P-008. Evaluating the Effect of Environmental Factors on Pathogen Regrowth in Dairy Compost Extract

J. Kim, X. Jiang;
Clemson Univ., Clemson, SC.

Composting has been used as a practical way for pathogen reduction in animal wastes. However, complete destruction of pathogens may not be assured, since composting is subject to various environmental factors which are varying and uncontrollable. A few survived pathogenic cells during composting may have a chance to repopulate when they encounter proper conditions for growth. The objective of this study was to identify the optimal conditions for Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes regrowth in composts. Water extract of commercially available dairy compost was used as a model system. The cocktails of five rifampin-resistant strains of each pathogen grown previously in reduced nutrient media were inoculated into water extract of compost which was extracted with sterile water at different ratios (1:2,1:5 and 1:10), and then stored at 35 or 22oC for 7 days. Both the growth and survival of pathogens in compost extract were enumerated by plate count method. The most grown and survived strains were identified by BOX-PCR or PFGE. The plate count data at 35oC indicated that both Salmonella spp. and E. coli O157:H7 regrew in compost extract with ratios of 1:2 and 1:5 with population increase ranging from 0.34 to 2.07 log CFU/ml, whereas no regrowth was detected for L. monocygenes in all compost extracts. At 22oC, both E. coli O157:H7 and L. monocytogenes multiplied in all compost extracts, whereas Salmonella spp. regrew in both 1:2 and 1:5 compost extracts. When tested with cultures grown previously in reduced nutrient media, all three pathogens regrew to higher (p<0.05) populations than the control cultures. Without competition for nutrients with background microflora, all pathogens regrew even in most diluted compost extract (1:10) with varying growth rates among strains. In summary, the low nutrient-adapted cultures can regrow well in compost extract with limited nutrients and less microbial flora at 22oC. These results clearly demonstrated that nutrient availability, species and physiological stage of pathogens, competitive microbial flora in compost, and incubation temperature were important factors affecting the regrowth of foodborne pathogens in compost ecosystem.