P-052. Variations in Chlorine and Acid Resistance among Escherichia coli O157:H7 strains Implicated in Foodborne Disease Outbreaks

S. Wang1, X. Deng1, D. S. Stewart2, M. Tortorello2, W. Zhang1;
1Illinois Inst. of Technology, Chicago, IL, 2US FDA Natl. Ctr. for Food Safety and Technology, Summit, IL.

Disinfection systems involving chlorinated wash-water are commonly used in industrial operations to inactivate bacterial pathogens on fresh produce. To better understand the mechanism how some Escherichia coli O157:H7 strains survive minimal processing treatments, we analyzed 42 outbreak strains that represented 9 distinct genetic lineages (or clades) of the O157 population (Manning et al, manuscript under review). A quantitative measurement of strain resistance was made by comparing the extended lag phase of each strain under the exposure to sublethal concentrations of chlorine (200 ppm) or HCl (pH 4.5) in a BioScreen apparatus. Statistical analysis revealed that strains in clade 7 and clade 8 (including the 2006 spinach outbreak strains) are significantly (p<0.05) more resistant to chlorine than strains in clade 9 (an ancestral lineage of E. coli O157:H7). In contrast, clade 8 strains are less resistant to low pH than clade 9 strains. To determine how clade 8 strains respond to chlorine oxidation at transcriptional levels, we analyzed the global gene expression profiles of a newly sequenced O157 strain TW14359 (clade 8) isolated from the spinach outbreak in 2006, and compared to those of the Sakai strain (clade 3) implicated in a radish sprout outbreak in Japan in 1995. The RNA was extracted and the labeled cDNA was hybridized on Affymetrix E. coli Array 2.0. Approximately 140 genes were identified to be differentially regulated (>2 fold p<0.1) after exposure to chlorine in the spinach outbreak strain compared to the Sakai strain. These included several antioxidant regulatory genes and genes associated with outer membrane lipoprotein biosynthesis, carbon starvation, osmotically inducible lipoproteins, putative oxidoreductase, as well as genes associated with the soxRS regulons and other oxidative defense activities. It is our hypothesis that inadequate chlorine treatments (which impose sublethal oxidative stress) may allow adaptive resistance of some E. coli O157:H7 strains (e.g. clade 8) to better survive the acidic human gastrointestinal tract, therefore, cause infectious disease.