Thermal Resistance and Gene Expression of both Desiccation-Adapted and Rehydrated Salmonella enterica Serovar Typhimurium Cells in Aged Broiler Litter
Authors: Chen Zhao , Jiang Xiuping 
Published: April 2017

The objective of this study was to investigate the thermal resistance and gene expression of both desiccation-adapted and rehydrated
Salmonella enterica
serovar Typhimurium cells in aged broiler litter.
S
. Typhimurium was desiccation adapted in aged broiler litter with a 20% moisture content (water activity [a
w
], 0.81) for 1, 2, 3, 12, or 24 h at room temperature and then rehydrated for 3 h. As analyzed by quantitative real-time reverse transcriptase PCR (qRT-PCR), the
rpoS
,
proV
,
dnaK
, and
grpE
genes were upregulated (
P
< 0.05) under desiccation stress and could be induced after 1 h but in less than 2 h. Following rehydration, fold changes in the levels of these four genes became significantly lower (
P
< 0.05). The desiccation-adapted Δ
rpoS
mutant was less heat resistant at 75°C than was the desiccation-adapted wild type (
P
< 0.05), whereas there were no differences in heat resistance between desiccation-adapted mutants in two nonregulated genes (
otsA
and P
agfD
) and the desiccation-adapted wild type (
P
> 0.05). Survival characteristics of the desiccation-adapted ΔP
agfD
(rdar [red, dry, and rough] morphotype) and Δ
agfD
(saw [smooth and white] morphotype) mutants were similar (
P
> 0.05). Trehalose synthesis in the desiccation-adapted wild type was not induced compared to a nonadapted control (
P
> 0.05). Our results demonstrated the importance of the
rpoS
,
proV
,
dnaK
, and
grpE
genes in the desiccation survival of
S
. Typhimurium. By using an Δ
rpoS
mutant, we found that the
rpoS
gene was involved in the cross-protection of desiccation-adapted
S
. Typhimurium against high temperatures, while trehalose synthesis or rdar morphology did not play a significant role in this phenomenon. In summary,
S
. Typhimurium could respond rapidly to low-a
w
conditions in aged broiler litter while developing cross-protection against high temperatures, but this process could be reversed upon rehydration.

IMPORTANCE
Physical heat treatment is effective in eliminating human pathogens from poultry litter used as biological soil amendments. However, prior to physical heat treatment, some populations of microorganisms may be adapted to the stressful conditions in poultry litter during composting or stockpiling, which may cross-protect them against subsequent high temperatures. Our previous study demonstrated that desiccation-adapted
S. enterica
cells in aged broiler litter exhibited enhanced thermal resistance. However, there is limited research on the underlying mechanisms of the extended survival of pathogens under desiccation conditions in animal wastes and cross-tolerance to subsequent heat treatment. Moreover, no information is available about the thermal resistance of desiccation-adapted microorganisms in response to rehydration. Therefore, in the present study, we investigated the gene expression and thermal resistance of both desiccation-adapted and rehydrated
S
. Typhimurium in aged broiler litter. This work will guide future research efforts to control human pathogens in animal wastes used as biological soil amendments.