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.

Keywords: Foodborne pathogen; Listeria; Listeria monocytogenes; Microbial competition; Regulatory microbiology; Selective enrichment.

Figure 1

Mean populations for 10 strains of Listeria following competitive growth with C. braakii in four selective enrichment media. Error bars reflect the standard error associated with three replications.

Figure 2

Incubation temperature effects on the mean Listeria population following competitive growth with C. braakii in four selective enrichment media. Lowercase letters at the base of the bars denote statistical significance at the 95% probability level as determined by Tukey's mean comparison test. Error bars reflect the standard deviation associated with 10 Listeria strains and three replications (n = 30). Means are presented in ascending order.

Figure 3

Comparison of selective conditions on the mean populations of Listeria following competitive growth with C. braakii. Nonselective basal formulations (A); basal formulations supplemented with sodium nalidixate only (B), acriflavine-HCl only (C), NaCl only (D), LiCl only (E), and cycloheximide only (F); and fully selective enrichment formulations (G). Incubation times were all 48 h. Incubation temperatures were 30°C for BLEB and its selective derivatives, and for basal UVM-1 broth and its selective derivatives. An incubation temperature of 35°C was used for basal Fraser broth and its selective derivatives. Lowercase letters at the base of the bars denote statistical significance at the 95% probability level as determined by Tukey's mean comparison test. Error bars reflect the standard deviation of nine Listeria strains and three replications (n = 27). One strain, Ls003, was omitted from the analysis because it could not be recovered from the UVM-1 or Fraser broths. Open bars, BLEB and BLEB selective derivatives; gray bars, UVM-1 broth and UVM-1 broth selective derivatives; dotted bars, Fraser broth and Fraser broth derivatives.