Modified nitrocefin‐EDTA method to differentially quantify the induced L1 and L2 β‐lactamases in Stenotrophomonas maltophilia

Aim: To estimate the ethylene diamine tetraacetic acid (EDTA) concentration at which the L1 enzyme activity in the cell extracts of Stenotrophomonas maltophilia can be mostly inhibited. Methods and Results: The effective inhibition concentration of EDTA against the L1 enzyme in the cell extracts was firstly evaluated by using the L2 isogenic mutant of S. maltophilia KJ, KJΔL2, as the assayed strain. Approximately 92% L1 activity was inhibited by 10 mmol l^?1 EDTA, which is 100‐fold higher than that from previously reported protocols (0·1 mmol l^?1). Three phylogenetic clusters of L1 proteins were revealed from 11 clinical S. maltophilia isolates, with a L1 protein divergence of 0–11%. The EDTA concentration required to inhibit the L1 enzymes of different phylogenetic clusters was estimated to be 10 mmol l^?1. Conclusion: The previous nitrocefin‐EDTA protocol for differentially quantifying the L1 and L2 activity in the cell extracts has been modified by raising the added EDTA concentration to 10 mmol l^?1. Significance and Impact of the Study: A rapid and accurate method for determination of L1 and L2 activity will provide a convenient tool for enzyme characterization and induction mechanism study of S. maltophilia.