Effect of the Concentrations of Magnesium and Calcium on the In-Vitro Susceptibility of Pseudomonas aeruginosa to Gentamicin

Abstract

The concentrations of magnesium and calcium in culture media, which differ markedly among various media and among lots of the same media, significantly affect in-vitro susceptibility of Pseudomonas aeruginosa to gentamicin. The MIC for 18 isolates was compared in Mueller-Hinton broth (MHB) and on four solid media. Defining sensitivity as an MIC of 5 µg/ml or less, the incidence of P. aeruginosa resistant to gentamicin varied between zero and 72%. The average MIC for 128 isolates was 3.7 ± 0.2 (se) µg/ml with MHB solidified with agar (Mg++, 1.3 mg/100 ml, and Ca+ +, 4.3 mg/100 ml) compared with 9.3 ± 0.5 (se) µg/ ml with Mueller-Hinton agar (MHA). By increasing the magnesium concentration from 0.3 mg/100 ml to 4.5 mg/ 100 ml in MHB, the average MIC increased from 1.8 µg/ml to 5.6 µg/ml (Ca++ constant). When the calcium concentration was increased from 1.2 mg/100 ml to 10.5 mg/100 ml in MHB, the average MIC increased from 0.57 µg/ml to 2.1 µg/ml (Mg++ constant). These data indicate the need to standardize the concentrations of magnesium and calcium in media if consistent, reproducible results are to be achieved in testing susceptibility of P. aeruginosa to gentamicin.