Preliminary evaluation of a procedure for improved detection of Shiga toxin-producing Escherichia coli in fecal specimens.

Culture confirmation of Shiga toxin-producing Escherichia coli (STEC) is very important for epidemiologic analysis. However, isolation of non-O157 STEC on conventional selective media such as sorbitol-MacConkey agar (SMAC) can be difficult because of heavy growth of competing bacteria and its phenotypical similarity to commensal nonpathogenic E. coli. An acid enrichment procedure was introduced in this study to facilitate detection of STEC from patients who were symptomatic. Forty-seven clinical fecal broths, which tested positive for Shiga toxin by commercial immunoassay, were processed for the isolation of STEC by both conventional and the acid enrichment methods. The acid enrichment method and conventional culture recovered STEC from 91% (43/47) and 70% (33/47) of the fecal broths, respectively. Neither method retrieved STEC in 3 specimens. Thirty-six STEC were successfully serogrouped, which included O26 (n = 11), O157 (n = 9), O103 (n = 7), O121 (n = 3), O111 (n = 2 each), O28AC, O146, O76, and O undetermined (n = 1 each). The analysis of STEC isolates by real-time PCR indicated that all 9 E. coli O157 contained stx2 gene alone or in combination with stx1. Non-O157 STEC more frequently contained stx1 only, and about one-third possessed stx2. The novel acid enrichment protocol greatly reduced the growth of competitor colonies on RTN and TCSMAC. The study demonstrated that incorporation of an acid enrichment procedure in clinical testing improved the isolation of STEC in fecal specimens.