School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemStudies on the prevalence, persistence and antibiotic resistance of enterococci from Australian dairy sources( 2016)This work was undertaken to determine the incidence of enterococci in raw milk and dairy products in the dairying regions of south-eastern Australia over a one-year period and to assess the ability of enterococci to persist in the dairy environment, survive pasteurisation and grow in dairy products. In addition, the risks associated with the presence of enterococci in dairy products was also considered. Enterococci were widespread in milk prior to manufacture, with lower counts occurring in raw milk in winter, which has not been reported in other studies internationally. The level of detection of enterococci in pasteurised milk was low and did not coincide with higher raw milk counts, indicating that the main enterococci population was heat sensitive. Although Enterococcus faecalis was the predominant species found in the raw milk, E. faecalis, E. malodoratus and E. faecium were detected in the greatest number of butter, Cheddar cheese and milk powder samples, respectively, suggesting that these species have inherent properties that enable them to survive processing conditions used in the manufacture of these dairy products. Thermal death determinations of the four main species identified from laboratory pasteurised milk (63 °C, 30 min) revealed that the E. faecalis isolates were the most heat sensitive, with the lowest z values (5.0-7.5 °C), while E. faecium, E. hirae and E. durans had the highest z values (8.5-9.8 °C). Thermoduric enterococci were able to grow at 4 and 7 °C in UHT milk. They were able to survive but not grow in butter, Cheddar cheese, cottage cheese, milk powder and yoghurt and in acidic and higher salt concentrations at 4 °C, indicating that dairy product environments limit the growth of enterococci. Genetic fingerprinting of enterococci at one of the dairy factories identified a persistent population of E. faecalis, which had streptomycin resistance and increased biofilm production, and inferred a link between these traits and persistence. The most frequently encountered antibiotic resistances were to streptomycin and tetracycline, which, along with chloramphenicol and erythromycin resistance, could be transferred to both E. faecalis and E. faecium but in limited situations to E. hirae and Listeria monocytogenes. Transfer to E. hirae has not been previously demonstrated, which is significant in context of this species increased resistance to heat treatment. The isolates were also mated in a multi-strain biofilm on stainless steel coupons in milk incubated at 10 and 25 °C. No transconjugants were detected under these experimental conditions. Results of this study found that enterococci in the south-eastern Australian raw milk supply possessed multiple virulence factors and antibiotic resistances, which were at times transferrable between species and genera. While the majority of enterococci were eliminated with pasteurisation, some enterococci were heat-resistant, meaning that enterococci with virulence factors and antibiotic resistance could gain entry into the food supply if these traits were present in such isolates. However, resistance to the antibiotic of most concern, vancomycin, was not found in E. faecalis, a major species associated with medical issues. Although there is a theoretical risk of enterococci containing medically-undesirable traits entering the food supply, their ability to grow in products may be limited by the nature and storage conditions of the products.