Contribution of Selective Conditions to Microbial Competition in Four Listeria Selective Enrichment Formulations

Abstract

Microbial competition during selective enrichment negatively affects Listeria monocytogenes populations and may hinder the subsequent detection or recovery of this organism. Competition assays among 10 selected strains of Listeria and Citrobacter braakii were performed in buffered Listeria enrichment broth, 3-(N-morpholino)propanesulfonic acid–buffered Listeria enrichment broth, University of Vermont medium–modified Listeria enrichment broth, and Fraser broth. The individual contributions of each selective agent in these media were also assessed, as well as the contribution of incubation temperature. Acriflavine hydrochloride and sodium nalidixate were ineffective at preventing the overgrowth of C. braakii; this resulted in substantially lower populations of Listeria than when the competitor was absent. At the higher levels, both of these selective agents were detrimental to Listeria populations. The highest enrichment populations of Listeria were observed when either NaCl or LiCl was present. In the absence of selective agents, the final populations of Listeria following competitive growth with C. braakii were not substantially affected by temperature; however, in the presence of selective agents, the Listeria populations were statistically higher at the higher incubation temperature. There are a limited number of selective agents available for use in Listeria-specific enrichment media, resulting in formulations that are only somewhat selective for this species. The optimization of current formulations may help researchers to improve Listeria recovery, particularly from products with a high microbial load. The understanding of the behavior and interactions between target and nontarget microorganisms in the presence of these available selective agents is a necessary step in the optimization of Listeria selective enrichment formulations.

Assessment of Reference Method Selective Broth and Plating Media with 19 Listeria Species Highlights the Importance of Including Diverse Species in Listeria Method Evaluations

“…L. seeligeri detection with the USDA method also may be challenging; our data indicated limited growth in the primary UVM enrichment culture. These data for L. seeligeri are consistent with those of three other studies, which indicated that L. seeligeri may not be recovered after enrichment in UVM or Fraser (31) and may not be detected on ALOA (68) or BBL CHROMagar Listeria (another selective and differential agar targeting PI-PLC activity) (24). We located only one study in which growth of L. ivanovii was evaluated in Fraser, and L. ivanovii did not grow after 24 h in this medium (59).…”

Soil Collected from a Single Great Smoky Mountains Trail Contains a Diversity of Listeria monocytogenes andListeriaspp.

“…In the Liao et al study, the distribution areas of phylogroups L1 ( L. monocytogenes lineage III), L3 ( L. monocytogenes lineage I), L6 ( L. innocua ), and L8 ( L. welshimeri ) contained the GSMNP region, and L3 and L6 were actually isolated from the GSMNP region ( 15 ); however, none of these phylogroups was isolated in the present study, which may indicate that they were not present in the samples. Alternatively, these phylogroups may have been present in the samples but not recovered; for example, they could have been suppressed or outcompeted by other strains during the enrichment step ( 47 – 53 ), or their colonies may have simply not been selected off the modified Oxford agar plates for isolation and characterization.…”

The Saprophytic Lifestyle of Listeria monocytogenes and Entry Into the Food-Processing Environment

“…No serotype nor lineage bias was observed in buffered Listeria enrichment broth (BLEB) in a study by Gorski et al (2006). Keys et al (2016) obtained the highest Listeria populations in selective enrichments by adding sodium chloride and lithium chloride at higher temperatures (35°C versus 30°C). The authors pointed out the limited number of selective substances for use in Listeria-specific enrichment media and thus resulting in only limited selective formulations.…”

“…The selective enrichment step is further complicated in challenging environments such as soils, where bacterial competition may play a role and lead to the loss of L. monocytogenes and other Listeria species ( Keys et al, 2016 ; Zilelidou et al, 2016 ). It has even been observed that certain L. monocytogenes genotypes outcompete each other: In selective enrichment UVM, lineage I strains are displaced by lineage II strains ( Etty et al, 2019 ).…”