P3-135 Microbiological Assessment and Testing of Organic Pre- and Post-harvest Fresh Produce and Irrigation Water on Maryland Farms for Salmonella, Listeria monocytogenes and Shiga Toxin-producing Escherichia coli

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Aixia Xu, University of Maryland-College Park, College Park, MD
Donna Pahl, University of Maryland-College Park, College Park, MD
Robert Buchanan, University of Maryland-College Park, College Park, MD
Shirley Micallef, University of Maryland-College Park, College Park, MD
Introduction: Consumption of locally, organically-grown produce is increasingly popular, but microbial safety remains under-researched.  Microbiological data necessary to evaluate organic produce safety risks remain scarce. 

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.