Survival of Clostridioides difficile in finished dairy compost under controlled conditions

The survival of Clostridioides difficile (previously Clostridium difficile) vegetative cells and endospores was compared at different levels of indigenous microflora using autoclaved and unautoclaved dairy composts with different moisture contents (MCs).

Both types of composts adjusted to 20, 30 and 40% MCs were inoculated with a suspension of C. difficile that contained both vegetative cells (c. 5-6 log CFU per gram) and endospores (c. 5·0 CFU per gram), and then stored aerobically inside a humidity-controlled chamber at room temperature 22·5 ± 0·8°C for 1 year. The level of indigenous microflora was very stable during the storage after day 7 in both types of compost. The greatest reductions of C. difficile vegetative cell counts occurred during the first 24 h of storage in autoclaved and unautoclaved composts, which had 4·7 and 5·5 log CFU per gram with 20% MC, 1·8 and 2·1 log CFU per gram with 30% MC, and 2·3 and 1·3 log CFU per gram with 40% MC, respectively. Both MC and the duration of storage have significant (P < 0·05) effects on the survival of vegetative cells for first 120 days of storage. The slow inactivation of C. difficile vegetative cells at higher MCs during aerobic storage was confirmed by exponentially decaying modelling data during the early stage of aerobic exposure. The reduction of endospore counts (<1·0 log CFU per gram) during the storage for both types of compost at all MCs was not significant (P > 0·05) except for the autoclaved compost with 30% MC.

The highly resistant C. difficile endospores to the unfavourable environmental conditions survived for more than a year while vegetative cells died off exponentially upon the initial aerobic exposure.

The long-term survival of C. difficile endospores in contaminated compost may transmit the pathogen to fresh produce, animals or water in pre-harvest conditions.