Evaluation of parameters affecting quantitative detection of Escherichia coli O157 in enriched water samples using immunomagnetic electrochemiluminescence.

We report here the use of immunomagnetic (IM) electrochemiluminescence (ECL) for quantitative detection of Esherichia coli O157:H7 in water samples following enrichment in minimal lactose broth (MLB). IM beads prepared in-house with four commercial anti-O157 monoclonal antibodies were compared for efficiency of cell capture. IM-ECL responses for E. coli O157:H7 (strain SEA13B88) were similar for all four commercial anti-O157 LPS monoclonal antibodies. The ECL signal was linearly correlated with E. coli O157:H7 cell concentration, indicating a constant ECL response per cell. Twenty-two strains of E. coli O157:H7 or O157:NM gave comparable ECL signals using IM beads prepared in-house. To assess the potential for interference from background bacteria in MLB-enriched water samples, 10(4) cells of E. coli O157:H7 (strain SEA13B88) were added to enriched samples prior to analysis. There was considerable variability in recovery of E. coli O157:H7 cells; net ECL signals ranged from 1% to 100% of expected values (i.e., percent inhibition from 0% to 99%). Cultures of Klebsiella pneumoniae, Klebsiella oxytoca, and Enterobacter cloacae, subsequently isolated from MLB-enriched water samples via IM separation (IMS), were observed to interfere with the binding of E. coli O157:H7 cells to IM beads. Recoveries of 10(4) E. coli O157:H7 cells were </=10% in the presence of ca. 10(8) K. pneumoniae, K. oxytoca, or E. cloacae cells. None of these strains gave a positive IM-ECL signal. Although competitive binding decreased sensitivity, there still was a linear correlation between ECL signal and higher E. coli O157:H7 cell concentrations. These studies indicate that IM-ECL in conjunction with MLB enrichment is capable of quantitatively detecting as few as 10(3) to 10(5) E. coli O157:H7 cells ml(-1), depending on percent recoveries, in enriched samples that contain ca. 10(9) total lactose-fermenting bacteria ml(-1). Assuming comparable growth rates for E. coli O157:H7 and other lactose-fermenting bacteria in MLB, it may be possible to detect as few as one E. coli O157:H7 in 100 ml of raw water containing as many as 10(4) to 10(6) lactose-fermenting bacteria (i.e., total coliforms).