Listeria innocua Hinders Recovery of Listeria monocytogenes Serotype 4b during Selective Enrichment in Buffered Listeria Enrichment Broth

Introduction: Selective enrichment procedures are typically used to increase the population of Listeria monocytogenes while inhibiting the growth of additional non-pathogenic microorganisms that might be present in the test sample. Enrichment broth formulations rarely demonstrate absolute target organism selectivity. It is possible that when more than one species of Listeria are present in a food sample the non-pathogenic species could be preferentially recovered.

Purpose: In order to gauge the extent of preferential recovery during the selective enrichment procedure, ten isolates each of L. monocytogenes and L. innocua were cultured together under selective conditions for a total of 100 isolate pairings.

Methods: Hemolytic patterns on blood agar plates were used to obtain presumptive 48 hour populations of each species. Species confirmation was performed by conventional PCR. Isolated colonies were also obtained using the streak plating method on a variety of selective and differential media. Ten typical colonies were selected from each plate for species confirmation.

Results: The initial inoculum ranged from 1.3 to 2.4 log CFU/mL with a mean of 2.2 ± 0.3 for L. innocua and ranged from 1.7 to 2.6 log CFU/mL with a mean of 2.1 ± 0.1 for L. monocytogenes. The final population of L. monocytogenes range from 7.0 to 9.0 log CFU/mL with a mean of 7.8 ± 0.6 and for L. innocua ranged from 8.3 to 9.7 with a mean of 9.2 ± 0.2. In 98 of 100 isolate pairings, L. innocua reached higher levels than did L. monocytogenes with a difference ranging from 0.2 to 2.4 logs and a mean log difference of 1.5 ± 0.6. Species confirmation following colony selection from non-differential media confirmed the preferential recovery of L. innocua from 48-hour enrichments.

Significance: The presence of non-pathogenic Listeria species may be indicative of low levels of L. monocytogenes which can go undetected with traditional streak plate procedures. Additional alternative testing should be used to minimize false negative sample reporting.