Detection and analysis of indicator and pathogenic bacteria in conventional and organic fruits and vegetables sold in retail markets

Produce-associated food-borne outbreaks have been increasingly implicated as the significant proportion of the annual incidence of food-borne illness worldwide. The objectives of this study were to determine the concentrations of indicator bacteria and the presence of Salmonella spp., Shigella spp., Escherichia coli (E. coli) O157:H7, and Listeria monocytogenes (L. monocytogenes), and to characterize predictors associated with Salmonella contamination of retail produce from fresh markets and supermarkets in Bangkok, Thailand. A total of 503 samples were collected during May 2018 and February 2019, comprised of sweet basil, spring onion, coriander, cabbage, lettuce, cucumber, and tomato, with conventional items from fresh open-air markets (n=167), conventional items from supermarkets (n=168), and organic items from supermarkets (n=168). The overall prevalence in these 503 items for fecal coliforms and E. coli was 84.3% and 71.4%, with mean concentrations (+/- standard deviation) of fecal coliforms and E. coli being (3.0x10(5)+/- 1.3x10(6)) most probable number (MPN)/g and (1.8x10(5)+/- 1.1x10(5)) MPN/g, respectively. The concentrations of fecal coliforms and E. coli were higher in produce sampled from fresh open-air markets than produce from supermarkets; similarly, these bacterial indicators were higher from produce grown under conventional methods than certified organic produce. The prevalence of Salmonella and Shigella was 4.8% and 0.4%, respectively, but no positives were found for E. coli O157:H7 and L. monocytogenes. The predominant Salmonella serovar was Stanley (30.8%). Based on logistic regression, the odds of Salmonella contamination were significantly (P<0.05) higher during the rainy versus dry season, produce grown using conventional versus organic agriculture, sweet basil versus other commodities, and using ice tank versus dry refrigeration for overnight retail storage. This study indicated that fruits and vegetables are important sources of microbial contamination. Hence, monitoring and surveillance of pathogen contamination to produce is needed to strengthen food safety.