Several outbreaks of food poisoning have been linked to the consumption of contaminated fresh vegetables and fruits. Enterohemorrhagic E. coli O157 (O157) and Salmonella are the most common causes of these illnesses. Listeria is also supposed to be a cause of food-borne illness. Detection methods for these three pathogenic bacteria from fresh vegetables and fruits were studied by enrichment culture procedure, PCR detection and selective detection. The first step was enrichment by BPW (buffered peptone water) for 20-24 hr at 36°C. After 6 hr of enrichment, 1ml of enrichment solution was transferred to NmEC (EC broth with Novobiocin and bile acid) for separation of O157. After enrichment for 20-24 hr, 1 ml of enrichment solution was treated as a PCR sample. Fewer than 10 cells of O157 or Salmonella per 25 g food samples could be detected by our methods. Listeria (1, 000 cells/25 g) were also detectable by these methods. Criteria for the multiplex PCR (O157, Salmonella and Listeria) were key factors for further testing (Limitations of No. detected were2.3×104, 2.7×105 and2.2×105 cfu/ml, respectively). Selective cultures were used when PCR was positive for pathogenic bacteria. These methods are economical and effective for detection of these pathogenic bacteria from fresh vegetables and fruits.
We examined 82 imported raw vegetables and 14 imported raw fruits for contamination with Salmonella, Enterohemorrhagic Escherichia coli O 157: H 7, and Listeria. Of 12 bean sprout samples, one was contaminated with L. monocytogenes serotype 1/2a, and two with L. innocua. One of two kiwi fruit samples was contaminated with L. seeligeri. Aeromonas caviae was isolated from one of five young field pea samples. One of three alfalfa samples was contaminated with Enteropathogenic E. coli 08. Three months later, we examined an additional 10 imported bean sprout samples obtained from the store that had sold the L. monocytogenes-contaminated bean sprouts described above. L. monocytogenes serotype 1 /2a was isolated from three samples, and L. innocua from one. The AFLP technique (Amplified Fragments Length Polymorphism) was used for genetic analysis of four isolates of L. mono-cytogenes serotype 1/2a and no homology was found among the isolates. The high L. monocytogenes-contamination rate for bean sprouts indicates poor hygiene control in growth, harvest, washing, distribution, and storage of the product. The washing and handling of imported raw vegetables and fruits should be improved to prevent food-borne disease.