Microbiological quality of fresh leafy vegetables, salad components and ready-to-eat salads: an evidence of inhibition of Listeria monocytogenes in tomatoes

Abstract

A total of 116 samples of 11 different fresh vegetables from a local market generally consumed in raw form as well as 12 samples of ready-to-eat green salads procured from three grade 1 and 2 restaurants in Mumbai, India were examined for overall microbial quality in terms of bacterial, mold and coliform levels and incidence of pathogens such as Listeria and Yersinia. Standard procedures and media were used for isolation and identification studies. Thoroughly washed samples of 26 leafy vegetables, 12 roots, 62 tomatoes and four samples each of cabbage, capsicum and cucumber showed total bacterial and yeast-mold count in 106-107 cfu/gm and 102-105 cfu/gm range respectively. On the other hand, higher range of bacterial (106-108 cfu/gm) and mold (104-107 cfu/gm) count were noticed in ready-to-eat salads from restaurants. The MPN index/gm for coliforms for vegetables from a local market ranged from <3 to >1100 whereas for ready-to-eat salads it was 11 to 460. Evidence of higher number of coliforms was observed mostly in green leafy vegetables. All (100%) local vegetables exhibited the incidence of Listeria and Yersinia. On the other hand, ready-to-eat salads showed 20 and 73% presence of Yersinia and Listeria respectively. Higher occurrence of fecal coliforms was (65.6%) found in raw vegetables while they were absent in ready-to-eat salad samples. Non-pathogenic species like Y. intermedia and L. innocua were predominating species in most of the samples. Nevertheless, presence of L. monocytogenes was observed in 7 out of 62 tomatoes, 5 out of 10 coriander leaves, 2 out of 4 spinach samples and one from 4 cabbage samples. Studies conducted to understand the ability of L. monocytogenes 036 and 35152 to grow in tomato in the presence of naturally occurring bacteria suggested that artificially inoculated (103 cfu/ml) cells are killed after 3 days, 12 days and 14 days of incubation at 37°C, 8-10°C and 2-4°C respectively.