Variation in Resistance to High Hydrostatic Pressure and rpoS Heterogeneity in Natural Isolates of Escherichia coli O157:H7

Abstract

Several natural isolates of Escherichia coli O157:H7 have previously been shown to exhibit stationary-phasedependent variation in their resistance to inactivation by high hydrostatic pressure. In this report we demonstrate that loss of the stationary-phase-inducible sigma factor RpoS resulted in decreased resistance to pressure in E. coli O157:H7 and in a commensal strain. Furthermore, variation in the RpoS activity of the natural isolates of O157:H7 correlated with the pressure resistance of those strains. Heterogeneity was noted in the rpoS alleles of the natural isolates that may explain the differences in RpoS activity. These results are consistent with a role for rpoS in mediating resistance to high hydrostatic pressure in E. coli O157:H7.In recent years there has been growing interest in the use of high hydrostatic pressure (HHP) as a means of food preservation. The attraction of HHP lies in the production of microbiologically safe foodstuffs with minimal use of chemical additives and without adversely affecting the organoleptic qualities of the food (11). HHP processing could replace traditional thermal pasteurization or be used in conjunction with existing techniques (3,10,14,22). However, exploitation of this potential requires a better understanding of the effects of HHP on microorganisms. HHP resistance varies among genera and species and is dependent on the physiological state of the organisms at the time of pressurization (6, 18). It is of concern that certain E. coli O157:H7 strains are among the most pressureresistant vegetative bacteria known (2, 18). It is therefore critically important to characterize the innate HHP resistance in these strains.We demonstrated recently that the pressure resistance of certain natural isolates of E. coli O157:H7 varied greatly (2, 17). Strains C9490 and 30-2C4 were the most pressure resistant and were able to withstand 500 MPa for 5 min with little viability loss; strains NCTC 12079 and W2-2 were of intermediate pressure resistance (ca. 3 to 4 log units decrease under the same conditions), whereas H1071 and an O124 strain, NCTC 8003, were the least pressure resistant (5 to 6 log decrease). However, this variation in pressure resistance was stationary-phase dependent, with the strains exhibiting similar pressure resistance in exponential phase (2). This led us to speculate that the differences in pressure resistance among the isolates were related to differences in RpoS. This sigma factor changes the specificity of RNA polymerase, allowing it to activate more than 30 genes, some of which are involved in stationary-phase stress survival (8). Stationary-phase bacteria are generally more resistant to other stresses, such as oxidative and osmotic stress (21). It is thus possible that resistance to inactivation by HHP is also controlled by mechanisms used to survive stationary-phase stress. These studies attempted to identify the genetic basis, and specifically the role of RpoS, in the wide variation observed in HHP resistance of natural isolates of E. coli O157:H7 stra...

Genotype-by-Environment Interactions Influencing the Emergence of rpoS Mutations in Escherichia coli Populations

“…Consistent with this notion, E. coli strains with high endogenous RpoS levels accumulated rpoS mutations at a higher rate during growth under nutrient limitation and disruption of rpoS indeed improves nutritional capability with many poor substrates of E. coli (King et al 2004). Of course, the trade-off in losing RpoS function is that rpoS mutants exhibit reduced resistance to encountered stresses such as prolonged starvation, high pressure, high osmolarity, and low pH (Cheville et al 1996;Waterman and Small 1996;Robey et al 2001;Dodd and Aldsworth 2002). Survival in the gastrointestinal 1 tract is reduced (Price et al 2000) and rpoS mutants also exhibit a reduced virulence potential in organisms in which virulence gene expression is under RpoS control (Wiedmann et al 1998;Nadon et al 2002).…”

The Growth Advantage in Stationary-Phase PhenotypeConferred by rpoS Mutations Is Dependent on the pH andNutrientEnvironment

“…The diversity of rpoS alleles found in LB batch cultures suggests that the activities of various sigma factors can be modulated in a variety of ways to efficiently promote protection from conflicting stresses that arise during stationary phase. The phenotypic diversity we observed in batch culture correlates well with the heterogeneity of rpoS alleles of E. coli and Salmonella found in natural, laboratory, and clinical environments, implying that the shifting, sometimes conflicting stresses found in many environments often lead to the emergence of rpoS mutants in many populations via natural selection (2,35,40,41,45,47,49). Furthermore, the fact that both wild-type and attenuated rpoS alleles are found in clinical and other natural settings suggests that the LB batch culture model mirrors important aspects of the evolution of bacteria in nature.…”

Sensitivity of Escherichia albertii , a Potential Food-Borne Pathogen, to Food Preservation Treatments

“…There were no significant differences in pressure tolerances between other E. albertii strains and strains 19982 and 12502. Significant differences were not observed in pressure tolerances of wild-type and rpoS-deficient strains of E. coli O157:H7, a result which is not in agreement with previous work which reported a reduction of Ͼ2 log 10 CFU/ml for the rpoS-deficient compared to wild-type strain results (17). Our work shows a similar reduction in populations of wild-type and rpoS E. coli O157:H7 but lacks statistical significance.…”