coli soil survival. The results showed that E. coli O157 isolates capable of long-term survival learn more (longer than 200 days) in manure-amended soil were characterized by the absence of mutations in their rpoS gene. In contrast, the strains not capable of long-term survival all possessed mutations in their rpoS gene. In addition, the long-term surviving strains showed significantly higher levels of acid resistance in simulated gastric fluid (pH 2.5). Sequencing of the rpoS gene of bovine, food

and clinical isolates revealed a skewed distribution of rpoS wild-type and mutant strains among the different sources. Bovine and food isolates had low numbers of mutants (< 1.4 and 6.9%, respectively), while a relatively high number of mutants was observed among human isolates (32.9%). The results indicate that a fully functional RpoS system is an advantage for

survival in Y-27632 mouse the manure-amended soil environment. Further deletion and complementation studies should provide more evidence on the role of RpoS in the long-term survival of E. coli O157 in diverse environments. Shiga toxin-producing Escherichia coli (STEC) O157 is considered a serious pathogen due to its low infectious dose, severe clinical consequences, and the potential for food- and waterborne outbreaks (Caprioli et al., 2005). Its long-term survival in manure and soil can be considered a significant risk factor for the (re)contamination of cattle, food crops and ultimately human infection

(Franz & van Bruggen, 2008; Fremaux et al., 2008). Escherichia coli O157 may respond to unfavourable conditions by expressing adaptive responses. Stationary-phase and almost any environmental stress that slows the growth rate of E. coli induce the RpoS-controlled general GPX6 stress response (Battesti et al., 2011). Escherichia coli strains with attenuated RpoS levels have lower levels of resistance to external stress but have broader nutritional abilities and increased competitive abilities with low nutrient concentrations, and vice versa (King et al., 2004). The appearance of rpoS mutants seems to be driven by the increased ability of such mutants to scavenge for scarce nutrients (King et al., 2004; Ferenci, 2005). This RpoS regulatory trade-off between stress resistance and metabolic capacity provides a means of broadening the ecological and phenotypic properties which might especially be advantageous to E. coli as this bacterium generally experiences a biphasic lifestyle with a relatively constant and optimal host-associated phase and a fluctuating non-optimal host-independent phase (Van Elsas et al., 2011). Variation in rpoS alleles has also been observed among pathogenic E. coli strains (e.g. O157) and have been linked to variation in the level of stress resistance and metabolic capacity (Waterman & Small, 1996; Robey et al., 2001; Parker et al., 2012). However, very little is known about the role of RpoS in the long-term survival of E.