The distribution of Listeria in pasture-raised broiler farm soils is potentially related to University of Vermont medium enrichment bias toward Listeria innocua over Listeria monocytogenes.

Authors: LOCATELLI, AUDE - Oak Ridge Institute For Science And Education (ORISE); Lewis, Micah; Rothrock, Michael

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2017
Publication Date: 12/21/2017
Citation: Locatelli, A., Lewis, M.A., Rothrock Jr, M.J. 2017. The distribution of Listeria in pasture-raised broiler farm soils is potentially related to University of Vermont medium enrichment bias toward Listeria innocua over Listeria monocytogenes. Frontiers in Veterinary Science. 4:227. 10.3389/fvets.2017.00227.
DOI: https://doi.org/10.3389/fvets.2017.00227

Interpretive Summary: The occurrence of Listeria monocytogenes (LM) has been widely investigated in the poultry production chain from the processing plant to the final product. However, limited data are available on Listeria spp., including LM, in the poultry farm environment. Therefore, fecal and soil samples from 37 pastured poultry flocks from ten all-natural farms over three years were assessed to determine the prevalence and diversity of Listeria within these alternative poultry farm environments using standard cultural and molecular methods. Listeria spp. was isolated in 15% of poultry farm samples and included Listeria innocua (LI) (65.7%), LM (17.4%) and L. welshimeri (LW) (15.1%). Additional multiplex PCR serotyping showed group 1/2a-3a to be the most dominant LM serovar group. Based on these results, monoculture growth experiments were conducted on four representative Listeria soil isolates (3 LM, 1 LI) to determine if enrichment media (Tripticase Soy Broth (TSB) and University of Vermont modified Listeria enrichment broth (UVM)), initial concentration (102 or 105 cells per ml), or incubation temperature (20, 30, 42°C) differentially effected these two Listeria species. Overall, no significant growth differences were observed between the growths of the three LM isolates (representing the 3 recovered serovar groups) under any of the growth conditions tested. Alternatively, at 30°C in UVM with the lower initial concentration, the LI isolate had a significantly shorter lag phase than the LM isolates, which was also demonstrated in LI:LM co-culture growth studies under these same incubation conditions. Co-cultures in TSB did not show preferential growth of LI over LM under similar incubation conditions. These results indicate that the use of UVM as an enrichment media may preferentially allow LI to outcompete LM when at low concentrations, biasing the Listeria prevalence from these farm samples towards LI and potentially underreporting the presence of LM in these environments.

Technical Abstract: The occurrence of Listeria monocytogenes (LM) has been widely investigated in the poultry production chain from the processing plant to the final product. However, limited data are available on Listeria spp., including LM, in the poultry farm environment. Therefore, fecal and soil samples from 37 pastured poultry flocks from ten all-natural farms over three years were assessed to determine the prevalence and diversity of Listeria within these alternative poultry farm environments using standard cultural and molecular methods. Listeria spp. was isolated in 15% of poultry farm samples and included Listeria innocua (LI) (65.7%), LM (17.4%) and L. welshimeri (LW) (15.1%). Additional multiplex PCR serotyping showed group 1/2a-3a to be the most dominant LM serovar group. Based on these results, monoculture growth experiments were conducted on four representative Listeria soil isolates (3 LM, 1 LI) to determine if enrichment media (Tripticase Soy Broth (TSB) and University of Vermont modified Listeria enrichment broth (UVM)), initial concentration (102 or 105 cells per ml), or incubation temperature (20, 30, 42°C) differentially effected these two Listeria species. Overall, no significant growth differences were observed between the growths of the three LM isolates (representing the 3 recovered serovar groups) under any of the growth conditions tested. Alternatively, at 30°C in UVM with the lower initial concentration, the LI isolate had a significantly shorter lag phase than the LM isolates, which was also demonstrated in LI:LM co-culture growth studies under these same incubation conditions. Co-cultures in TSB did not show preferential growth of LI over LM under similar incubation conditions. These results indicate that the use of UVM as an enrichment media may preferentially allow LI to outcompete LM when at low concentrations, biasing the Listeria prevalence from these farm samples towards LI and potentially underreporting the presence of LM in these environments.