Effect of media composition on the susceptibility of Xanthomonas maltophilia to beta-lactam antibiotics.

The susceptibility of Xanthomonas maltophilia strains to beta-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 x 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. This strain behaved inconsistently on Iso-Sensitest agar. Previous studies have shown that medium-dependent susceptibility in X. maltophilia is not related to beta-lactamase expression. The present study demonstrated that 22 and 25 kDa outer membrane proteins were induced on media with higher nutrient concentrations. The possible relationship of these proteins to sensitivity is considered.