Incidence and survival of Escherichia coli O157:H7 and Listeria monocytogenes on salad vegetables

Salad vegetables contaminated with pathogens can cause food poisoning. The isolation of Escherichia coli O157:H7 and Listeria monocytogenes from different salad vegetables; cucumber, cabbage, carrot, and lettuce was carried out. E. coli O157:H7 was isolated from all the salad vegetables while L. monocytogenes was isolated from only cabbage and lettuce. The effect of different storage temperatures on the survival and growth of E. coli O157:H7 and L. monocytogenes on cabbage and lettuce was determined. Known population of each isolate was inoculated into sliced cabbage and lettuce separately and stored at 5°C (refrigerator temperature) and 28°C (room temperature) for 10 days. Bacteria were counted on daily basis. Result showed initial increase in most cases on second and/or third day followed by decrease in bacterial population all through storage. There was no growth towards the end of storage at 28°C. In all cases there was decrease in pH. The count of E. coli O157:H7 ranged from 3.6×10³ – 4.0×10² cfu/g in salad vegetables stored at 5°C and the pH, 7.11 - 5.66 while the count at 28oC was 3.9×103 – 1.0×10² cfu/g with pH 7.11 – 4.06. L. monocytogenes count in salad vegetables stored at 5oC was 1.51×10^4 – 4.8×10² cfu/g and pH was 7.12 – 5.84. At 28°C, the count was 1.49×10^4 – 2.0×10² cfu/g and pH was 7.14 – 4.14. Both bacterial pathogens survived at storage temperatures at which salad vegetables are normally stored in practice. Their presence and survival in salad vegetables call for public health concern because salad vegetables receive little or no heat treatment before consumption.