Purpose: This study aimed to determine pathogen prevalence for Salmonella, Shiga toxin-producing Escherichia coli (STEC) and Listeria monocytogenes, and quantify indicators on pre- and post-harvest organic produce and irrigation water on Maryland farms.
Methods: Six farms were sampled 2-4 times in 2012. Organic produce samples from fields, harvested samples from packing houses, and irrigation water were analyzed quantitatively for E. coli, total coliforms (TC), aerobic bacteria (APC), yeasts (Y) and molds (M), using 3MTMpetrifilms. Pathogens were isolated by enrichment and selective media. The identity of presumptive pathogens was confirmed by PCR amplification using species-specific primers.
Results: A total of 20 water and 94 produce samples (17 varieties; 74 pre-harvest, 20 post-harvest) were collected. E. coli was detected on 4/74 (5.4%) of pre-harvest produce, 2/20 (10%) of post-harvest produce and 11/20 (55%) of water samples. E. coli was detected in pond water at 1.16 log CFU/ml (n = 7), but only -0.03 log CFU/ml in groundwater (n = 13). Comparing pre- and post-harvest samples, respectively, average counts in log CFU/g detected were TC: 1.67 and 3.05; E. coli: 0.09 and 0.40; APC: 5.27 and 4.60; Y: 4.31and 3.83; and M: 2.89 and 2.72. APC (P < 0.001) and Y (P = 0.01) counts were significantly higher in pre-harvest versus post-harvest tomatoes, and TC (P < 0.05) was lower for lettuce. In general, TC and E. coli counts were significantly higher in post-harvest produce when compared to pre-harvest (P < 0.05). Salmonella and L. monocytogenes were not detected. No shigatoxin genes were detected, but intimin (eae) genes were detected in E. coli isolates from 4 produce samples and 4 water samples.
Significance: This study has generated new data on the microbiological quality of field and packing house organic produce; and can be used to assess efficacy of current pre- and post-harvest organic practices.