Sensitivity of two techniques to detect Escherichia coli O157 in naturally infected bovine fecal samples

The aim of this study was to investigate the sensitivity and reliability of two techniques commonly used for the isolation of Escherichia coli O157: (i) buffered peptone water (BPW) containing vancomycin, cefsulodin and cefixime followed by immunomagnetic separation (IMS-VCC) and (ii) modified E. coli (EC) broth supplemented with novobiocin (m ECn), both followed by culturing on cefixime tellurite sorbitol McConkey (ctSMAC) agar plates. Over a 2-year period, 24 feedlots located over a large geographical area (∼600×450 km) were screened for the presence of E. coli O157. A total of 194 E. coli O157 isolates were identified; 151 (77.4%) using IMS-VCC and 108 (55.4%) using m ECn. The recovery rates of IMS-VCC varied from 100% to 47%, whereas for m ECn ranged from 100% to 16%. All isolates were grouped, using randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR), into 3 major clusters that comprised 39 different subtypes, providing evidence of considerable genetic heterogeneity. The results from this study revealed false negatives in IMS-VCC technique, most probably due to the high genetic diversity of environmental E. coli O157 isolates and antibiotic sensitivity. Using only IMS-VCC as a method for detection may result in significant underestimation of the pathogen. Performing two different enrichment steps in parallel can lead to markedly improved recovery rates of E. coli O157 isolates from naturally infected samples.