Binding of APEC-LsrB to AI-2 using an AI-2 assay. A The activity of APEC-LsrB binding to endogenous AI-2 was evaluated by an AI-2 assay, which showed that the recombinant APEC-LsrB (BL21) bound to endogenous AI-2 produced by wild-type BL21 and released AI-2 at 55 °C. However, the luxS mutant BL21∆luxS did not produce AI-2 so that no AI-2 could be released from APEC-LsrB (BL21∆luxS). B The activity of APEC-LsrB binding to exogenous AI-2 was evaluated by an AI-2 assay, which showed that APEC-LsrB (BL21∆luxS) could bind to exogenous AI-2, resulting in low AI-2 activity in cell-free culture fluid (CF). However, APEC-LsrB (BL21) could not bind to exogenous AI-2 as it was hindered by endogenous AI-2 (produced by wild-type BL21), which resulted in high AI-2 activity in CF.