Abstract
The occurrence of Listeria monocytogenes has been widely investigated in the poultry production chain from the processing plant to the final product. However, limited data are available on Listeria species, including Listeria monocytogenes, in the poultry farm environment. Therefore, fecal and soil samples from 37 pastured poultry flocks from 10 all-natural farms over 3 years were assessed to determine the prevalence and diversity of Listeria within these alternative poultry farm environments using standard cultural and molecular methods. Listeria species were isolated in 15% of poultry farm samples and included Listeria innocua (65.7%), L. monocytogenes (17.4%), and Listeria welshimeri (15.1%). Additional multiplex PCR serotyping showed group 1/2a-3a to be the most dominant L. monocytogenes serovar group. Based on these results, monoculture growth experiments were conducted on four Listeria soil isolates (three L. monocytogenes isolates representing the three recovered serovar groups and one L. innocua isolate) to determine if culture medium [tripticase soy broth (TSB) and University of Vermont modified Listeria enrichment broth (UVM)], inoculum concentration (102 or 105 CFU/ml), or incubation temperature (20, 30, and 42°C) differentially affected these Listeria species. Overall, very few significant growth differences were observed between the behavior of the three L. monocytogenes isolates (representing the three recovered serovar groups) under the growth conditions tested. Alternatively, at 30°C in UVM with the lower inoculum concentration, the L. innocua isolate had a significantly shorter lag phase than the L. monocytogenes isolates. In coculture growth studies under these same incubation conditions, the lag phase of L. innocua and L. monocytogenes was similar, but the final concentration of L. innocua was significantly higher than L. monocytogenes. However, cocultures in UVM for high inoculum concentration did not show preferential growth of L. innocua over L. monocytogenes. These results indicate that the use of UVM as an enrichment medium may preferentially allow L. innocua to outcompete L. monocytogenes at low concentrations, biasing the Listeria prevalence from these farm samples toward L. innocua and potentially underreporting the presence of L. monocytogenes in these environments.
Highlights
The genus Listeria is currently comprised of 17 species, including 11 Listeria species described since 2009 [1]
The overall prevalence of L. monocytogenes on these 10 farms was low compared with other grow-out farm environments where 0–46.2% of the environmental and feces samples were L. monocytogenes positive [45], but the distribution of the serotypes was consistent with other studies that have characterized L. monocytogenes serotypes in broiler flocks [26, 41, 43]
Using the cultural conditions for the initial enrichment step for the USDA-Food Safety Inspection Service (FSIS) Microbiology Laboratory Guide (MLG) 8.10 L. monocytogenes enrichment method (UVM, 30°C) we found that L. innocua exhibited a significantly shorter lag time than the L. monocytogenes strains in monocultures, especially for low initial inoculum concentrations (102 CFU/ml)
Summary
The genus Listeria is currently comprised of 17 species, including 11 Listeria species described since 2009 [1]. The genus Listeria sensu stricto includes six species: Listeria innocua, Listeria ivanovii, Listeria grayii, Listeria monocytogenes, Listeria seeligeri, and Listeria welshimeri. These species are well documented and are known to be commonly found in different environments throughout the world [2,3,4,5,6]. Among all the Listeria species, L. monocytogenes is recognized as one of the most important foodborne pathogens in many industrialized countries This pathogen is responsible for listeriosis, a potentially fatal disease that may lead to abortion or serious cases of meningitis, encephalitis, and septicemia [7, 8]. In 2015 in the United States, L. monocytogenes was responsible for an estimated 116 cases of listeriosis, 111 hospitalizations, and 15 deaths [12]
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