IN-VITRO SUSCEPTIBILITY OF 124 XANTHOMONAS-MALTOPHILIA (STENOTROPHOMONAS-MALTOPHILIA) ISOLATES - COMPARISON OF THE AGAR DILUTION METHOD WITH THE E-TEST AND 2 AGAR DIFFUSION METHODS

The in vitro susceptibility of 124 Xanthomonas maltophilia isolates wa s tested by four methods: Agar dilution (reference method), E-test, a disk diffusion and a tablet diffusion method. Trimethoprim-sulfamethox azole had the highest activity against X. maltophilia, followed by a c ombination of aztreonam-clavulanic acid at different ratios, the ratio 1:1 being the most active with a susceptibility rate of 85% as compar ed to 2% for aztreonam alone. Addition of the beta-lactamase inhibitor tazobactam to piperacillin enhanced the rate of susceptible isolates from 31% to 53%. Relatively few isolates were susceptible to ciproflox acin (27%) and gentamicin (9%). Generally, the disk diffusion method h ad a considerably higher frequency of ''very major'' discrepancies whe n compared with the agar dilution method than with the other methods. The susceptibility of X. maltophilia to trimethoprim-sulfame-thoxazole and ciprofloxacin could reliably be determined by all the diffusion m ethods tested, but otherwise the agar dilution method is to be preferr ed. A standardized and reliable diffusion method for susceptibility te sting of X. maltophilia remains to be found. Trimethoprim-sulfamethoxa zole must be considered the drug of choice in the treatment of severe X. maltophilia infections. The combination aztreonam-clavulanic acid i s promising, but must be proved in a clinical setting.