Potential for growth of E. coli in aerobic compost extract

Aerobic compost extract (ACE) is a watery extract of compost and a type of compost tea that is applied by horticultural producers to fruit and foliage of crops for improved plant health and crop protection. ACE was prepared using compost sampled from commercial open windrows during the cooling phase of composting when the internal windrow temperature was approximately 50°C. Escherichia coli, Listeria monocytogenes and Bacillus cereus were not detected (<1 cfu per 100 g) in this compost. E. coli M23 strepr is a nonpathogenic strain that has growth characteristics similar to strains of E. coli pathogenic to humans. ACE was inoculated with E. coli M23 strepr at 1 x 107 cfu ml^-1 at the beginning of extraction. No significant change in the number of E. coli M23 strepr was observed up to 72 h later. However, there was a significant increase in E. coli M23 strepr numbers by 72 h when 0.8% fish hydrolysate or 1% molasses were introduced to ACE 24 h after extraction commenced. Introduction of 0.5-2% liquid kelp or a mixture of 1.7% liquid kelp and 0.8% fish hydrolysate lead to a decline in the number of E. coli M23 strepr. There was no relationship between the number of E. coli M23 strepr and the abundance of culturable bacteria and fungi in ACEs amended with nutrients, although a low oxygen concentration, pH and high conductivity was associated with increased E. coli M23 strepr numbers in an ACE amended with 1% molasses. The results imply that methods should be identified and imposed to assure that human enteric pathogens do not contaminate amended ACEs, during or after preparation, and/or are prevented from attaining levels that pose a risk to humans consuming fruit and vegetables treated with amended ACE.