In vitro susceptibility of clinically important bacteria to amikacin: correlation of results of broth dilution and disk sensitivity tests and effect of medium composition.

Abstract

Minimal inhibitory concentrations of amikacin for 296 strains of gram-positive and gram-negative bacteria determined in Mueller-Hinton broth by the method recommended by the International Collaborative Study Group confirmed previous observations that amikacin is effective against current clinical isolates sensitive to gentamicin and that strains moderatly sensitive or resistant to gentamicin are often inhibited by clinically achievable concentrations. A simple linear regression analysis was used for comparison of zone sizes obtained with 10-mug disks of amikacin with minimal inhibitory concentrations obtained by the tube dilution technique. For 152 strains, including seven species of Enterobacteriaceae, the correleation coefficient (r) was -0.79 on Oxoid diagnostic senstivity test agar (which yields zone sizes similar to those obtained on Mueller-Hinton medium) and -0.78 on Oxoid isotonic sensitest agar (which contains cations in isotonic concentrations); the figures were -0.74 and -0.66, respectively, for 80 strains of Pseudomonas aeruginosa. Zone sizes for enteric gram-negative bacilli were on the average 2-3 mm smaller on isotonic sensitest agar than on diagnostic sensitivity test agar, and about 6 mm smaller for strains of P. aeruginosa and Providencia. Minimal inhibitory concentrations of amikacin can be estimated satisfactorily by extrapolation of results of agar diffusion tests, but the composition of the medium, particularly its calcium and magnesium content, should be defined precisely.