Limited Dissemination of Extended-Spectrum ß-Lactamase- and Plasmid-Encoded AmpC-Producing Escherichia coli from Food and Farm Animals, Sweden.

Extended-spectrum ß-lactamase (ESBL)- and plasmid-encoded ampC (pAmpC)-producing Enterobacteriaceae might spread from farm animals to humans through food. However, most studies have been limited in number of isolates tested and areas studied. We examined genetic relatedness of 716 isolates from 4,854 samples collected from humans, farm animals, and foods in Sweden to determine whether foods and farm animals might act as reservoirs and dissemination routes for ESBL/pAmpC-producing Escherichia coli. Results showed that clonal spread to humans appears unlikely. However, we found limited dissemination of genes encoding ESBL/pAmpC and plasmids carrying these genes from foods and farm animals to healthy humans and patients. Poultry and chicken meat might be a reservoir and dissemination route to humans. Although we found no evidence of clonal spread of ESBL/pAmpC-producing E. coli from farm animals or foods to humans, ESBL/pAmpC-producing E. coli with identical genes and plasmids were present in farm animals, foods, and humans.

The effect of undissociated lactic acid on Staphylococcus aureus growth and enterotoxin A production.

The potential of Staphylococcus aureus cheese isolates to grow and produce staphylococcal enterotoxin A under conditions typical for cheese making was investigated in three broth experiments. The effect of the concentration of undissociated lactic acid (HLac) in conjunction with specific pH values was studied by adjusting pH at a single concentration of lactic acid. First, the time-to-growth of S. aureus was modelled by using survival analysis and absorbance data obtained from an automated turbidity reader. The fitted model describes the time to growth and indicates the growth / no growth boundary of S. aureus as a function of HLac concentration, temperature and water activity. Second, growth rates and lag times of S. aureus were estimated after two different pre-treatments in skim milk at three HLac concentrations and two temperatures based on optical detection times of serial dilutions of bacterial solutions. Growth rates differed between strains, and increased with increasing temperature and decreasing HLac concentration. Preliminary results indicate that lag times were dependent on pre-treatment suggesting that the growth potential of S. aureus in cheese curd may be greater if milk is used immediately after milking compared to holding at 4°C after milking. Third, growth, inactivation, and enterotoxin A production of S. aureus strains were investigated at twelve combinations of HLac concentration and temperature. Concentrations of enterotoxin A increased linearly during the first four days, with a production rate increasing with increasing temperature and decreasing HLac concentration. Significant amounts of enterotoxin A were produced during extended incubation, up to 14days, but then initial pH had changed. This highlights a potential limitation of modelling based on the initial environmental conditions in batch experiments. In summary, ranges of time-to-growth, growth rates, lag times and enterotoxin A production rates of S. aureus in the presence of HLac were estimated. The results can be used together with process data to indicate the range and magnitude of growth and enterotoxin A production during initial stages of cheese production.

Occurrence of foodborne pathogens and characterization of Staphylococcus aureus in cheese produced on farm-dairies.

The objective of this study was to address knowledge gaps identified in an earlier risk assessment of Staphylococcus aureus and raw milk cheese. A survey of fresh and short-time ripened cheeses produced on farm-dairies in Sweden was conducted to investigate the occurrence and levels of S. aureus, Listeria monocytogenes and Escherichia coli, to characterize S. aureus isolates with special emphasis on enterotoxin genes, antibiotic resistance, bio-typing and genetic variation, and to collect information related to production practices. In general, the hygienic quality of farm-dairy cheeses appeared to be of an acceptable microbiological quality, e.g. L. monocytogenes and staphylococcal enterotoxin were not detected in cheese samples. However, E. coli and enterotoxigenic S. aureus were frequently found in raw milk cheeses and sometimes at levels that are of concern, especially in fresh cheese. Interestingly, levels in raw milk fresh cheese were significantly lower when starter cultures were used. Up to five S. aureus colonies per cheese, if possible, were characterized and about 70% of isolates carried one or more enterotoxin genes, most common were sec and sea. The Ovine biotype (73%) was most common among isolates from goat milk cheese and the Human biotype (60%) from cow milk cheese. Of all isolates, 39% showed decreased susceptibility to penicillin, but the proportion of isolates from cows' cheese (66%) compared to isolates from goats' cheese (27%) was significantly higher. S. aureus isolates with different properties were detected in cheese from the same farm and, sometimes even the same cheese. Isolates with the same pulsed-field gel electrophoresis (PFGE)-pattern were detected on geographically distant dairies. This indicates that multiple sources and routes of contamination are important. To improve the safety of these products efforts to raise awareness of the importance of hygiene barriers and raw milk quality as well as improved process control can be suggested, e.g. use of starter cultures and monitoring of fermentation with a pH-meter. For future safety assessments, a better understanding of factors determining toxin production in these cheeses is needed.