Association of Listeria spp. Contamination in the Dairy Processing Plant Environment with the Presence of Staphylococci

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Abstract
Monitoring the dairy processing plant environment for Listeria spp. has become common practice for determining sanitation effectiveness. Improved effectiveness and efficiency of sanitation programs could be achieved through the use of an indicator test. The objective of this research was to select a suitable indicator for the presence of Listeria spp. in the dairy processing plant and to develop criteria to guide its use. Microbial groups selected for study included: total aerobes, anaerobes, acid producers, salt tolerant aerobes, Enterobacteriaceae, lactobacilli, staphylococci, enterococci, gram-negative bacteria, and yeast and mold. Environmental swabs from 15 dairy plants were analyzed for these microbial groups and the presence of Listeria spp. Staphylococci were determined to be the best indicator group by using discriminant analysis selection. A discriminant model based on staphylococcus data from 409 environmental swabs grouped 85% of the 47 Listeria spp.-containing swabs correctly using 6,600 CFU staphylococci as the break point. Of the 362 swabs not contaminated with Listeria spp. 63% were correctly grouped. Of those swabs classified as negative using the 6,600 CFU criterion, 97% were confirmed negative. Of those swabs classified as positive, 23% were confirmed positive. Swabs classified as positive by the indicator test had a 7.7 times greater risk of listeria contamination than swabs classified as negative.

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