Glucosinolates-Derived Compounds as a Green Manure for Controlling Escherichia coli O157:H7 and Salmonella in Soil
Environmental Microbial and Food Safety Laboratory
2012 Annual Report
1a.Objectives (from AD-416):
ARS is interested in the role of pre-harvest sources of contaminations to fresh produce. The Cooperator is interested in improving the microbial safety of fresh produce.
1b.Approach (from AD-416):
Soil (soil amendments, compost) and irrigation water are major sources of pre-harvest contamination of fresh produce. We will utilize plant natural defense mechanism (glucosinolate-derived compounds) as a green manure to control attenuated strains of enteric pathogens in soil. Brassica plants will be grown in hoop houses. After crop harvest, remnant plant tissues will be tilled over as a green manure. Soil inoculated with attenuated strains will be sampled periodically for their persistence in presence of green manure.
Antibacterial activity of GDC, mainly, isothiocyanates, indole methyl oxazolidinone, and methyl propyl pyrazole carboxylic acid were evaluated against E. coli O157:H7 and its mutants deficient in surface appendages, nonpathogenic E. coli, and Salmonella. Sterile filter disc (6 mm) impregnated with 10-20 µL of GDC (10 mg/ml) were placed on Tryptic soy agar (TSA) plates seeded with respective bacteria. After 48 h incubation at 37°C, the diameter (mm) of the inhibitory zone around the disc was recorded. Antibacterial effect of GDC was dose-dependent, increasing with the dose applied. Salmonella were more sensitive to these compounds than E. coli O157:H7 or non-pathogenic E. coli. Benzyl isothiocyanate exhibited significantly higher zone of inhibition than other compounds or gentamicin (positive control) against Salmonella strains. The antibacterial effects of benzyl- and phenethyl isothiocyanate against E. coli O157:H7 were comparable to that of gentamicin. The antibacterial effect of 3-Methyl-1-propyl-1H-pyrazole-4-carboxylic acid and 1H-Indol-3-ylmethyl-2-oxazolidinone was not evident on test pathogens. A nonpathogenic E. coli O157:H12 strain enriched in bovine fecal slurry (7 log CFU/ml) was sprayed on five plots (5’ x 4’ plot, 1 L per plot). Plots were marked as (1) inoculated control (2) acetic acid (3) cinnamaldehyde (4) Sporan® and (5) BIT, and treated with respective antimicrobials. Soil samples were analyzed periodically for E. coli O157:H12 populations by spiral plating on MacConkey agar containing 50 mg/L nalidixic acid or MPN procedure. Populations of E. coli O157:H12 reduced in soil with time irrespective of antimicrobial treatment. BIT was superior to acetic acid, cinnamaldehyde, and Sporan® in reducing E. coli O157:H12 in soil. None of these compounds had any effect on native microorganisms of soil.