Purpose: This study compared the survival patterns of known acid-resistant O157:H7 strains (5 strains, including O157:H7 ATCC 43895), to those of acid-adapted but not acid-resistant non-O157:H7 STEC (3 strains each of serogroups O26, O45, O103, O111, O121, O145) in a model stomach system (synthetic gastric fluid).
Methods: Each strain was grown in Brain Heart Infusion broth (BHIB) for 24h to induce acid adaptation. Inocula were prepared by making serial dilutions in BHIB (to ca. 104 CFU/ml). Each strain (2ml) was combined with 18ml of pre-warmed SGF (pH 2.0; 37 °C) and incubated (shaken @ 100 rpm) at 37 °C for 6h. Sampling was conducted at 0h, 0.5h, 1h, 2h, 3h, 4h and 6h and cell counts were determined on modified Eosin Methylene Blue agar (MEMB). For each trial (n=3) of each strain, an exponential curve with detection limit was modeled to fit the data and generate two survival parameters: initial population (log CFU/ml) and decay rate (log CFU/ml/h).
Results: Decay rates ranged from 0.085 log CFU/ml/h to 10.11 log CFU/ml/h. Analysis of variance showed significant differences (P < 0.05) in initial log CFU/ml and decay rate, with significant differences between serogroups O103 and O157; O111 and O121; O121 and O45, O26, O145, O157; and O157 and O45. Significant differences (P < 0.05) were found in decay rates between the five O157:H7 strains. Overall, the lowest decay rate was exhibited by one O111 strain; one O121 strain had the highest decay rate.
Significance: These results suggest that non-O157:H7 STEC may survive well in the low pH environment of the human stomach, with the potential to produce illness. Further research is needed to investigate the survival patterns of non-O157 STEC in food products under similar conditions.