Effect of zinc concentration in Mueller-Hinton agar on susceptibility of Pseudomonas aeruginosa to imipenem

The susceptibility of Pseudomonas aeruginosa to imipenem has been shown to vary according to zinc concentration in the media. MICs of imipenem for 68 unique clinical isolates of P. aeruginosa were determined in media supplemented with zinc at concentrations between 0.5 and 6.0 mu g/ml. In agar containing up to 3 mu g of zinc/ml, 75 to 82% of the strains were susceptible to imipenem at an MIC of less than or equal to 4 mu g/ml. In agar supplemented to contain 6 mu g of zinc/ml, however, only 40% of the strains were susceptible to imipenem. Manufacturers should ensure that the concentration of zinc in commercial media is below 3 mu g/ml to avoid false classification of isolates as resistant to imipenem.