Purpose: The objectives of this study were to optimize culturing methods for detecting STEC in dairy compost and also to compare growth rates of different STEC serotypes during enrichment of dairy compost.
Methods: The finished dairy compost with 30% moisture content was inoculated with a cocktail of six STEC serotypes at a final concentration of ca. 102 CFU/g. Afterwards, bacterial cells in the inoculated compost were enriched by four methods: pre-enrichment by universal pre-enrichment broth (UPB) followed by selective enrichment using STEC enrichment broth or modified Tryptic Soy Broth with novobiocin (mTSB+n), direct selective enrichment by STEC-EB or mTSB+n, and the enrichment efficiencies were compared by plating enriched cultures onto Cefixime-Tellurite Sorbitol MacConkey Agar with noviobiocin (CTN-SMAC) and modified Rainbow Agar (mRBA). Immunomagnetic separation (IMS) was applied to separate STEC serotypes after enrichment.
Results: There was no significant difference (P > 0.05) between CTN-SMAC and mRBA for STEC enumeration. Applying selective enrichment methods alone recovered ca. 0.5 log CFU/g more cells as compared to the two-step enrichment methods. Among six STEC serotypes, serotypes O45 and O145 grew faster and the cell populations reached up to 7.29 and 7.83 log CFU/g, respectively, after 16-hour enrichment, suggesting these two as the potential persistent serotypes surviving in dairy compost.
Significance: Our results demonstrated that low levels of STEC could be detected from dairy compost by culturing method through optimizing enrichment procedure followed by IMS, which can be useful to study the growth and survival of STEC during composting or in dairy manure-based soil amendments.