The resistance of Listeria monocytogenes to cadmium and arsenic has been used extensively for strain subtyping. However, limited information is available on the prevalence of such resistance among isolates from the environment of food-processing plants. In addition, it is not known whether the resistance of such isolates to heavy metals may correlate with resistance to quaternary ammonium compounds extensively used as disinfectants in the food-processing industry. In this study, we characterized 192 L. monocytogenes isolates (123 putative strains) from the environment of turkey-processing plants in the United States for resistance to cadmium and arsenic and to the quaternary ammonium disinfectant benzalkonium chloride (BC). Resistance to cadmium was significantly more prevalent among strains of serotypes 1/2a (or 3a) and 1/2b (or 3b) (83% and 74%, respectively) than among strains of the serotype 4b complex (19%). Resistance to BC was encountered among 60% and 51% of the serotype 1/2a (or 3a) and 1/2b (or 3b) strains, respectively, and among 7% of the strains of the serotype 4b complex. All BC-resistant strains were also resistant to cadmium, although the reverse was not always the case. In contrast, no correlation was found between BC resistance and resistance to arsenic, which overall was low (6%). Our findings suggest that the processing environment of turkeyprocessing plants may constitute a reservoir for L. monocytogenes harboring resistance to cadmium and to BC and raise the possibility of common genetic elements or mechanisms mediating resistance to quaternary ammonium disinfectants and to cadmium in L. monocytogenes.Listeria monocytogenes causes listeriosis, a relatively rare but serious food-borne disease with symptoms such as stillbirths or abortions, septicemia, and meningitis or encephalitis. It has a high mortality rate (ca. 20%) for at-risk populations, which include pregnant women and their fetuses, neonates, and those who are elderly and/or immunocompromised (25). Strains of three serotypes (1/2a, 1/2b, and 4b) are associated with most human listeriosis cases. A substantial fraction of sporadic cases and the majority of food-borne outbreaks are due to serotype 4b strains (12,25). However, strains of serotype 4b tend to be underrepresented in foods, and the majority of food isolates are of serotype 1/2a or 1/2b (11,14,27). The environment of food-processing plants is considered to play a key role in contamination of processed ready-to-eat foods, and substantial evidence points to the ability of certain strains to colonize the processing-plant environment and to persist there, frequently over extended periods of time (for reviews, see references 12 and 13).Benzalkonium chloride (BC) and other quaternary ammonium compounds (QACs) are extensively used as disinfectants in the food-processing industry (19,22). Frequent use of BC and other QACs in food-processing plants may constitute a selective pressure for the emergence and establishment of resistance to these compounds among L. monocytogenes isolates...

Even though at least 400 Listeria phages have been isolated from various sources, limited information is available on phages from the food processing plant environment. Phages in the processing plant environment may play critical roles in determining the Listeria population that becomes established in the plant. In this study, we pursued the isolation of Listeria-specific phages from environmental samples from four turkey processing plants in the United States. These environmental samples were also utilized to isolate Listeria spp. Twelve phages were isolated and classified into three groups in terms of their host range. Of these, nine ( Listeria monocytogenes is an important food-borne pathogen responsible for listeriosis, an illness with severe symptoms and relatively high mortality rates (20 to 30%). Individuals at risk are primarily pregnant women and their fetuses, immunocompromised patients, and the elderly. Most cases of human listeriosis involve bacteria of serotypes 1/2a, 1/2b, and 4b. The food processing plant environment is of key importance for the contamination of ready-to-eat foods by L. monocytogenes (15,16). However, attributes of the bacteria that determine their distribution, prevalence, and persistence in the processing plant environment remain poorly characterized.The ability of L. monocytogenes to form biofilms and to resist disinfectants commonly employed for processing plant sanitation has been postulated to be an important determinant of the organism's ecology in the processing plant environment (16). Differences in ability to form biofilms and to tolerate disinfectants used in processing plants may contribute to the higher relative prevalence of certain serotypes. In several studies, strains of serotype 1/2a were significantly more frequently isolated from environmental samples of processing plants than were strains of serotype 4b (17,27,30,31). There is evidence that serotype 1/2a strains were more likely to form biofilms and to be resistant to the disinfectant benzalkonium chloride (BC) than were strains of serotype 4b (4,24,25).Another potentially important determinant of Listeria's ecology in the processing plant (and other) environments would be expected to be the susceptibility of the organisms to Listeria-specific bacteriophage (listeriaphage). The first listeriaphage report was published in 1945 (28), and to date, at least 400 phages have been isolated from various sources, including foods, sewage, silage, and lysogenic strains (21). The differential susceptibility of Listeria strains to selected phages has been utilized extensively as a strain-typing tool in epidemiological studies (22). Recent studies suggest the potential of phages as biological control agents for Listeria in foods and in the processing plants (13,18,19), and in 2006, the U.S. FDA approved the application of a listeriaphage mixture on the surface of ready-to-eat meat and poultry products (11). GRAS (generally regarded as safe) status has been granted to listeriaphage P100, isolated from a sewage effluent sampl...

