Enhanced Recovery of Listeria from Dairy-Plant Processing Environments through Combined Use of Repair Enrichment and Selective Enrichment/Detection Procedures

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Abstract
The efficacy of using a repair step to increase sensitivity of recovery of injured Listeria from environmental sources in dairy processing plants was investigated. The USDA-FSIS Listeria isolation protocol using UVM-modified Listeria enrichment broth medium University of Vermont (UVM) for primary enrichment was the standard method chosen for comparison. UVM broth was used in conjunction with rapid methods (Organon Teknika and Gene-Trak™), following manufacturer's guidelines. Listeria Repair Broth (LRB) was used as the repair enrichment medium in modified protocols of the standard and rapid procedures. LRB employs a nonselective period (2–5 hours) for repair of injured Listeria prior to selective-agent addition. Of 80 environmental sites positive by any method, UVM and LRB showed similar recovery rates (87.5% and 88.8%, respectively). Thus LRB provided little advantage over current procedures for use in contaminated sites. UVM was superior when used in conjunction with either rapid method. The USDA and modified USDA (mUSDA) procedures gave identical recovery rates (93%), but 10 additional positive sites were attributed to the use of two enrichment broths. The culture method combined with either rapid method from each broth increased the sensitivity to 97.5–98.8% when data from UVM and LRB was combined. False negative rates in the USDA method (7.1%) were attributed to the lack of color change in Fraser secondary broth. Fraser broth also yielded many false positive results (overall 66.2%) making this broth of limited value as a screening tool for highly contaminated samples. In order to optimize methodology for detection of Listeria, suppression of background flora and the recovery of potentially injured Listeria in the processing environment must be addressed. Overgrowth occurring during the nonselective enrichment period was suspected of causing suboptimal sensitivity in LRB; however, the combination of UVM and LRB showed promising recovery rates. Ceftazidime was evaluated against 68 background isolates that survived throughout the various enrichment and detection methods. Inhibition of 57 of the contaminants indicates a potential role for ceftazidime in the LRB selective-agent regime for sites with high microbial background.

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