DETECTION AND CHARACTERIZATION OF ESCHERICHIA COLI O157:H7 AND SALMONELLA IN FOOD
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Escherichia coli O157:H7 and Salmonella are among the most important foodborne pathogens that cause millions cases of infections and hundreds deaths each year in the United States. Beef and poultry products are frequently recognized transmission media for these two organisms. Rapid detection and isolation methods applied to beef or chicken products are expected for these two bacteria. A rapid sample preparation method for E. coli O157:H7 detection by PCR method in ground beef samples was developed by combining different techniques, including filtration, centrifugation, enzyme digestion, and DNA extraction. The detection limit of this method was 103 cells/g without enrichment, and 100 cells/g can be detected after 6 h en-richment. For Salmonella, a poultry specific isolation method was modified from the USDA/FSIS manual by considering the specific characteristics of poultry products. Higher than 95% of the suspect colonies isolated by using the modified method were confirmed as Salmonella by PCR/API 20 E tests. This method was applied on retail organic and conventional chicken samples for Salmonella isolation. All Salmonella isolates were further characterized by serotyping, PFGE and antibiotics susceptibility tests. The results indicated that organic and conventional chicken samples were frequently contaminated with Salmonella, and that Salmonella from organic chicken were more susceptible to antimicrobials commonly used in human and veterinary medicine. High acid resistance capability is another unique characteristic of E. coli O157:H7 which is correlated with low infection dose of this pathogen. Survival mechanism of E. coli O157:H7 cells in gastric juice or acidified LB (pH 2.5) was studied, it was found that the limited availability of glutamate and/or arginine creates an illusion of cell-density-dependent acid sensitive phenotype of E. coli O157:H7 during acid-challenge.