Development of acid adaptation in Escherichia coli O157:H7, Listeria monocytogenes and Salmonella in şavak tulumi cheese during production and ripening

The present study was undertaken to investigate the effects of production of Şavak tulumi cheese on survival and acid resistance of Salmonella, Listeria monocytogenes, and E. coli O157:H7. Production procedures of commercial Şavak tulumi producers in Elazığ were evaluated and the shortest processing was selected as a modal. In each of triplicate experiments per pathogen, raw ewe?s milk was inoculated with 5-strain mixture of Salmonella, Listeria monocytogenes or E. coli O157:H7 (average 7.0 log10 cfu/ml) and processed (at 25oC, 3 days) and ripened at (6oC, 90 days) in plastic containers. Microbiological and chemical analysis and exposure experiments to synthetic gastric fluid (SGF, pH 2.0-2.5) were carried out during processing (days 1 and 2) and on certain days (0, 15, 30, 45, 60, 90) during ripening. During ripening, range of pH for tulumi cheese samples was 4.46-4.75, aw was 0.899-0.915, and salt level was 2.88-4.05 %. The numbers of Salmonella decreased relatively faster as of the second day of curd-processing and dropped below detection limit (1.0 log10 cfu/g) on day 90. However, numbers of L. monocytogenes and E. coli O157:H7 showed a gradual decrease during ripening and their numbers were 2.91+0.27 log10 cfu/g and 3.15+0.51 log10 cfu/g, respectively, on day 90. In SGF experiments, Salmonella did not survive 90 min exposure even on the day 2 of processing while viable cells of 0.90+0.02 log10 cfu/ml of E. coli O157 and 1.36+0.52 log10 cfu/ml of L. monocytogenes were recovered after 90 min exposure on day 90 of ripening. These results indicate that compared to Salmonella cells, E. coli O157:H7 and L. monocytogenes were more resistant to the intrinsic conditions occurred during production of Şavak tulumi cheese and these two pathogens could survive to 90 min exposure to SGF throughout the processing and ripening periods. This increased survival could be due to activation of acid resistance mechanisms of E. coli O157:H7 and L. monocytogenes by subletal conditions occurred during production and ripening period.