Effect of media composition on the susceptibility of Xanthomonas mahophilia to β-lactam antibiotics

The susceptibility of Xanthomonas maltophilia strains to β-lactams was shown to depend on the concentrations at which individual media were prepared. MIC and disc susceptibility tests were performed on solidified Mueller-Hinton and Iso-Sensitest media prepared at 0.1, 0.3, 1, and 3× the concentrations recommended by the manufacturers. Nine of ten X. maltophilia strains tested showed increasing sensitivity to meropenem, cefotaxime, cefoperazone, piperacillin and latamoxef as the nutrient concentrations of the two media were increased. The opposite pattern was found with one strain (NCTC 10257) on Mueller-Hinton agar