Agriculture Reference
In-Depth Information
to heat and substantially increase before signifi cant inactivation temperatures are
reached.
Under low-key composting conditions such as composting in a small heap or with
a low C:N ratio, the transition from the mesophilic to thermophilic phase of compost-
ing may be very slow or not occur at all, especially during winter months. Using a
bioreactor, Lung and others (2001) and Jiang and others (2003a) have demonstrated
that the mesophilic composting either caused the initial growth of pathogen or main-
tained the pathogen populations for at least 4 days. Hutchison and others (2005)
reported the extended survival of
E. coli
O157 in heaped mixtures of dairy and beef
cattle manure for 32 and 93 days, respectively, probably due to a longer mesophilic
phase occurring in the fi eld. Droffner and Yamamoto (1991) isolated thermal-resistant
E. coli
,
S
. Typhimurium, and
Pseudomonas
mutants from compost samples, and those
mutants were capable of growth at 54 °C. Both
E. coli
and
S.
Typhimurium mutants
can survive for at least 56 and 44 days, respectively, in an outdoor industrial compost
and for at least 9 days in laboratory compost at ca. 60 °C when ca. 10
7
CFU/g were
inoculated into food waste and municipal biosolids (Brinton and Droffner 1994).
Most composting studies so far used nonstressed cultures, not representative of
the microorganisms in complex composting systems, for pathogen challenge
studies (Lung and others 2001; Jiang and others 2003a; Huchinson and others 2005;
Nicholson and others 2005). Therefore, future studies should examine pathogen sur-
vival during composting using a range of conditions typically encountered during the
composting.
Other improper composting practices may permit the extended survival of patho-
gens, including adding fresh manure to the compost that has gone through the ther-
mophilic cycle, lack of temperature and moisture monitoring, less turnings of heaps,
changes in composting raw materials, manure deepstacking, cross-contamination of
equipment handling raw materials and fi nished compost, and the presence of wildlife
around the farm (FDA 2001; Rangurajan and others 2002). Therefore, risks associated
with the practices above should be assessed scientifi cally, which can help to develop
practical composting practices being easily adopted by small farmers to produce
compost of acceptable quality free of viable human pathogens.
Pathogen Regrowth
Even though proper composting can effectively inactivate most enteric pathogens, due
to the heterogeneity of composting heaps, cold spots inside the heaps, and surface
contamination by wildlife and the environment, the cured or fi nished compost may
allow a few survived or newly introduced pathogenic cells to regrow to higher
numbers under favorable environmental conditions. Studies have shown that pathogen
repopulation (growth) in compost is affected by moisture level, temperature, and
nutrient content of the composted solids (Soares and others 1995; Hay 1996; Tiquia
2005). Studies on the microbiological safety of biosolid composting revealed that
Salmonella
and
E. coli
can survive the composting process and regrow in compost
and stored biosolids when held under favorable conditions (Russ and Yanko 1988;
Sidhu and others 1999).
E. coli
can regrow to substantial levels in compost samples,
which were initially very dry, ca. 19% moisture content (Soares and others 1995). To
