Survival of Shiga-toxigenic Escherichia coli in Flour

Introduction:  Historically, dry ingredients have been viewed as lower risk due limited ability for survival given the low water activity (a_w). This paradigm is changing, however, given outbreaks of Salmonella and Shiga toxigenic Escherichia coli (STEC) with a_w controlled ingredients and foods. Although a low a_w can inhibit the growth of bacteria, certain foodborne pathogens such as STEC can persist in these products for extended periods of time. Survival data for STEC in dry ingredients, such as flour, is necessary for proper risk assessment.

Purpose: Evaluate the ability of various STEC serotypes to survive in dry flour.

Methods:  Representative serotypes from four STEC serogroups (O45, O103, O145, and O157) were used to inoculate separate samples of sterilized all-purpose flour. Flour was stored in an environmental chamber (25°C, RH=57%) to simulate a dry storage environment, then sampled routinely for water activity measurement and enumeration of STEC. All experiments were independently replicated (n=9).

Results: After five weeks of storage, O45 and O145 serotypes showed the greatest survival in flour with log reductions of 0.63 (±0.47) log CFU/g and 0.77 (±0.48) log CFU/g, respectively. Populations of O157 and O103 had significantly reduced (P<0.05) over five weeks with 1.2 (±0.29) log CFU/g and 1.2 (±0.26) log CFU/g, respectively. After six weeks, all serotypes except O45 were below the limit of detection (50 CFU/g), but positive after enrichment. After seven weeks, all serotypes were only positive by enrichment.

Significance: These data suggest that STEC can remain viable over many weeks stored at room temperature in a low a_w environment. More studies will be needed to further characterize the survival of other STEC serotypes in this environment for longer periods of time.