Assessment on the Sensitivity and Specificity of Five Culture Media in the Detection of Environmental Escherichia coli O157

Introduction: Escherichia coli is a large and diverse group of bacteria. Most of them are harmless and are naturally found in the human intestinal tract, but others can be deadly. E. coli O157:H7 has become an important problem in human health in the United States. Scientific studies have indicated that cattle herds are the primary reservoirs of E. coli O157:H7. The ability of E. coli O157:H7 to survive in soil and manure for extended periods of time could explain its spread into the water supply and onto crops. Prior to confirm the presence of E. coli O157:H7 in a sample, proper isolation of E. coli O157 is necessary.  

Purpose: The purpose of this study was to establish the sensitivity and specificity of five commercial media, and establish which of them is/are the best option for isolation for presumptive identification of environmental E. coli O157 from cattle farms.

Methods: One hundred thirty-eight samples of fresh cattle fecal matter on the ground, water from water troughs and ponds, and swabs from troughs, salt and hay bunks were tested for E. coli O157. For the isolation of the E. coli O157, samples were enriched in TSB, followed by an Immunoseparation and then plating onto SMAC, CT-SMAC, CHROMagar^TM, T CHROMagar^TM, and VCC CHROMagar^TM. RT-PCR was used to identify the strains by using the FDA’s BAM methodology and the genes were stx₁, stx₂, and eae.

Results: SMAC tested positive to E. coli O157 in 109 samples, an 81% of these samples were false positive. Using logistic regression CT-SMAC and CHROMagar® was the best combination for the isolation of E. coli O157, showing a 79% of positive and only 0.05% of false negative results.

Significance: The findings of this study can be used by regulatory agencies for detection of environmental E. coli O157:H7 in cattle and produce farms.