Disruption of mpl Activates β-Lactamase Production in Stenotrophomonas maltophilia and Pseudomonas aeruginosa Clinical Isolates.

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.