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Evaluation of indicator microorganisms in blueberry and raspberry production systems Aliphtiras, George J.

Abstract

Recent outbreaks of foodborne disease have been attributed to the consumption of fresh produce. Enteric pathogens derived from animal and human faecal material have been identified as the dominant etiological agents in these outbreaks. The objective of this study was to identify significant sources of such microbial contamination in blueberry and raspberry production systems. Study participants (berry farms) were selected according to agronomic practices, including fertilization, harvesting, source of irrigation water and method of distributing irrigation water. The study was conducted over two berry-growing seasons. Enumeration of coliforms and Escherichia coli on fruit, fruit contact surfaces and environmental samples from growers and processors were conducted using traditional, violet red bile agar-based (TSA-VRBA[sub MUG] overlay, Petrifilm™ E coli count plates) and newly developed chromogenic assays (Chromocult® coliform agar and XM-G agar). The TSA-VRBA[sub MUG] overlay method, however, was found to be unsuitable and was discontinued after the first sampling year. Results suggested that contamination of berry fruit surfaces occurred at the grower level. Harvesting and processing practices did not contribute significant coliform bacteria (P > 0.05) to the fruit surface at the grower and processor levels. Based upon Kruskal-Wallis and multiple comparison analyses on E coli counts, ditch water-overhead sprinkler irrigation was deemed to be a significant source of microbial contamination in blueberry production. The inability to detect a difference in the sanitary or faecal indices of berry samples using coliform populations re-affirmed the unsuitability of coliform counts as indicators of sanitation for fresh produce. E coli isolates from berry samples were non-verocytotoxicogenic based upon PCR analyses. However, the frequency of E coli occurrence in berry samples indicates a potential risk from other enteric pathogens. Comparison of selective agars revealed that efficiencies of coliform and E coli recovery for CCA and PEC were significantly different (P < 0.05). However, E coli counts within the accuracy and estimated range on CCA and PEC correlated to each other (P < 0.05), likely due to the shared β-D-glucuronidase- based differentiation reaction. CCA is a suitable alternative to TSA-VRBA[sub MUG] overlay and PEC for coliform and E coli enumeration due to greater indicator recovery and differentiation capability.

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