Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2004
Publication Date: 5/1/2005
Citation: Kenney, S.J., Anderson, G.L., Williams, P.L., Millner P.D., Larry R. Beuchat. 2005. Persistence of Escherichia coli O157:H7, Salmonella Newport, and Salmonella Poona in the gut of a free-living nematode, Caenorhabditis elegans, and transmission to progeny and uninfected nematodes. International Journal of Food Microbiologyl. 101:227-236. Interpretive Summary: Foodborne illness outbreaks have raised interest in identifying pre- and postharvest sources that can contaminate raw and minimally processed fruits and vegetables. Soil that contacts produce may become contaminated with human pathogenic bacteria in a variety of ways: raw or improperly composted manure, contaminated irrigation water, runoff from pastureland, or excreta from wild animals that visit the crop areas. Higher populations of bacteria and nematodes are known to reside in the rhizosphere of plants compared to bulk soil. Free-living nematodes (microscopic worms) such as Caenorhabditis elegans are attracted to areas in soil in which large populations of bacteria are present because they feed on them, including human pathogenic bacteria. In the research reported here, we tested the hypothesis that free-living nematodes may ingest human enteric pathogens present in soil matrices and harbor them in their gut, where they could survive even after the worm dies and produce has be treated with sanitizer. Ingested pathogens may then colonize the gut and be protected against environmental stresses imposed by produce sanitizers, even after the worm has died. Worms were fed cells of E. coli OP50, E. coli O157:H7, and salmonellae then incubated at 4, 20, and 37°C for up to 5 days. Initial populations of ingested pathogens significantly increased nearly 100-fold per worm within 1 day at 20°C and remained constant for an additional 4 days. Results also show that E. coli O157:H7 and salmonellae grew from about 230 per worm to approx. 200,000 per worm and survived at least 5 days after ingestion by C. elegans. Infected worms may come into contact with pre-harvest fruits and vegetables and contaminate their surfaces by excreting pathogens. If worms on the surface of produce become attached to the surface and die, the pathogens in their gut can remain viable for several days. A strong sanitizer, 1% sodium hypochlorite, pH 13, successfully disinfected the worms and their eggs of E. coli and salmonellae Evidence also showed that worms infected with S. enterica serotype Newport could transfer this salmonellae strain to uninfected wild type , and the pathogen persisted in the gut of adult wild type C. elegans two generations removed from exposure to the pathogen. This information will help inform produce packers, scientists, and others interested in determining the mechanisms of pre-harvest produce contamination and issues of concern at post-harvest disinfection.
Technical Abstract: A study was done to determine the persistence of Escherichia coli O157:H7 and Salmonella enterica in the gut of a free-living nematode, Caenorhabditis elegans, as affected by temperature and relative humidity and to determine if infected worms transmit Salmonella enterica serotype Newport to progeny and uninfected worms. Worms were fed cells of E. coli OP50, E. coli O157:H7, and salmonellae followed by incubating at 4, 20, and 37°C for up to 5 days. Initial populations of ingested pathogens significantly increased by up to 2.93 log10 cfu/worm within 1 day at 20°C on K agar and remained constant for an additional 4 days. When plated on Bacto agar, populations of pathogens remain constant at 4°C, decreased significantly at 20°C, and increased significantly at 37°C within 3 days. Worms fed E. coli OP50 or S. enterica serotype Newport were also incubated at 4 or 20°C at relative humidities of 33, 75 or 98% for 24 h. Populations of ingested E. coli OP50 and S. enterica serotype Newport decreased by up to 1.65 and 3.44 log10 cfu/worm, respectively, in worms incubated at 20°C and 33% relative humidity for 24 h. Placement of adult worms labeled with green fluorescent protein (gfp) that had ingested gfp-labeled S. enterica serotype Newport on K agar with uninfected wild type worms resulted in transfer of the pathogen to gut of wild type worms. S. enterica serotype Newport was isolated from adult C. elegans two generations removed from exposure to the pathogen.