Genetic diversity of Listeria monocytogenes strains from a high-prevalence dairy farm

Listeria monocytogenes is a significant food-borne human and veterinary pathogen. Contaminated silage commonly leads to disease in livestock, but the pervasive nature of the bacterium can make it difficult to identify the source of infection. An investigation of bovine listeriosis that occurred on a Pacific Northwest dairy farm ("farm A") revealed that the clinical strain was closely related to fecal strains from asymptomatic cows, and that farm environment was heavily contaminated with a diversity of L. monocytogenes strains. In addition, the farm A clinical strain was closely related to clinical and environmental strains obtained 1 year prior from a second Northwest dairy farm ("farm B"). To investigate the source(s) of contamination on farm A, environmental samples were collected from farm A at two time points. Pulsed-field gel electrophoresis characterization of 538 isolates obtained from that farm identified 57 different AscI pulsovars. Fecal isolates obtained from individual cows were the most genetically diverse, with up to 94% of fecal samples containing more than one pulsovar. The maximum numbers of pulsovars and serotypes isolated from a fecal sample of one cow were 6 and 4, respectively. Serotype 1/2a was isolated most frequently at both time points. Microarray genotyping of bovine listeriosis, fecal, and silage strains from both farms identified four probes that differentiated listeriosis strains from environmental strains; however, no probe was common to both bovine listeriosis strains.