Cefoxitin is both an inhibitor of class A beta-lactamase of Xanthomonas campestris pv. campestris str. 17 and an inducer of its gene

In Xanthomonas campestris pv. campestris (Xcc), the chromosomally encoded class A β-lactamase (Blaxcc) is expressed at a high basal level in the absence of an inducer and its expression is inducible by ampicillin. Like most of the class A β-lactamases, Blaxcc cannot digest the β-lactam ring of cefoxitin. However, Xcc exhibits high basal resistance to cefoxitin. A promoter activity assay with Pblaxcc-lacZ transcriptional fusion from a plasmid and western blotting using anti-Blaxcc polyclonal antibodies demonstrated that a sublethal concentration of cefoxitin can induce expression of the blaxcc gene. Cefoxitin can be used as an inducer to study blaxcc expression in blaxcc-deficient mutants such as Xcbla and XcampR. Addition of cefoxitin to the Bla enzyme solution blocks β-lactamase activity, suggesting that cefoxitin is an inhibitor of Blaxcc. This explains why there is no β-lactamase activity in cefoxitin-induced Xcc. A significant synergistic effect was also observed between cefoxitin and other β-lactam antibiotics. A homology model demonstrated that the methoxy-group in the β-lactam ring of cefoxitin tends to displace the conserved catalytic water molecule into the active cavity of Blaxcc, thus leading to formation of a stable but inactive acyl-Blaxcc intermediate.