Susceptibilities of 123 strains of Xanthomonas maltophilia to eight beta-lactams (including beta-lactam-beta-lactamase inhibitor combinations) and ciprofloxacin tested by five methods

Abstract

This study evaluated the susceptibility of 123 Xanthomonas maltophilia strains to ticarcillin, ticarcillin-clavulanate, ampicillin, amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin, piperacillin-tazobactam, imipenem, and ciprofloxacin by Kirby-Bauer disk, E test, and Sensititre dehydrated microdilution MIC and conventional agar dilution MIC methodology. Intermediate susceptibility breakpoints for members of the family Enterobacteriaceae were used. When results were analyzed as MICs for 50 and 90% of the strains tested and percentages of strains susceptible at the breakpoint, good correlation between the methods was observed, with ticarcillin-clavulanate clearly the most active beta-lactam by all four methods. However, when the various methods were compared with the agar dilution methodology by regression analysis, poor r2 values (0.3 to 0.7) were obtained for compounds with sufficient on-scale values to permit analysis. When the number of strains with log2 ratios of reference agar dilution MICs to test MICs of +3 to -3 were analyzed, correlation was also poor, with many major and very major discrepancies for all methods tested. Results obtained with time-kill studies of nine strains with discrepant ticarcillin-clavulanate MICs appeared to correlate best when compared at 24 h with agar dilution MICs. The concentration of ticarcillin-clavulanate required to reduce the colony count by > or = 2 log10 reduction values for eight of nine strains compared with that for growth controls was < or = 16.0/2.0 micrograms/ml at 6 h and ranged from 16.0/2.0 micrograms/ml to 128.0/2.0 micrograms/ml at 24 h. The susceptibility method of choice for X. maltophilia has not yet been standardized, but time-kill studies correlated best with agar dilution MICs.