Affiliations: Laboratory of Soil Microbiology, Faculty of
Agriculture, Saga University, Saga 840-8502, Japan | The United Graduate School of Agricultural Sciences,
Kagoshima University, Kagoshima 890-0065, Japan
Abstract: Application of compost in agricultural practice could potentially
cause contamination of foodstuffs with pathogenic bacteria such as Escherichia
coli O157: H7 (E. Coli O157). We investigated pathogenic bacteria in compost
collected from the compost facilities, and evaluated the survival of E. coli
K12 and O157 in laboratory experiments. Out of 19 compost product samples,
coliform bacteria and salmonella were detected in 7 and 3 samples respectively.
The number of coliform bacteria was 1.8 × 10^2 to 2.5
× 10^6 CFU/g dw and that of salmonella was 4.2 ×
10^1 to 6.0 × 10^3 CFU/g dw.
Moreover, coliform bacteria, fecal coliform, E. coli and salmonella were
detected during composting at 54°C to 67°C. The results indicated that
moisture content was a very important factor to the heat sensitivity of
pathogenic bacteria in compost, E. coli in compost of high moisture content was
more sensitive than that in compost of low moisture content, cells harvested in
logarithmic phase was more sensitive than these in stationary phase, and E.
coli K12 was more sensitive than E. coli O157. Based on the D values, the
lethal time of E. coli K12 and O157 from 10^8 to
10^0 CFU/g dw were 16.3 and 28.8 min, respectively, at
60°C in compost with 40% moisture content. However, some E. coli
cells survived in composting process at 54°C to 67°C.
Water potential (low moisture content) and physiological aspects of bacteria
(stationary phase) could explain only in part of the prolonged survival of E.
coli in compost, and there should be some other factors that are conducive to
bacterial survival in compost.
