Antibacterial activity of the monobactam SQ 26,776 against antibiotic resistant enterobacteria, including Serratia spp.

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Abstract
The activity in vitro of the new monobactam SQ 26,776 (SQ) was determined against 493 clinical bacterial isolates by an agar dilution technique. Ampicillin, cefazolin, cefotaxime, cefoperazone, ceftazidime, gentamicin, mezlocillin, moxalactam, netilmicin and piperacillin were used as comparator agents. The activity of SQ and other agents against a control strain of Escherichia coli and its R-plasmid-bearing transconjugants was also determined. SQ showed no useful activity against Gram-positive species and Bacteroides fragills , but was highly active against Gram-negative aerobes with an MIC 50 , lower than 1 mg/l for all species tested, excepting Actinetobacter species and Pseudomonas aeruginosa , even at an inoculum of 10 6 cfu. The MIC 90 , was generally within one or two dilutions of the MIC 50 ,. The activity of SQ against E. coli , Citrobacter spp, Salmonella spp, and Proteus mirabilis was very high (MIC 50 , 0.06 mg/l) and exceeded that of the so-called third generation cephalosponns tested. SQ was generally active against enterobacteria resistant to older antimicrobials, including Serratia species, inhibiting all strains tested of that genus at 8 mg/l with MIC 50 and 90 and 0.12 and 4mg/l, respectively. The MIC 90 , for Ps aeruginosa was 8 mg/l. Activity against Haemophilus influenzae was very high. The above results implied a high degree of stability to β -lactamases, and this was borne out by the similar MICs to control strain of E. coli transconjugants with plasmids coding for TEM-1, TEM-2, OXA-1, OXA-2, and SMV-1 β -lactamases. We concluded that SQ was a highly active agent deserving clinical investigation in infections caused by enterobacteria and Ps. aeruginosa , particularly in situations where strains resistant to other agents are a problem.

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