The authors compared the in vitro activity of
imipenem,
ceftazidime and
cefotaxime against 100 strains of Acinetobacter calcoaceticus isolated in 1986 and 1987. The minimal inhibitory and bactericidal concentrations (MICs and MBCs) were determined by the
agar dilution and broth microdilution methods respectively, with and without 50 per cent of human serum in the medium to evaluate the possible influence of protein binding.
Imipenem was the most active of the three drugs against Acinetobacter, including
beta-lactamase producing strains. The MICs 50 and 90 of
imipenem were 0.18 micrograms/ml and 0.48 micrograms/ml respectively, as opposed to 5.16 and 14.61 micrograms/ml for
ceftazidime, 16 and 75.6 micrograms/ml for
cefotaxime. No change was noted in the susceptibility of Acinetobacter to
imipenem from 1981 to 1987. The geometric mean MIC of
imipenem was 0.25 micrograms/ml. Susceptibility remained unchanged for
ceftazidime and
cefotaxime but the geometric mean MICs were higher, being 7.29 and 22.8 micrograms/ml respectively.
Imipenem had the highest bactericidal activity, with a mean MBC/MIC ratio of 1.16. The presence of human serum did not influence the results, due to the low protein binding of all three
antibiotics. It is concluded that
imipenem is one of the major
antibiotics available for the treatment of nosocomial
Acinetobacter infections. However, a few resistant strains have recently been isolated, confirming the need for epidemiological surveillance of bacterial resistance to this
antibiotic.