Submitted to: Compost Science and Utilization
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/1/2003
Publication Date: 1/20/2004
Citation: Duffy,B., Sarreal, C., Ravva, S., Stanker, L. 2004. Effect of molasses on regrowth of E. coli O157:H7 and Salmonella in compost teas. Compost Science & Utilization. 12(1):93-96. Interpretive Summary: Composting is widely regarded as effective methods for reducing human pathogen numbers in animal waste before application to agricultural land and gardens. Compost teas are gaining in popularity, particularly among organic growers and home gardeners because of its plant disease suppressive activity and plant nutritional associated with foliar or soil applications of teas. Our study demonstrates that care should be taken to avoid regrowth of pathogens that pose a potential public health risk. Several important human pathogenic bacteria commonly occur in animal waste used as starting materials for compost. Although the composting process usually kills these, we have demonstrated that certain conditions can negate this benefit by allowing regrowth from essentially undetectable numbers of surviving bacteria. Specifically, use of molasses as an amendment to compost teas for the purpose of stimulating beneficial microbial populations, also stimulates populations of fast-growing, pathogenic Salmonella and E. coli, and this practice be avoided, or pursued with utmost caution.
Technical Abstract: Compost water extracts (compost teas) are gaining popularity among organic growers, largely because of their disease suppressive activity when applied to foliage or soil. Production methods often include addition of supplemental constituents, particularly molasses, to stimulate plant-beneficial microbial populations. We have found that molasses amendments also favor regrowth of human pathogenic bacteria, raising public health concerns about potential contamination of treated crops, particularly produce intended for fresh consumption. Using disease outbreak strains marked with green fluorescent protein (GFP) and spontaneous antibiotic-resistance, we found that regrowth of Salmonella enterica serovar Thompson and Escherichia coli O157:H7 was positively correlated with molasses concentration. For Salmonella, regrowth was also dependent on the type of starter compost material used. Salmonella populations increased from 1 at time 0 to over 1000 CFU ml-1 in dairy manure compost tea with 1% molasses, and from 1 at time 0 to over 350,000 CFU ml-1 in chicken manure compost tea by 72 h. E. coli populations increased from 1 at time 0 to approximately 1000 CFU ml-1 in both types of tea by 72 h. Pathogen regrowth did not occur when molasses was eliminated or kept to 0.2%.