Occurrence of Salmonella-specific bacteriophages in swine feces collected from commercial farms

Salmonella is one of the leading causes of human foodborne illness and is associated with swine production. Bacteriophages are naturally occurring viruses that prey on bacteria and have been suggested as a potential intervention strategy to reduce Salmonella levels in food animals on the farm and in the lairage period. If phages are to be used to improve food safety, then we must understand the incidence and natural ecology of both phages and their hosts in the intestinal environment. This study investigates the incidence of phages that are active against Salmonella spp. in the feces of commercial finishing swine. Fecal samples (n¼60) were collected from each of 10 commercial swine finishing operations. Samples were collected from 10 randomly selected pens throughout each operation; a total of 600 fecal samples were collected. Salmonella spp. were found in 7.3% (44=600) of the fecal samples. Bacteriophages were isolated from fecal samples through two parallel methods: (1) initial enrichment in Salmonella Typhimurium; (2) initial enrichment in Escherichia coli B (an indicator strain), followed by direct spot testing against Salmonella Typhimurium. Bacteriophages active against Salmonella Typhimurium were isolated from 1% (6=600) of the individual fecal samples when initially enriched in Salmonella Typhimurium, but E. coli B-killing phages were isolated from 48.3% (290=600) of the fecal samples and only two of these phages infected Salmonella Typhimurium on secondary plating. Collectively, our results indicate that bacteriophages are widespread in commercial swine, but those capable of killing Salmonella Typhimurium may be present at relatively low population levels. These results indicate that phages (predator) populations may vary along with Salmonella (prey) populations; and that phages could potentially be used as a food safety pathogen reduction strategy in swine.