Survey for psychrotrophic bacterial pathogens in minimally processed lettuce

A total of 120 minimally processed, cut and packaged lettuce samples mere purchased from retail supermarkets or provided by a salad production facility over an 8-month period. The samples mere tested for total aerobic plate counts and for the presence of potentially pathogenic species belonging to the genera of Listeria, Aeromonas and Yersinia. The aerobic prate counts ranged from 10(3) to 10(9) colony forming units (cfu) g(-1). Most samples (76%) contained between 10(5) and 10(7) cfu g(-1) total aerobic bacteria. Listeria monocytogenes was isolated from three samples, Aeromonas hydrophila or Aeromonas caviae from 66 samples, and Yersinia enterocolitica from 71 samples. The pathogenic potential of Y. enterocolitica isolates was determined by screening for an array of biochemical, serological and genetic traits (heat-stable enterotoxin gene, the attachment and invasion gene locus, the invasin gene locus and the virulence plasmid). The Y. enterocolitica isolates lacked many of the phenotypic and genetic markers associated with virulence in primary pathogenic strains. As the roles of the reputed virulence factors of Aeromonas spp. in human infection are uncertain, the pathogenic potential of the Aeromonas isolates in lettuce remains unclear.