Antibiotic Susceptibility Testing of Pseudomonas aeruginosa: Selection of a Control Strain and Criteria for Magnesium and Calcium Content in Media

Because of the influence of Mg+ + and Ca+ + on the susceptibility of Pseudomonas aeruginosa to aminoglycosides and polymyxins, we studied the practical effects of variations in these cations in commercial Mueller-Hinton media. Batches of Mueller-Hinton agar sold by three firms contained 9.4–32.6 mg of Mg+ + and 31.8–86.0 mg of Ca + + per liter; Mueller-Hinton broth had 2.0–5.6 mg of Mg+ + and 4.1–35.8 mg of Ca++ per liter. Aminoglycoside MICs increased 16- to 32-fold with increases in Mg+ + or Ca+ + in media. Dilution tests with currently available Mueller-Hinton broth were unreliable. Disk diffusion and agar and broth dilution methods gave comparable results with media prepared to contain equivalent amounts of Mg+ + and Ca+ +. We recommend that Mueller-Hinton media contain 20–35 mg of Mg++ and 50–100 mg of Ca++ per liter. Acceptable performance of Mueller-Hinton media with a control P. aeruginosa should be assured by the manufacturer before sale and by the clinical laboratory during use.