Spatial Distribution of Salmonella, Escherichia coli O157:H7, and Other Bacterial Populations in Commercial and Laboratory-Scale Sprouting Mung Bean Beds

Abstract

The reliability of testing spent irrigation water to assess the microbiological status of sprouting mung bean beds has been investigated. In commercial trials, the distribution of opportunistic contaminants within 32 bean sprout beds (25 kg of mung beans per bin) was assessed 48 h after germination. The prevalence of generic Escherichia coli, thermotolerant coliforms, and Aeromonas in sprouts (n = 288) was 5, 11, and 39%, respectively, and 57, 70, and 79% in the corresponding spent irrigation water samples (n = 96). Contamination was heterogeneously distributed within the seedbed. In laboratory trials, beans inoculated with a five-strain cocktail of either Salmonella or E. coli O157:H7 (10(3) to 10(4) CFU/g) were introduced (1 g/500 g of noninoculated seeds) at defined locations (top, middle, or base), and the beans were then sprouted for 48 h. When seeds inoculated with pathogens were introduced at the base or top of the seedbed, the pathogens were typically restricted to these sites and resulted in 44% of the spent irrigation water samples returning false-negative results. Introducing inoculated beans into the middle or at the presoak stage enhanced the distribution of both pathogens within the subsequent sprout bed and resulted in comparable levels recovered in spent irrigation water. The study demonstrated that even though screening a single sample of spent irrigation water is more reliable than testing sprouts directly, it does not provide an accurate assessment of the microbiological status of sprouting mung bean beds. Such limitations may be addressed by ensuring that bean batches are mixed prior to use and by taking spent irrigation water samples from multiple sites at the latter stages of the sprouting process.

Scientific Opinion on the risk posed by Shiga toxin‐producing Escherichia coli (STEC) and other pathogenic bacteria in seeds and sprouted seeds

“…In laboratory experiments with artificial inoculation of seeds (Charkowski et al, 2002;Liu and Schaffner, 2007b), and with naturally contaminated seeds (Stewart et al, 2001), the number of pathogens in the sprouting water was similar to that on the sprouts, after some times of sprouting. In contrast, (Hora et al, 2005) showed that Salmonella artificially inoculated in a few seeds placed in the seedbed before sprouting did not always result in Salmonella being found in the water irrigating the seeds. In addition, inoculation of seeds during the sprouting process with Salmonella or L. monocytogenes, instead of inoculation at the start of sprouting, resulted in no multiplication of these pathogenic bacteria (Castro-Rosas and Escartin, 2000;Schoeller et al, 2002).…”

“…Evidence for heterogenous distributions of bacterial pathogens have been observed for: seeds (FDA, 1999c;ISS, 2011;Van Beneden et al, 1999); irrigation water (Liu and Schaffner, 2007a;McEgan et al, 2008); sprouts and spent irrigation water (Hora et al, 2005;Liu and Schaffner, 2007a;McEgan et al, 2008).…”

“…However despite being more sensitive than testing sprouts directly, a single sample of spent irrigation water has been shown not to give reliable information on the status of a sprouting mung bean bed (Hora et al, 2005) and thus composite samples must be formed from multiple sampling points from each production lot and analysed for microbial pathogens of concern for this approach to be effective. Even so it can still not be assured with absolute certainty that the pathogens (Salmonella, STEC) will be detected (Hora et al, 2005). However, the distribution of contaminating enteropathogens in spent irrigation water is considered to be more homogenous than in seeds and therefore fewer samples are needed to form a representative sample.…”

Quantitative Assessment of Contamination of Fresh Food Produce of Various Retail Types by Human-Virulent Microsporidian Spores

“…Inactivation of potential biological contaminants without altering the freshness of the product is impractical, and therefore water of a high microbiological quality is essential for the production of safe fresh food (22). Unfortunately, considerable evidence indicates the involvement of contaminated water in the production of berries, sprouts, and green-leafed vegetables (2,12,19,26). Microsporidian spores resist standard wastewater treatment and can be found in sewage sludge end products commonly used for fertilization of ready-to-eat crops or in runoff-impacted surface water used for irrigation (15).…”

“…Microsporidian spores have been reported to be found in samples from groundwater and surface water (3, 4, 7-10, 13, 24), including water used for irrigation of fresh-produce operations (26). Although berries, sprouts, and vegetables have been contaminated with a variety of human pathogens (12,17,19,21,22,25), reports on contamination with microsporidian spores are scant (1). Because E. intestinalis has been identified in irrigation water used for ready-to-eat crop production (26) and since unspecified spore species have been found in samples from strawberries, lettuce, and parsley (1), we initiated testing of commercial fresh produce at the retail level to quantitatively assess contamination with human-virulent microsporidian spores.…”