Comparison of the incidence of Listeria on equipment versus environmental sites within dairy processing plants.

This study was undertaken to compare the incidence of Listeria contamination of processing equipment with that of the general dairy processing environment. A total of 378 sponge samples obtained from 21 dairy plants were analyzed for Listeria using three different enrichment media. Use of extended microbiological analysis allowed us to identify 26 Listeria positive sites which would have not been identified had a single test format been employed. Eighty (80) of 378 sites (21.2%) were identified as Listeria positive. Listeria innocua was isolated from 59 of the 80 (73.8%) positive samples, L. monocytogenes was identified in 35 (43.8%) of the positive samples, and L. seeligeri was isolated from 5 (6.3%) of the Listeria positive samples. Positive equipment samples were obtained from 6 of the 21 (28.6%) plants and 19 of the 21 (90.5%) plants had positive environmental sites. Seventeen of the 215 (7.9%) samples from equipment were positive for Listeria species. Eleven of these sites, including 3 holding tanks, 2 table tops, 3 conveyor/chain systems, a pasta filata wheel, a pint milk filler and a brine pre-filter machine, were positive for L. monocytogenes. Nineteen of the 21 (90.5%) plants had positive environmental sites. Sixty-three of the 163 (41.1%) samples from environmental sites were Listeria positive and 24 were positive for L. monocytogenes. Two-tailed student t-test analysis of the mean frequencies indicated that the level of contamination was significantly higher (p < 0.001) in 'environmental' (49.7%) as opposed to 'equipment' samples (7.0%). Our study indicates that environmental contamination with Listeria does not necessarily translate into contamination of equipment within the same plant, and that greater emphasis needs to be placed on the cleaning and sanitizing of the plant environment.