Antimicrobial resistance in gram-positive cocci isolated from poultry in Western Australia : an assessment of poultry meat as a vehicle for the transmission of resistant strains via the food chain.

Abstract

The aim of this study was to examine whether Gram-positive cocci isolated from processed poultry in Western Australia provided a potential risk for the transfer of antimicrobial-resistant organisms to humans via commercially prepared ready-to-eat chicken. Research in this study was conducted in three phases: the characterisation of Gram-positive cocci isolated from poultry, an assessment of the isolates' thermal tolerance and the development of a Hazard Analysis Critical Control Points (HACCP) based food-safety program. In the first phase of the study, three specific objectives were investigated. The first determined the presence of Gram-positive cocci on poultry and on processing equipment from poultry-processing plants. The findings confirm the presence of staphylococci and enterococci on incoming live and slaughtered birds and processed carcasses. The data also indicate that carcasses probably become cross-contaminated during processing, when these bacteria are present on the incoming live birds and equipment. The second objective was to characterise staphylococcal isolates by antimicrobial susceptibility testing, and chromosomal and plasmid DNA analysis. The susceptibility of isolates to antimicrobial agents was tested by the disk diffusion method according to the NCCLS (National Committee for Clinical Laboratory Standards) guidelines. Isolates were typed by contour-clamped homogeneous electric field (CHEF) gel electrophoresis of SmaI digested chromosomal DNA, and plasmids were isolated by the cetyltrimethylammonium bromide (CTAB) method. Approximately 37% of Staphylococcus aureus and 16% of coagulase-negative staphylococcal (CNS) isolates were resistant to six or more of the antimicrobial agents tested. Many isolates exhibited resistance to antibiotics that are commonly used in human medicine and registered for veterinary use in Australia.Among the S. aureus isolates there were twenty-four epidemiologically unrelated SmaI CHEF groups. All staphylococcal isolates, except three CNS, were found to harbour from one to seven plasmids. Some staphylococcal isolates with epidemiologically related CHEF patterns had similar plasmid profiles and resistance patterns. The third objective was to determine the antimicrobial susceptibility of enterococci isolates to the glycopeptide antibiotics. The isolation of two vancomycin-resistant E. faecalis isolates is the first report of VRE outside the health-care setting in Western Australia. Additionally the detection of the vanA gene in an E. gallinarum isolate, a motile enterococcus, has potentially important implications for infection control practices in hospitals. In the second phase of the study, three specific objectives were established to investigate the practical implications of these findings for the chicken industry. The first objective of this phase of the study was to determine the thermal tolerance (D and Z-values) of antimicrobial-resistant, Gram-positive cocci in ground chicken meat. The results indicate that these isolates do not exhibit enhanced thermal-resistance characteristics compared to antimicrobial-susceptible bacteria. The second objective established the internal time-temperature profiles for cooking commercially prepared chicken and estimated the process lethality (F-values).From three cooking trials, it was confirmed that the internal temperature of at least 70°C was achieved for at least thirty-eight minutes. The third objective of this phase assessed the effectiveness of the thermal process in reducing the risk of the transfer of antimicrobial-resistant cocci via the food chain. The data confirm that the lethal effect (F-values) of the thermal process destroyed these antimicrobial-resistant cocci in commercially prepared ready-to-eat chicken. In the third phase of the study, the data obtained in the earlier parts of the study was incorporated into a model food-safety program for a fast-food chicken chain. The model was based upon the internationally accepted HACCP system, adopted by the Codex Alimentarius Commission. Mindful that the thermal-process step represents only one critical control point in the safe preparation of chicken, this preventative approach ensures that all hazards are controlled at every other step of the process. The data suggest that antimicrobial-resistant, Gram-positive cocci will be present on some ready-to-cook poultry meat processed in Western Australia. This creates opportunities for the potential spread of resistant strains or resistance genes to humans via the food chain. The information from this study will be useful in providing background data and direction for future planning in preventing antimicrobial-resistant bacteria from poultry meat being transmitted through the food chain. The full implementation of the HACCP program would offer substantial benefits and protection to consumers.