Prevalence of Salmonellae, Listeria monocytogenes, and Fecal Coliforms in Bulk Tank Milk on US Dairies

Abstract

The objective of this study was to determine the prevalence of Salmonella, Listeria monocytogenes, and fecal coliforms in bulk tank milk in the United States. As part of the NAHMS Dairy 2002 survey, 861 bulk tank milk samples were collected from farms in 21 states. Milk was directly plated on selective agars for direct bacterial enumeration and was enriched in selective broths to increase detection sensitivity. Somatic cell counts (SCC) and standard plate counts (SPC) were also determined. Coliforms were detected in 95% (818 of 860) of the samples, and the average SCC was 295,000 cells/mL. Twenty-two samples (2.6%) were culture-positive for Salmonella, and 9 serotypes were identified: Montevideo (n = 7), Newport (n = 4), Muenster (n = 2), Meleagridis (n = 2), Cerro (n = 2), 44:Z36 (Z38) (n = 2), Dublin (n = 1), Anatum (n = 1), and 9, 12:nonmo-tile (n = 1). Listeria monocytogenes was isolated from 56 (6.5%) samples, and serotyping of these isolates yielded 5 serotypes (1/2a, 1/2b, 3b, 4b, and 4c). Of the L. monocytogenes isolates, 93% were serotypes 1/2a, 1/2b, and 4b, the most common human clinical isolates. Regional differences in L. monocytogenes and Salmonella prevalence were observed, but more studies are needed to determine the validity of these differences. There were no apparent relationships between SCC or SPC and incidence of Salmonella or L. monocytogenes. Although the prevalence of L. monocytogenes and Salmonella was low, these pathogens represent a potential risk to consumers of raw milk and raw milk products.