Submitted to: Composting and Compost Utilization International Symposium
Publication Type: Proceedings
Publication Acceptance Date: 3/5/2002
Publication Date: 5/6/2002
Citation: Duffy, B., Sarreal, C., Stevenson, R., Ravva, S. and Stanker, L., 2002. Regrowth of pathogenic bacteria in compost teas and risk of transmission to strawberry plants. p. 1142-1149. In F.C. Michel, Jr., R.F. Rynk and H.A.J. Hoitink (ed.), Proceedings of 2002 International Symposium: Composting and Compost Utilization, The JG Press, Inc., Emmaus, PA. (CD-ROM from JG Press).
Interpretive Summary: Composting is an effective approach to reduce pathogen numbers in manure. We are concerned with potential regrowth of pathogens when composts are handled inappropriately. Processing of composts to produce watery extracts called compost teas is gaining in popularity among organic growers and home gardeners. Preparation methods often call for the addition of carbon sources such as molasses to stimulate a rapid increase in microbial populations. Recipes calling for as much as 2% of such ammendments are common in trade literature. Our work monitored the risk potential for regrowth of E.coli O157:H7 and Salmonella Thompson in teas made from various types of compost. We used strains marked with antibiotic resistance and carrying stable GFP plasmids. Amendments greatly increased regrowth of both bacteria in dairy compost, yard waste compost and chicken manure compost teas (approximately 2.5 log increase with 0.5% molasses in most cases). However, regrowth potential depended on the pathogen and also on the type of compost used to prepare teas. For example, with 0.5% molasses, Salmonella increased from 1 to over 1000 CFU/ml in teas made from dairy or yard compost but exceeded 350,000 in chicken manure compost tea, whereas, E.coli regrowth was the same in all types of compost teas. Using Confocal Laser Scanning Microscopy, we observed that GFP marked pathogens were transferred to tomato foliage by spray application of contaminated teas, and that pathogen populations proliferated in veins and on glandular trichomes.
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 call for organic ammendments, such as molasses, to stimulate plant-beneficial microbial populations. We have found that certain ammendments 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 Salmonella echerica serovar Thompsaon and Escherichia coli )157:H7 colonized strawberry leaves after foliar application of contaminated compost teas. Regrowth of these pathogens from 1 CFU ml(-1) to populations over 4 log greater was dependent on the type of starter compost material used. There was a positive correlation between molasses concentration and pathogen regrowth in all teas. The potential risk of pathogen regrowth was minimized by avoiding ammendments altogether.