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, ticarcillinclavulanate, 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%Yo 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 P-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 .2 log10 reduction values for eight of nine strains compared with that for growth controls was <16.0/2.0 ,ug/ml at 6 h and ranged from 16.0/2.0 ,ug/ml to 128.0/2.0 ,ug/ml at 24 h. Parallel kill curves with piperacillin4azobactam yielded .2 log1o reduction values ranging from -16.0/4.0 ,ug/ml to 128.0/4.0 ,ug/ml at 6 h and >128.0/4.0 ,ug/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.Infections with Xanthomonas maltophilia are increasingly encountered, especially in immunocompromised patients (9,(14)(15)(16)25). This organism is inherently resistant to most P-lactam and non-13-lactam agents by virtue of permeability barriers and elaboration of at least two ,B-lactamases; trimethoprim-sulfamethoxazole and, perhaps, moxalactam are currently suggested as the drugs of choice for treating infections with this organism (4, 7, 9-11, 14-16, 25). P-Lactam resistance in X maltophilia is mediated by production of at least two P-lactamases-a zinc-dependent metalloenzyme that breaks down carbapenems and is resistant to P-lactamase inhibitors and a cephalosporinase that is susceptible to 1-lactamase inhibitors (3,23,24). A previous preliminary study by our group, as well as reports by other workers, have documented apparent increased susceptibility of X maltophilia strains to ticarcillin-clavulanate by disk diffusion and MIC methodology compared with susceptibility to ticarcillin, ampicillin, amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin, and piperacillin-tazobactam (8, 19, 21 ampicillin, amoxicillin-clavulanate, amp...

Comparison of different antimicrobial susceptibility testing methods for Stenotrophomonas maltophilia and results of synergy testing

“…However, as results of this study also revealed, there are discrepant susceptibility data, particularly with disc diffusion for colistin, ticarcillin/clavulanate, and ciprofloxacin and with the E-test for ticarcillin/clavulanate, ciprofloxacin and, to a lesser extent, ceftazidime. There have been several reports related to the discordance between different susceptibility testing methods for S. maltophilia [6][7][8][23][24][25]. It has been previously reported by the NCCLS that for an acceptable performance of susceptibility tests, the overall categorical error rate should be \10%, of these B1.5% being VMEs and B3% being MEs [17].…”

Ability of Latin America laboratories to detect antimicrobial resistance patterns: experience of the SENTRY antimicrobial surveillance program (1997-2000)

“…Pankuch et al [14] also found a poor correlation among five susceptibility testing methods evaluated against eight β-lactams (including β-lactam-β-lactamase inhibitor combinations) and ciprofloxacin antimicrobial agents against S. maltophilia.…”

<i>Stenotrophomonas (Xanthomonas) maltophilia:</i> In vitro Susceptibility to Selected Antimicrobial Drugs, Single and Combined, with and without Defibrinated Human Blood

“…Agar dilution and microtiter broth dilution tests yielded MICs that correlated rather well (table 2). Previously, Pankuch et al [21] found time-kill curve assays to correlate best with MICs obtained with the agar dilution technique. Determination of FICs (table 3) with the aid of microtiter checkerboard titrations in broth and utilizing various antibiotic combinations revealed the combinations of rifampin + polymyxin B, ceftazidime plus ofloxacin, and cotrimoxazole plus timentin to exert additive/synergistic effects against the four test strains of S. maltophilia: these findings were expected as based on data presented by previous investigators [12][13][14][15]27].…”