BAL30072 is a new monocyclic beta-lactam antibiotic belonging to the sulfactams. Its spectrum of activity against significant Gram-negative pathogens with beta-lactam-resistant phenotypes was evaluated and was compared with the activities of reference drugs, including aztreonam, ceftazidime, cefepime, meropenem, imipenem, and piperacillin-tazobactam. BAL30072 showed potent activity against multidrug-resistant (MDR) Pseudomonas aeruginosa and Acinetobacter sp. isolates, including many carbapenem-resistant strains. The MIC(90)s were 4 microg/ml for MDR Acinetobacter spp. and 8 microg/ml for MDR P. aeruginosa, whereas the MIC(90) of meropenem for the same sets of isolates was >32 microg/ml. BAL30072 was bactericidal against both Acinetobacter spp. and P. aeruginosa, even against strains that produced metallo-beta-lactamases that conferred resistance to all other beta-lactams tested, including aztreonam. It was also active against many species of MDR isolates of the Enterobacteriaceae family, including isolates that had a class A carbapenemase or a metallo-beta-lactamase. Unlike other monocyclic beta-lactams, BAL30072 was found to trigger the spheroplasting and lysis of Escherichia coli rather than the formation of extensive filaments. The basis for this unusual property is its inhibition of the bifunctional penicillin-binding proteins PBP 1a and PBP 1b, in addition to its high affinity for PBP 3, which is the target of monobactams, such as aztreonam.