-lactam antibiotics results in the selection of bacteria showing resistance toward this class of antibiotics. The review focuses on the increasing importance of methicillin resistant staphylococci and broad-spectrum β-lactamase-producing Enterobacteriaceae in horses. Diagnostic protocols that optimize accurate identification of these bacteria from both clinical samples and samples obtained from putative carrier animals are described. In addition, the opportunities and pitfalls for preventive and curative measures are discussed.

Mutation in the C-di-AMP cyclase dacA affects fitness and resistance of methicillin resistant Staphylococcus aureus

Faster growing and more virulent strains of methicillin resistant Staphylococcus aureus (MRSA) are increasingly displacing highly resistant MRSA. Elevated fitness in these MRSA is often accompanied by decreased and heterogeneous levels of methicillin resistance; however, the mechanisms for this phenomenon are not yet fully understood. Whole genome sequencing was used to investigate the genetic basis of this apparent correlation, in an isogenic MRSA strain pair that differed in methicillin resistance levels and fitness, with respect to growth rate. Sequencing revealed only one single nucleotide polymorphism (SNP) in the diadenylate cyclase gene dacA in the faster growing but less resistant strain. Diadenylate cyclases were recently discovered to synthesize the new second messenger cyclic diadenosine monophosphate (c-di-AMP). Introduction of this mutation into the highly resistant but slower growing strain reduced resistance and increased its growth rate, suggesting a direct connection between the dacA mutation and the phenotypic differences of these strains. Quantification of cellular c-di-AMP revealed that the dacA mutation decreased c-di-AMP levels resulting in reduced autolysis, increased salt tolerance and a reduction in the basal expression of the cell wall stress stimulon. These results indicate that c-di-AMP affects cell envelope-related signalling in S. aureus. The influence of c-di-AMP on growth rate and methicillin resistance in MRSA indicate that altering c-di-AMP levels could be a mechanism by which MRSA strains can increase their fitness levels by reducing their methicillin resistance levels.

Dutch guideline on the laboratory detection of methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) has rapidly emerged worldwide, affecting both healthcare and community settings, and intensive livestock industry. The efficient control of MRSA strongly depends on its adequate laboratory detection. This guideline provides recommendations on the appropriate use of currently available diagnostic laboratory methods for the timely and accurate detection of MRSA in patients and healthcare workers. Herewith, it aims to standardise and improve the diagnostic laboratory procedures that are used for the detection of MRSA in Dutch medical microbiology laboratories.

A diversity of Antibiotic-resistant Staphylococcus spp. in a Public Transportation System

OBJECTIVES

Our goal was to determine the diversity and abundance of Staphylococcus bacteria on different components of a public transportation system in a mid-sized US city (Portland, Oregon) and to examine the level of drug resistance in these bacteria.

METHODS

We collected 70 samples from 2 cm × 4 cm sections from seven different areas on buses and trains in Portland, USA, taking 10 samples from each area. We isolated a subset of 14 suspected Staphylococcus spp. colonies based on phenotype, and constructed a phylogeny from16S rRNA sequences to assist in identification. We used the Kirbye-Bauer disk diffusion method to determine resistance levels to six common antibiotics.

RESULTS

We found a range of pathogenic Staphylococcus species. The mean bacterial colony counts were 97.1 on bus and train floors, 80.1 in cloth seats, 9.5 on handrails, 8.6 on seats and armrests at bus stops, 3.8 on the underside of seats, 2.2 on windows, and 1.8 on vinyl seats per 8 cm(2) sample area. These differences were significant (p < 0.001). Of the 14 isolates sequenced, 11 were staphylococci, and of these, five were resistant to penicillin and ampicillin, while only two displayed intermediate resistance to bacitracin. All 11 isolates were sensitive to trimethoprim-sulfamethoxazole, vancomycin, and tetracycline.

CONCLUSIONS

We found six different strains of Staphylococcus, and while there were varying levels of drug resistance, we did not find extensive levels of multidrug-resistant bacteria, and no S. aureus was found. We found floors and cloth seats to be areas on buses and trains that showed particularly high levels of bacteria.

Evaluation of salt concentrations, chromogenic media and anatomical sampling sites for detection of methicillin-resistant Staphylococcus aureus in pigs

The performance of chromogenic media for the detection of methicillin-resistant Staphylococcus aureus (MRSA) in humans, has been evaluated in numerous studies. However, few comparative studies have been performed for the detection of MRSA in pigs. In this study two different salt concentrations (2.5% or 7.5% NaCl) were evaluated in the enrichment nutrient broth and three selective chromogenic media (chromID MRSA, BrillianceMRSA and MRSASelect) for their ability to detect MRSA in swabs from 29 pigs obtained from three different anatomical sampling sites (anterior nares, skin behind both ears and perineum). ChromID MRSA showed the highest relative sensitivity and specificity after enrichment in 7.5% NaCl, followed by MRSASelect and BrilianceMRSA. For all chromogenic media more MRSA-positive results were obtained for specimens collected from skin behind the ears than for specimens taken from both nares and perineum. The results with regard to the anatomical sampling sites were confirmed in a larger study on three different pig farms involving 60 pigs per farm. Skin behind the ears was the anatomical site with the highest relative sensitivity (91.4%) for MRSA detection compared to perineum and anterior nares, with a relative sensitivity of 76.5% and 75.3%, respectively. An increased relative sensitivity could be achieved when combining two anatomical sites. Sampling of anterior nares and skin behind the ears appeared to be the most sensitive combination with a relative sensitivity of 98.2%. These results show that sampling of only the anterior nares underestimates the real pig MRSA prevalence.

