Evaluation of an in vitro model with a novel statistical approach to measure differences in bacterial survival of extended-spectrum β-lactamase-producing Escherichia coli on an inanimate surface

BACKGROUND: The role of environmental contamination in the transmission of Enterobacteriaceae is increasingly recognized. However, factors influencing the duration of survival in the environment have not yet been extensively studied. In this study, we developed and evaluated an in vitro model with a novel statistical approach to accurately measure differences in bacterial survival, that can be used to model the effects of multiple factors/conditions in future experiments. METHODS: Two extended-spectrum β-lactamase (ESBL)-producing Escherichia coli (E. coli) isolates were used for this in vitro experiment: a CTX-M-15-producing E. coli sequence type (ST) 131 and a CTX-M-1-producing E. coli ST10 isolate. Each strain was 1:1 diluted in sterile water, sterile saline or sheep blood. Cover glasses (18 × 18 mm) were inoculated with the dilution and subsequently kept at room temperature. Bacterial survival on the glasses was determined hourly during the first day, once daily during the following 6 days, and from day 7 on, once weekly up to 100 days. The experiment was repeated six times for each strain, per suspension fluid. RESULTS: Viable bacteria could be detected up to 70 days. A biphasic survival curve for all suspension fluids was observed, whereby there was a rapid decrease in the number of viable bacteria in the first 7 h, followed by a much slower decrease in the subsequent days. CONCLUSIONS: We found a difference in survival probability between E. coli ST10 and ST131, with a higher proportion of viable bacteria remaining after 7 h for ST131, particularly in sheep blood.