Survey for psychrotrophic bacterial pathogens in minimally processed lettuce

A total of 120 minimally processed, cut and packaged lettuce samples mere p urchased from retail supermarkets or provided by a salad production facilit y over an 8-month period. The samples mere tested for total aerobic plate c ounts and for the presence of potentially pathogenic species belonging to t he genera of Listeria, Aeromonas and Yersinia. The aerobic prate counts ran ged 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. Liste ria monocytogenes was isolated from three samples, Aeromonas hydrophila or Aeromonas caviae from 66 samples, and Yersinia enterocolitica from 71 sampl es. The pathogenic potential of Y. enterocolitica isolates was determined b y screening for an array of biochemical, serological and genetic traits (he at-stable enterotoxin gene, the attachment and invasion gene locus, the inv asin 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 facto rs of Aeromonas spp. in human infection are uncertain, the pathogenic poten tial of the Aeromonas isolates in lettuce remains unclear.