Novel Technology to study co-evolution of humans and Staphylococcus aureus: consequences for interpreting the biology of colonisation and infection

Human nasal carriage of Staphylococcus aureus is a textbook example of an apparently neutral interaction between humans and a bacterial species that can still lead to (severe) opportunistic infections. The co-evolutionary aspects of this interaction are slowly surfacing, facilitated by the emergence of a diversity of diagnostic, epidemiological and molecular research tools. Basic microbiology has helped define persistent vs. non-carriage and the genotype of both host and guest has been explored in search of genetic markers for bacterial persistence. This chapter summarises the current state of affairs relating to artificial human colonisation studies with S. aureus, large-scale human cohort studies and innovative assessment of the humoral immune status of (non-)nasal carriers. These experimental approaches have recently assisted in identifying bacterial and human determinants and risk factors for staphylococcal carriage. Further refinement of the model by describing the interactions between S. aureus and its human host in molecular detail is important since it may pinpoint novel anti-infectious strategies.

Comparison of MRSASelect Agar, CHROMagar Methicillin-Resistant Staphylococcus aureus (MRSA) Medium, and Xpert MRSA PCR for detection of MRSA in Nares: diagnostic accuracy for surveillance samples with various bacterial densities

Rapid laboratory methods provide optimal support for active surveillance efforts to screen for methicillin-resistant Staphylococcus aureus (MRSA). Most laboratories struggle to determine the optimal use of resources, considering options to balance cost, speed, and diagnostic accuracy. To assess the performance of common methods, the first comparison of MRSASelect agar (MS) and CHROMagar MRSA (CA), with and without broth enrichment followed by a 24-h subculture to MS, was performed. Results were compared to those of the Xpert MRSA assay. For direct culture methods, the agreement between MS and CA was 98.8%. At 18 h, direct MS identified 93% of all positive samples from direct culture and 84% of those identified by the Xpert MRSA. For Trypticase soy broth-enriched MS culture, incubated overnight and then subcultured for an additional 24 h, the agreement with Xpert MRSA was 96%. The agreement between direct MS and Xpert MRSA was 100% when semiquantitative culture revealed a bacterial density of 2+ or greater; however, discrepancies between culture and Xpert MRSA arose for MRSA bacterial densities of 1+ or less, indicating low density as a common cause of false-negative culture results. Since 1+ or less was established as the most common MRSA carrier state, broth enrichment or PCR may be critical for the identification of all MRSA carriers who may be reservoirs for transmission. In this active-surveillance convenience sample, the use of broth enrichment followed by subculture to MS offered a low-cost but sensitive method for MRSA screening, with performance similar to that of Xpert MRSA PCR.

Chromogenic media vs real-time PCR for nasal surveillance of methicillin-resistant Staphylococcus aureus: impact on detection of MRSA-positive persons

Surveillance for methicillin-resistant Staphylococcus aureus (MRSA) colonization can be an important element for infection control programs when managing a multidrug-resistant pathogen such as MRSA. The sensitivity and speed of laboratory testing affects the proportion of appropriate isolation days captured, which determines the success or failure of a MRSA control program. Chromogenic culture, CHROMagar MRSA (BBL, Becton Dickinson, Sparks, MD) and MRSASelect (Bio-Rad, Hercules, CA), with and without broth enrichment and real-time polymerase chain reaction (PCR; BD GeneOhm MRSA, BD Diagnostics, San Diego, CA), were compared and found to have a wide range of sensitivities (78.5%-98.2%), specificities (91.6%-100.0%), and turnaround times (2-72 hours). Real-time PCR provided the most rapid results and demonstrated the highest sensitivity followed by broth-enriched culture and then direct plating for MRSA detection in nasal swabs. There was no substantial difference in the labor required for any of the 3 approaches.

Evaluation of four selective agars and two enrichment broths in screening for methicillin-resistant Staphylococcus aureus

To evaluate methicillin-resistant Staphylococcus aureus detection, we tested in vitro four selective agars and two enrichment broths apart and in combination. Tryptone soya broth with salt, aztreonam, and cefoxitin appeared to be the most sensitive medium. This broth was superior to a phenol red mannitol broth with aztreonam and ceftizoxime.

High occurrence of methicillin-resistant Staphylococcus aureus ST398 in equine nasal samples

Methicillin-resistant Staphylococcus aureus (MRSA) infections do occur in equine patients. Little is known, however, about their origin and the general equine MRSA colonization status. In West European horses in particular, neither the colonization rate nor the present strains or their antimicrobial susceptibility patterns are known. In the present study, a sample of 110 (Belgian, French, Dutch and Luxemburg) horses presented at a Belgian equine clinic was screened for nasal MRSA carriage. An indirect culturing protocol using a 0.001% colistin and nalidixic acid containing broth was compared to a direct agar method. Phenotypic identification following growth on a chromogenic MRSA screening agar (ChromID MRSA) was combined with genotypic analysis (PCR, PFGE, SCCmec, spa, and MLST typing). Antimicrobial susceptibility was tested through disk diffusion. Twelve (10.9%) horses carried MRSA, with the enrichment protocol resulting in a significantly higher isolation rate. None of the isolated strains were typeable through SmaI PFGE. They all harboured SCCmec type IVa or V and belonged to spa type t011 or t1451 of the ST398 lineage. All isolates were tetracycline resistant and sulfonamide and enrofloxacin susceptible. Macrolide, lincosamide, trimethoprim and aminoglycoside susceptibility varied and in total five different antimicrobial resistance patterns were distinguished. These results show that ST398 is certainly present in West European horses. Due to its known interspecies transmission and the structure of the equine industry, the presence of this clone in horses poses a substantial health hazard for both animals and humans.