Combined secondary enrichment of primary enrichment broths increases Listeria detection

The efficacy of combining dual primary enrichment cultures into a single se condary broth was evaluated for detecting Listeria in naturally contaminate d meats and environmental samples obtained from dairy processing plants. A total of 336 samples were tested using University of Vermont modified Liste ria enrichment broth (UVM) and Listeria repair broth containing selective a gents (LRBS) as primary enrichment media. Eighty samples (23.8%) yielded Li steria by at least one method. Neither primary enrichment broth was signifi cantly better (P > 0.05) than the other in identifying Listeria-positive sa mples. UVM media, when used as a primary enrichment broth, identified 66 Li steria-positive samples, while the use of LRBS as a primary enrichment brot h identified 65 Listeria-positive samples. Listeria detection improved sign ificantly (P < 0.01) when two primary enrichment media were used for sample analysis. It is not clear whether this improvement was due to simply repli cating the primary enrichment or to the particular pair of primary enrichme nt media used. The use of a dual secondary enrichment procedure was better (P < 0.05) than the use of either individual primary enrichment medium alon e. The overall rate of recovery increased from 81.3 to 82.5% for single sec ondary enrichment to 93.8% using a dual secondary enrichment technique. Ana lysis of results obtained when combining two independent isolation methods versus combining two primary enrichment media into one single secondary enr ichment 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 t he sensitivity of testing while decreasing the potential workload associate d with an increase in enrichment procedures.