Easy-to-perform modified Elek test to identify Shiga-like toxin-producing diarrhoeogenic Escherichia coli

We determined whether Shiga-like toxin I (SLT-I) -producing diarrhoeogenic Escherichia coli could be detected by a modified Elek test. The test (SLT Elek test) is based on the principle of the Elek test and the Ouchterlony double-gel diffusion. The development of the SLT Elek test was preceded by a preliminary study; the purpose of the latter was to establish whether a simplified purification procedure of SLT-I (involving bacterial sonic extract, “Affi-Gel Blue” chromatography and anion- and cation-exchange liquid chromatography) could be employed in the preparation of rabbit antisera to SLT-I. SLT-I-specific antisera were obtained after adsorption of sera with bacterial sonic extract from non-toxigenic E. coli. A total of 135 strains of E. coli were tested by the SLT Elek test (100 SLT-I-negative and 35 SLT-I-positive). The results of the SLT Elek test and the Vero cell test correlated well: 30 strains gave positive results and 100 strains gave negative results in both tests. Only 5 strains gave discrepant results: they were weakly positive in the Vero cell assay, whereas 3 gave borderline reactions and 2 were negative in the SLT Elek test. Positive predictive value was 1, negative predictive value was 0.98; the SLT Elek test was 91 % sensitive and 100 % specific.