P-041. Inability of Exponentially Growing Cells of Escherichia coli O157:H7 to Induce Glutamate-Dependant Acid Resistance and Survive pH 2 Acid Shock

S. Viazis1, A. Olstein2, F. Diez-Gonzalez1;
1Univ. of Minnesota, St. Paul, MN, 2Paradigm Diagnostics, Inc., St. Paul, MN.

Acid resistance (AR) in Escherichia coli is defined as the ability to survive an acid shock (AS) of pH 2.5 or less and is generally restricted to stationary-phase cells. However, it is believed that log-phase E. coli can become AR if cells are incubated at acidic pH. The purpose of this study was to determine the ability of log-phase E. coli O157:H7 strain ATCC 43895 to become AR and survive AS in glutamic acid media. We hypothesized that exposure to conditions typically observed with stationary cultures would induce AR in growing cells. Several conditions were utilized in an attempt to induce AR in log-phase E. coli O157:H7 cells. Cultures were incubated in tryptic soy broth (TSB) at 37oC for 24 h, diluted 100-fold into fresh TSB tubes and grown for 3 h at 37oC. Aliquots of log-phase cells were transferred to media containing different compositions, pH and ox/redox conditions. After incubations from 0.5 to 3 h, cell suspensions were subjected to AS in pH 2 glutamic acid solutions for 1 h. Survivors were determined by dilution, plating, and comparing cell counts before and after AS. The AR-inducing media included: peptone water (PW) and PBS at pH 4.0 to 6.0, supernatant from stationary phase cultures in TSB and EC, anaerobic LB broth with glucose (LBG) at pH 4, M9 minimal media at pH 5.5, and media containing different concentrations of ferric ammonium citrate, MgCl2, cumene hydroperoxide, ethanol, CdCl2, and/or N-hexanoyl-L-homoserine lactone. When TSB-growing cells were transferred into PW and PBS at any pH and incubated at 37oC for as long as 3 h, no growth was observed but the survival rates after AS were less than 0.1%. Survival rates (SR) greater than 20% were observed in the same TSB cultures grown overnight. Relatively low SR (<0.1%) were also observed when pH 4 TSB, EC and LBG were used. Addition of media supplements resulted in SR of less than 0.1%. These results indicated that acid sensitive E. coli O157:H7 could not easily become resistant to low pH and suggested that there may be additional factors regulating their acid tolerance response. The inability of exponentially-growing cells to become AR may have potential implications for food safety.