ABSTRACTThe persistence ofListeria monocytogenesin food processing plants and other ecosystems reflects its ability to adapt to numerous stresses. In this study, we investigated 138 isolates from foods and food processing plants for resistance to the quaternary ammonium disinfectant benzalkonium chloride (BC) and to heavy metals (cadmium and arsenic). We also determined the prevalence of distinct cadmium resistance determinants (cadA1,cadA2, andcadA3) among cadmium-resistant isolates. Most BC-resistant isolates were resistant to cadmium as well. Arsenic resistance was encountered primarily in serotype 4b and was an attribute of most isolates of the serotype 4b epidemic clonal group ECIa. Prevalence of the known cadmium resistance determinants was serotype associated:cadA1was more common in isolates of serotypes 1/2a and 1/2b than 4b, whilecadA2was more common in those of serotype 4b. A subset (15/77 [19%]) of the cadmium-resistant isolates lacked the known cadmium resistance determinants. Most of these isolates were of serotype 4b and were also resistant to arsenic, suggesting novel determinants that may confer resistance to both cadmium and arsenic in these serotype 4b strains. The findings may reflect previously unrecognized components of the ecological history of different serotypes and clonal groups ofL. monocytogenes, including exposures to heavy metals and disinfectants.

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Uncertainty exists concerning the key factors contributing to Campylobacter colonization of poultry, especially the possible role of vertical transmission from breeder hens to young birds. A longitudinal study of Campylobacter colonization was performed in two sibling pairs of turkey flocks (four flocks total). Each pair of sibling flocks shared breeder hen populations and was obtained from the same hatchery. One flock of each pair was grown on a commercial farm, and the other was grown in an instructional demonstration unit (Teaching Animal Unit [TAU]). Flocks were located within a 60-mi (96.8-km) radius. The time of placement, feed formulations, stocking density, and general husbandry were the same for both flocks, and each flock was processed at a commercial processing plant following standard feed withdrawal and transport protocols. Both flocks grown on the commercial farms became colonized with Campylobacter between weeks 2 and 3 and remained colonized until processing. Between 80 and 90% of isolates were Campylobacter coli, and the remainder were Campylobacter jejuni. In contrast, neither C. coli nor C. jejuni were isolated from either of the TAU flocks at any time during the production cycle. None of the fla types of Campylobacter from the breeders that provided poults to one of the commercial flocks matched those from the progeny. These results failed to provide evidence for vertical transmission and indicate that this type of transmission either did not occur or was not sufficient to render the TAU turkey flocks Campylobacter positive. Management practices such as proper litter maintenance, controlled traffic between the TAU farm and other turkey flocks, and other less well-defined aspects of turkey production were likely responsible for the absence of Campylobacter in the TAU flocks before harvest.