Evaluation of sterility testing procedures for laboratory animal rodent diets

The United States Food and Drug Administration (FDA) and the United States Department of Agriculture (USDA) recommend grinding or stomaching food for human consumption prior to Salmonella testing. The Quality Assurance Laboratory (QAL) at the National Institute of Environmental Health Sciences (NIEHS) routinely tests all incoming, unsterilized rodent feed for Salmonella spp. and total microbial load prior to use. Currently, the QAL cultures intact feed pellets in sterile Thioglycolate enrichment broth that sits stationary in the incubator for seven days. Under these conditions, a natural ingredient pelleted feed does not break down thoroughly possibly preventing detection of viable microbes at the pellet center. The purpose of this study was to determine if grinding pelleted rodent feed improves the sensitivity of microbial detection. Our study compared bacterial growth from an unautoclaved and autoclaved, natural ingredient rodent diet (NIH-31) processed as intact pellets or ground feed. Our results indicate that grinding unautoclaved feed for total aerobic bacteria testing does increase the sensitivity of microbial detection. However, when the feed is appropriately sterilized, grinding the feed provides no added benefit.