In vitro activity of aztreonam--a new monocyclic beta-lactam antibiotic

The in vitro antibacterial activity of aztreonam (SQ 26776), a new beta-lactam antibiotic, was measured by the agar dilution technique. Aztreonam is known to have a narrow spectrum with activity only against Gram negative bacteria. The strains tested were 223 clinical isolates from blood cultures obtained at Rigshospitalet, Copenhagen, Denmark. Its activity against E. coli, Klebsiella spp., Proteus spp., Enterobacter spp., Citrobacter, Salmonella typhimurium and Serratia marcescens was satisfactory with MIC values normally below 0.5 mg/l. However, six out of 135 strains of E. coli showed surprisingly high MIC values of eight and 16 mg/l. The activity against Pseudomonas spp. and Acinetobacter spp. were limited, but the majority were inhibited by concentrations of aztreonam between 2.0 and 8.0 mg/l. The MIC values for the tested anaeobic bacteria were high, ranging from 8.0 to above 512 mg/l. With its narrow spectrum of activity, aztreonam seems to be a valuable addition to the antibiotic arsenal. Clinical studies will determine its real value.