Non-Pathogenic Microorganisms Complicate Recovery of Listeria monocytogenes from Buffered Listeria Enrichment Broth

Introduction: The recovery of Listeria monocytogenes from buffered Listeria enrichment broth (BLEB) can be complicated by non-pathogenic Listeria species in the test sample which typically reach higher populations. Different species of Listeria demonstrate variation in their tolerance to background microorganisms during selective enrichment further increasing the inter-species post-enrichment population differentials and further hindering L. monocytogenes recovery.

Purpose:  This study evaluates the effects of Citrobacter braakii on the recovery of L. monocytogenes when additional species of Listeria are present in the test sample.

Methods: The effects of C. braakii on the final populations of L. monocytogenes (n = 100), Listeria innocua (n = 55), L. seeligeri (n = 14), and L. welshimeri (n = 31) were determined. Select pathogenic and non-pathogenic Listeria strains were paired based on sensitivity to C. braakii growth and the pairs were used to inoculate BLEB along with C. braakii. The population differentials between the Listeria pairings were determined. These same Listeria pairs were co-inoculated along with C. braakii into cheese, guacamole, and crab meat and L. monocytogenes recovery using Oxford and Rapid’L.mono™ chromogenic agars was evaluated. 

Results: The enrichment populations of individual strains of Listeria when grown in the presence of C. braakii ranged from 1.8 to 8.0 (mean = 7.0 ± 1.1) log CFU/ml for L. monocytogenes, 3.7 to 8.2 (mean = 7.5 ± 0.9) log CFU/ml for L. innocua, 1.3 to 7.8 (mean = 6.7 ± 1.6) log CFU/ml for L. seeligeri, and 1.3 to 8.1 (mean = 7.6 ± 1.2) log CFU/ml for L. welshimeri. The population differentials between paired species of Listeria were too great for recovery of L. monocytogenes from spiked food samples when using either recovery medium.

Significance: The presence of background microorganisms can further increase the population differentials between competing strains further increasing the difficulty of recovering L. monocytogenes even when using a species differentiating medium.