Purpose: To assess the survival of vegetative cells and spores of C. difficile in finished dairy compost by simulating pre-harvest contaminations of agro-production.
Methods: Both autoclaved and unautoclaved finished dairy composts at 20, 30, and 40% moisture levels, were inoculated with C. difficile, grown in brain heart infusion (BHI) broth supplemented with yeast extract and L-cysteine for 12 h, at concentrations of 5-6 log CFU/g of vegetative cells and spores and held at room temperature for up to 120 days. The surviving vegetative cells and spores in duplicate compost samples were enumerated on BHI agar and BHI agar supplemented with 0.1% sodium taurocholate, respectively, at selected time intervals. The experiment was repeated three times for each moisture level. Variance of cell and spore counts were analyzed using Tukey’s multiple-comparison procedure of the Statistical Analysis System at a P-value of 0.05.
Results: During the entire study, the change of moisture content of both composts at each set moisture level was not significantly different. The vegetative cell counts rapidly declined during storage for both types of compost and after 24 h, the declines were 4-5 log CFU/g at 20% moisture, and ca.1.5-2.5 log CFU/g for 30 and 40% moisture levels. However, vegetative cells were detectable for up to 30, 120, and 60 days, respectively, at 40, 30, and 20% moisture levels in both types of compost. In contrast, the spore counts were not significantly different for 120 days in both types of compost at all three moisture levels.
Significance: The extended survival of C. difficile spores in compost suggests the contaminated compost/manure-amended soil could serve as a potential source of C. difficile, that contaminates fresh produce, water or field workers on farm.