Prevalence of Listeria monocytogenes during production and postharvest processing of cabbage.

From November 1999 to May 2000, analyses of 425 cabbage, 205 water, and 225 environmental sponge samples from four cabbage farms with packing sheds and from two packing sheds in the Rio Grande Valley and Uvalde, Tex., were conducted to determine whether Listeria monocytogenes was present. Samples were tested by the Food and Drug Administration method for the isolation of Listeria spp., and confirmed isolates were DNA fingerprinted by repetitive-element sequence-based polymerase chain reaction (rep-PCR). L monocytogenes was isolated from 3% (26 of 855) of the samples. Twenty of these isolates were obtained from cabbage (7 isolates from farms and 13 from packing sheds). Three isolates were from water samples(two from farms and one from a packing shed), and three were from environmental sponge samples of packing shed surfaces. Rep-PCR-generated fingerprints of 21 of the isolates revealed 18 distinctive banding patterns. Four isolates from environmental sponge samples of conveyor belts and from cabbage samples shared identical banding patterns, suggesting common sources of contamination. These identical environmental isolates suggest that contact with packing shed surfaces may be a source of contamination of cabbage. However, the cabbage samples could have arrived contaminated, since they were not washed.

Antimicrobial resistance of Listeria monocytogenes isolated from various cabbage farms and packing sheds in Texas.

Twenty-one isolates of Listeria monocytogenes from cabbage, environmental, and water samples were evaluated for antimicrobial resistance by the disk diffusion method. Ninety-five percent (20 of 21) of the isolates tested were resistant to two or more antimicrobial agents. This finding is significant, since multiresistant strains of Listeria spp. are not commonly found in nature. Eighty-five percent (17 of 20) of the multiresistant strains were resistant to penicillin, and the remaining multiresistant isolates were somewhat sensitive to penicillin. A multiresistant strain showing intermediate sensitivity to penicillin was resistant to gentamicin. One isolate was susceptible to all antimicrobial agents except penicillin. Penicillin- and gentamicin-resistant L. monocytogenes have not previously been reported from human, food, or environmental samples. This study provides evidence of the emergence of multiresistant L. monocytogenes strains, pointing to an increase in the potential threat to human health posed by this pathogen.