Influence of medium composition on bacterial susceptibility testing to gentamicin and netilmicin

The composition of the growth medium, especially the presence of divalent cations Mg2+ and Ca2+, affects the susceptibility of bacteria to aminoglycoside antibiotics. Regression lines were calculated for gentamicin and netilmicin with 146 fresh clinical isolates and by the use of the standardized disc diffusion and agar dilution methods on PDM-agar or Mueller-Hinton agar. At the break-point less than or equal to 4 micrograms/ml, denoting susceptibility different zones of inhibition were obtained on the different agar media. When performing regression line analysis from tests with 56 isolates by the disc diffusion method on Mueller-Hinton agar versus MIC's in Mueller-Hinton broth, the resulting lines were different from those when performing the MIC's in agar. Furthermore, the bacteria were twofold or more susceptible in unsupplemented Mueller-Hinton broth than on Mueller-Hinton agar.