Purpose: The purpose of this study was to evaluate the effects of L. welshimeri and L. seeligeri on subsequent isolation of L. monocytogenes from buffered Listeria enrichment broth (BLEB).
Methods: 2% UHT milk was doubly spiked with L. monocytogenes and L. welshimeri (or L. seeligeri). Selective enrichments using BLEB were performed. Population differentials at 48h were determined based on qPCR enumeration. Select strain pairings were then used for additional spike-recovery studies in four distinct food matrices using Oxford agar as the recovery medium with confirmation of 10 colonies per enrichment.
Results: Population differentials as much as 2.7 ± 0.1 and 3.7 ± 0.2 logs were observed for L. monocytogenes paired with L. seeligeri and L. welshimeri, respectively. In all pairings, L. monocytogenes was the less predominant species. Population differentials of 2.7 and 3.7 logs would require confirmation of approximately 500 and 5000 colonies respectively for L. monocytogenes recovery. The presence of L. seeligeri had minimal effects while L. welshimeri completely masked the presence of L. monocytogenes in spiked guacamole, crab meat, and broccoli. No clear relationship between the predominant species and generation time or ability to produce an inhibitory activity was observed.
Significance: The presence of L. welshimeri or L. seeligeri during selective enrichment using BLEB can hinder recovery of L. monocytogenes on non-differentiating media such as Oxford agar. Additionally, the large population differentials suggest that in some cases isolation of L. monocytogenes would be difficult even using a species-specific chromogenic agar.