Purpose: To guide the development of recommendations for handling enrichment broths, we assessed the effect of storage temperatures on the survival of STEC in these samples.
Methods: Aliquots of a master stool sample spiked ~105 CFU/ml with one of 20 STEC strains (O157:H7/NM(6), O26:H11(2), O111:H8(2), O121:H19(2), O145:NM(2), O103:H2(2), O69:H11, O79:H7 and O165:NM) were used to prepare replicate stool enrichment broths. Half of the enrichment broths were stored at ambient temperature (25°C), while the other half were stored in the refrigerator (3-5°C). Broths were subcultured to either CT-SMAC (O157 STEC) or washed sheep blood agar with mitomycin C (non-O157 STEC) at days O-5, 14, 21, 28 and 56 and tested for STEC by EIA and PCR. The results for each strain on days 1 through 56 were compared to those obtained initially for the different storage conditions.
Results: Over the period of the study, there was no apparent difference in survival of the 20 strains in broths stored at 4°C and 25°C as assessed by EIA and PCR. There were differences in the number of target to non-target organisms in the broths stored under the different conditions. Target organisms in broths stored at 25°C were more likely to be overgrown by day 14 (17/20) compared to broths stored at 4°C (7/20).
Significance: The success of public health surveillance systems for identifying and controlling outbreaks is dependent upon STEC isolation. While STEC can survive in stool enrichment broths for extended periods at 4°C and 25°C, their isolation is less encumbered by competing flora when stored at 4°C.