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 (Bla(xcc)) 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, Bla(xcc) cannot digest the β-lactam ring of cefoxitin. However, Xcc exhibits high basal resistance to cefoxitin. A promoter activity assay with P(blaxcc)-lacZ transcriptional fusion from a plasmid and western blotting using anti-Bla(xcc) polyclonal antibodies demonstrated that a sublethal concentration of cefoxitin can induce expression of the bla(xcc) gene. Cefoxitin can be used as an inducer to study bla(xcc) expression in bla(xcc)-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 Bla(xcc). 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 Bla(xcc), thus leading to formation of a stable but inactive acyl-Bla(xcc) intermediate.