Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2012
Publication Date: 1/11/2013
Citation: Gurtler, J., Douds, D.D., Dirks, B.A., Quinlan, J.J., Nicholson, A., Phillips, J.G., Niemira, B.A. 2013. Survival of Salmonella and E.coli O157:H7 in soil and translocation into leek (allium porrum) as influenced by mycorrhizal fungi. Applied and Environmental Microbiology. 79:1813-1820. Interpretive Summary: Arbuscular mycorrhizal fungi (AMF) are beneficial molds that naturally occur in soil and form healthful relationships with most farm crops. AMF improves plant nutrition and growth and increases resistance to drought and disease. A study was conducted to determine if AMF affects the absorption of human pathogens such as Salmonella and E. coli O157:H7 into crops and if it improves the survival of pathogens in soil. The soil surrounding leek plants was colonized by AMF and the soil was then inoculated with either Salmonella or E. coli O157:H7. Plants were held and sampled over 22 days. Results showed that Salmonella survived better in plant tissues and soil than did E. coli 0157:H7, regardless of the presence of AMF. Between days 1 and 15, Salmonella and E. coli O157:H7 populations decreased more rapidly in leeks without AMF than in leeks with AMF. On days 15 and 22, only 10 of 24 samples were positive for Salmonella in AMF-negative leeks, versus 15 of 24 positive samples in AMF-positive leeks, suggesting that AMF may enhance pathogen survival. Further studies should address the influence of AMF on Salmonella and E. coli 0157:H7in other fresh produce commodities.
Technical Abstract: A study was conducted to determine the influence of arbuscular mycorrhizal (AM) fungi on survival of Salmonella and E. coli O157:H7 (EHEC) in soil and translocation into leek roots and shoot. AM fungi are naturally-occurring soil symbionts that form mutualistic relationships with most crop plants. Six-week-old leek plants (with [Myc+] or without [Myc-] AM fungi [Glomus intraradices]) were used as a model. Plants were inoculated with composite suspensions of either Salmonella or EHEC at ca. 8.2 log CFU/plant. Soil, root, and shoot samples were analyzed for pathogen presence (by enrichment) and populations (direct plating) on days 1, 8, 15 and 22, following inoculation. Initial populations (day 1) were ca. 3.1 and 2.1 log CFU/root and ca. 2.0 and 1.5 log CFU/shoot for Salmonella and EHEC, respectively. Day 1 soil populations were ca. 5.5 and 5.1 CFU/g for Salmonella and EHEC, respectively. Presence/absence results indicated that at days 8 and 22, only 31% of root samples were positive for EHEC, versus 73 percent positive for Salmonella. Overall mean Salmonella levels in soil (Myc+ and Myc- samples) were 3.4 log CFU/g at day 22, while EHEC populations dropped to less than or equal to 0.75 log CFU/g by day 15. Although populations did not change between days 1 and 15 in Myc+ shoots, pathogen numbers significantly declined (P is less than 0.05) in the Myc- shoots during the same period, suggesting that AM fungi may potentially enhance bacterial survival. EHEC was not detected by enrichment in Myc- shoots after day 8, but persisted in Myc+ shoots up to day 22; and, at the last two sampling times, only 10 of 24 samples were positive for Salmonella in Myc- shoots, while 15 of 24 Myc+ shoots were Salmonella-positive, again suggesting that AM fungi may potentially enhance pathogen survival. Further studies should address the influence of AM fungi on Salmonella and EHEC in other fresh produce commodities.