Purpose: This study evaluated the equivalence of reducing media and/or enrichment time to a widely used modified FDA Bacteriological Analysis Manual (BAM) method.
Methods: Environmental samples (n=2634) were collected from food, non-food, and transfer point contact surfaces from retail delis. Each sample was tested for L. monocytogenes and L. spp. using a modified BAM protocol. The results were recorded for two enrichment periods (24 h, 48 h) plated concurrently on modified Oxford agar (MOX) and Listeria monocytogenes Plating Medium (LMPM) incubated 48 h. Outcomes from reduced enrichment time and single media type were compared to responses from the complete modified BAM method; 90% confidence intervals (α=0.10) were constructed using two-tailed asymptotic tests to evaluate equivalence at ±1.0%, ±0.5%, and ±0.1% prevalence.
Results: To detect L. spp., equivalence at ±1% prevalence was achieved plating 24 h enrichments on both media and 24 h and 48 h enrichments plated on MOX. No abbreviated methods were equivalent at ± 0.5% prevalence for L. spp. To detect L. monocytogenes, plating 24 h and 48 h enrichments to LMPM was equivalent at ±0.5% (α=0.10). At ±0.1% prevalence, truncated methods lost equivalence due to increases in false positives. MOX did not significantly contribute to detection of L. monocytogenes when LMPM was used.
Significance: Truncated methods to isolate L. monocytogenes or L. spp. reduce costs and/or time to results and may be an appropriate option for environmental monitoring plans if increased false positives are more tolerable.