Isolation, DNA fingerprint analysis, and antimicrobial resistance of Listeria monocytogenes from various cabbage farms and packing sheds in Texas

Abstract

Analyses of 855 cabbage, water, and environmental sponge samples for Listeria monocytogenes from 6 cabbage farms and/or packing sheds in the Rio Grande Valley and Uvalde, TX were conducted from November 1999-May 2000. Samples were tested by the Food and Drug Administration method for isolation of Listeria spp., and positive isolates were DNA fingerprinted using repetitive element sequence-based PCR (rep-PCR). L. monocytogenes was isolated from 3% (26 of 855) of the samples. Twenty of these isolates were identified from cabbage on the farm and in the packing shed. Three isolates were from water samples (2 on the farm and 1 at the 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, indicating similar sources of contamination. Twenty-one isolates 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 multi-resistant strains of Listeria spp. are not commonly found in nature. Eighty-five percent (17 of 20) of the multi-resistant strains were resistant to penicillin, and the remaining multi-resistant isolates were intermediately (somewhat) sensitive to penicillin. A multi-resistant strain showing intermediate sensitivity to penicillin was resistant to gentamicin. One isolate was susceptible to all antimicrobial agents except penicillin. Penicillin and gentamicin resistance have not previously been reported in Listeria monocytogenes isolated from human, food, or environmental samples. This study provides evidence for the emergence of multiple-resistant L. monocytogenes strains, pointing to an increase in the potential threat to human health posed by this pathogen. This research establishes that L. monocytogenes contamination of cabbage is present at all stages of production and post-harvest processing and indicates that the ubiquitous distribution of this pathogen is the likely source of contamination on the farms and in the packing sheds. This study indicates the possible emergence of multi-resistant L. monocytogenes strains in nature, and it may point to the development of antibiotic resistance irrespective to whether the organism was exposed to antibiotics through animal husbandry practices.