[In vitro antibacterial activity of apalcillin on aerobic bacteria and the regression curve]

Minimal inhibitory concentrations (MICs) of apalcillin (APL) were evaluated by agar dilution on 1,201 bacterial strains isolated simultaneously in four university hospitals; agar diffusion tests (disks APL: 75 micrograms) were performed on these strains to establish relationship between MIC and zone diameters. For Enterobacteriaceae naturally non beta-lactamase-producing (E. coli and P. mirabilis), mode MIC was 0.5 microgram/ml; some acquired penicillinase-producing strains were only inhibited by concentrations greater than or equal to 16. Chromosomal penicillinase producing Klebsiella were inhibited by 2 to 8 micrograms/ml but APL was inactive on acquired penicillinase-producing strains. For chromosomal cephalosporinase-producing species (Enterobacter, Citrobacter, Serratia, indole + Proteus and Providencia) two populations of strains were observed: one sensitive and the second resistant to carboxypenicillins: on the first population, mode MIC of APL was 1 to 4 micrograms/ml; on the second MIC were generally greater than or equal to 64 micrograms/ml. P. aeruginosa strains sensitive to carboxypenicillins were inhibited by 1 and 2 micrograms/ml; this activity was diminished on strains resistant to these antibiotics (MIC APL 8-32). MIC of Acinetobacter varied to 0.25 to greater than 128 with a majority of strains inhibited by 4 to 64 micrograms/ml. APL was active against non penicillinase producing Staphylococci; mode MIC was 2 micrograms/ml for Enterococci. Correlation coefficient of regression curve was 0.87. For critical concentrations less than or equal to 8 and greater than 64 micrograms/ml, critical diameters could be greater than or equal to 19 and less than 12 mm.