Salmonella and Escherichia coli O157:H7 survival in soil and translocation into leeks (Allium porrum) as influenced by an arbuscular mycorrhizal fungus (Glomus intraradices)

Abstract

A study was conducted to determine the influence of arbuscular mycorrhizal (AM) fungi on Salmonella and enterohemorrhagic Escherichia coli O157:H7 (EHEC) in autoclaved soil and translocation into leek plants. Six-week-old leek plants (with [Myc+] or without [Myc-] AM fungi) were inoculated with composite suspensions of Salmonella or EHEC at ca. 8.2 log CFU/plant into soil. Soil, root, and shoot samples were analyzed for pathogens on days 1, 8, 15, and 22 postinoculation. Initial populations (day 1) were ca. 3.1 and 2.1 log CFU/root, ca. 2.0 and 1.5 log CFU/shoot, and ca. 5.5 and 5.1 CFU/g of soil for Salmonella and EHEC, respectively. Enrichments indicated that at days 8 and 22, only 31% of root samples were positive for EHEC, versus 73% positive for Salmonella. The mean Salmonella level in soil was 3.4 log CFU/g at day 22, while EHEC populations dropped to ≤ 0.75 log CFU/g by day 15. Overall, Salmonella survived in a greater number of shoot, root, and soil samples, compared with the survival of EHEC. EHEC was not present in Myc- shoots after day 8 (0/16 samples positive); however, EHEC persisted in higher numbers (P = 0.05) in Myc+ shoots (4/16 positive) at days 15 and 22. Salmonella, likewise, survived in statistically higher numbers of Myc+ shoot samples (8/8) at day 8, compared with survival in Myc- shoots (i.e., only 4/8). These results suggest that AM fungi may potentially enhance the survival of E. coli O157:H7 and Salmonella in the stems of growing leek plants.

Fig 1

Micrograph of an uncolonized (A) and an AM fungus-colonized (B) leek root after clearing in 10% (wt/vol) KOH and staining with trypan blue. The micrograph was taken at the microscopy facility of the USDA ARS Eastern Regional Research Center. The arrow denotes the location of trypan blue-stained fungal tissue in the root cortex.

Fig 2

Survival of E. coli O157:H7 and Salmonella enterica in leek (Allium porrum) shoots as influenced by the presence of mycorrhizal fungi (Glomus intraradices) for up to 22 days. Predicted regression curves, based on the exponential decay model with asymptote, represent Salmonella MYC+ (filled squares), Salmonella Myc− (open squares), E. coli MYC+ (closed circles), and E. coli MYC+ (open circles) strains. Nonlinear regression curves represent predicted values. Data points represent the means (log CFU) of the results of three independent replicate experiments with 2 or 3 observations per time point. Duplicate samples were analyzed for repetition 1 and triplicate samples for replicates 2 and 3.

Fig 3

Survival of E. coli O157:H7 and Salmonella enterica in leek (Allium porrum) roots as influenced by the presence of mycorrhizal fungi (Glomus intraradices) for up to 22 days. Predicted regression curves, based on the exponential decay model with asymptote, represent Salmonella MYC+ (filled squares), Salmonella Myc− (open squares), E. coli MYC+ (closed circles), and E. coli MYC+ (open circles) strains. Nonlinear regression curves represent predicted values. Data points represent the means (log CFU) of the results of three independent replicate experiments with 2 or 3 observations per time point. Duplicate samples were analyzed for repetition 1 and triplicate samples for replicates 2 and 3.

Fig 4

Survival of E. coli O157:H7 and Salmonella enterica in soil as influenced by the presence of mycorrhizal fungi (Glomus intraradices) for up to 22 days. Predicted regression curves, based on the exponential decay model with asymptote, represent Salmonella MYC+ (filled squares), Salmonella Myc− (open squares), E. coli MYC+ (closed circles), and E. coli MYC+ (open circles) strains. Nonlinear regression curves represent predicted values. Data points represent the means (log CFU) of the results of two independent replicate experiments with 3 observations per time point. Duplicate samples were analyzed for repetition 1 and triplicate samples for replicates 2 and 3.