Moxalactam-tobramycin-resistant Pseudomonas aeruginosa Isolates in Patients with Cystic Fibrosis

In vitro studies have shown synergistic killing activity against Pseudomonas aeruginosa (PA) with the combination of an aminoglycoside and moxalactam, a new beta-lactam agent. We describe three patients with cystic fibrosis (CF) with PA isolates that were resistant to all single-agent antibiotics, but sensitive to the combination of moxalactam-tobramycin. Initially, all patients had a good clinical response to this combination. However, during a second course of therapy, there was clinical deterioration coincident with the rapid emergence of moxalactam-tobramycin-resistant PA isolates.