Stenotrophomonas maltophilia KH has two acid β-lactamases with isoelectric points (pIs) of 4.6 and 5.4, and several basic β-lactamases (pIs >7.0) that produce a ladder-shaped pattern by IEF. An isogenic L2 mutant, KHL2xylE, was constructed by gene replacement. From IEF and native PAGE zymograms of strains KH and KHL2xylE, it was demonstrated that the basic β-lactamases and the acid β-lactamase with pI 5.4 are encoded by the same L2 gene and that the active types of these L2 charge variants were dependent on the buffer pH. The β-lactamase activities of these L2 charge variants in phosphate buffer at pH 7.0 and 8.0 were 1075±29 and 1114±81 U mg−1, respectively. These results indicate that L2 charge variants give S. maltophilia a better chance of adapting and surviving in response to changes in the environment.
