Combined secondary enrichment of primary enrichment broths increases Listeria detection

The efficacy of combining dual primary enrichment cultures into a single secondary broth was evaluated for detecting Listeria in naturally contaminated meats and environmental samples obtained from dairy processing plants. A total of 336 samples were tested using University of Vermont modified Listeria enrichment broth (UVM) and Listeria repair broth containing selective agents (LRBS) as primary enrichment media. Eighty samples (23.8%) yielded Listeria by at least one method. Neither primary enrichment broth was significantly better (P>0.05) than the other in identifying Listeria-positive samples. UVM media, when used as a primary enrichment broth, identified 66 Listeria-positive samples, while the use of LRBS as a primary enrichment broth identified 65 Listeria-positive samples. Listeria detection improved significantly (P<0.01) when two primary enrichment media were used for sample analysis. It is not clear whether this improvement was due to simply replicating the primary enrichment or to the particular pair of primary enrichment media used. The use of a dual secondary enrichment procedure was better (P<0.05) than the use of either individual primary enrichment medium alone. The overall rate of recovery increased from 81.3 to 82.5% for single secondary enrichment to 93.8% using a dual secondary enrichment technique. Analysis of results obtained when combining two independent isolation methods versus combining two primary enrichment media into one single secondary enrichment broth indicated that there was no significant difference (P>0.05) in either procedure. Inoculum size (0.1 ml versus 0.2 ml) did not have an effect on the overall rate of recovery. The procedure developed increased the sensitivity of testing while decreasing the potential workload associated with an increase in enrichment procedures.