A magnetite suspension-based washing method for immunoassays using Escherichia coli cells with autodisplayed Z-domains

•A new washing method for immunoassays was developed by using magnetite suspension.•The washing method was applied to immunoassay based on E. coli with autodisplayed Z-domain.•Washing parameters were optimized for concentration of magnetite, recovery of E.coli, etc.•The immunoassay with new washing method was effectively used in diagnosis of CRP.

Escherichia coli cells with autodisplayed Z-domains have been used for immunoassays of specific target analytes. In this study, a magnetite suspension was used for the washing step in immunoassays of E. coli cells with autodisplayed Z-domains. This approach enhanced the washing conditions for these immunoassays by determining (1) the optimal concentration of the magnetite suspension, (2) the capacity of the magnetite suspension-based washing method to recover E. coli cells, and (3) the level at which the activity of autodisplayed Z-domains is maintained. In immunoassays of C-reactive protein (CRP), the immunoassay incorporating the magnetite suspension-based washing method showed a sensitivity and limit of detection considerably higher than those of the conventional centrifugation-based washing method. The results indicated that immunoassays incorporating the magnetite suspension-based washing method are effective for medical diagnoses based on CRP assay.

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