Disruption of <em>mpl</em> Activates β-Lactamase Production in Stenotrophomonas maltophilia and Pseudomonas aeruginosa Clinical Isolates

ABSTRACT The hyperproduction of chromosomally encoded β-lactamases is a key method of acquired resistance to ceftazidime, aztreonam, and, when seen in backgrounds having reduced envelope permeability, carbapenems. Here, we show that the loss of Mpl, a UDP-muramic acid/peptide ligase, is a common and previously overlooked cause of chromosomally encoded β-lactamase hyperproduction in clinical isolates of Stenotrophomonas maltophilia and Pseudomonas aeruginosa , important pathogens notorious for their β-lactam-resistant phenotypes. </jats:p