In vitro activity of newer fluoroquinolones against Stenotrophomonas maltophilia.

Sir, We have read with interest the article published recently by Weiss et al., comparing the in vitro activity of seven fluoroquinolones against Stenotrophomonas maltophilia. Most published studies concerning this microorganism agree about its increasing importance as a nosocomial pathogen and about the therapeutic problems caused by its frequent resistance to most -lactam and aminoglycoside antibiotics. The most active -lactams, such as the combination ticarcillin/clavulanic acid and ceftazidime, and aminoglycosides are usually efficacious when infections are caused by susceptible strains. When strains are resistant to these groups, the only alternative is trimethoprim– sulphamethoxazole, a combination whose activity is only bacteriostatic against most strains. Older fluoroquinolones, such as ciprofloxacin and ofloxacin can be useful in some cases, but resistance rates are frequently high. Newer fluoroquinolones have been designed mainly to improve their activity against Gram-positives. The increase in activity against Gram-negatives, as compared with ciprofloxacin, is much lower. Among 326 clinical strains of S. maltophilia, Weiss et al. found MIC50s and MIC90s of 4 and 16 mg/L for ciprofloxacin. The increase in activity of newer fluoroquinolones, comparing MIC50s, ranges from four-fold (levofloxacin and gatifloxacin) to 16-fold (clinafloxacin), and comparing MIC90s ranges from two-fold (levofloxacin and gatifloxacin) to eight-fold (clinafloxacin). Although these newer fluoroquinolones may be a useful alternative for the treatment of infections caused by S. maltophilia, their main usefulness is probably derived from their activity against -lactam-resistant and aminoglycoside-resistant strains. We have studied the activity of levofloxacin, trovafloxacin, grepafloxacin, moxifloxacin, gatifloxacin and sitafloxacin against 22 ticarcillin-resistant, ceftazidime-resistant S. maltophilia clinical strains, by the agar dilution method. MICs obtained are given in the Table. MICs of most fluoroquinolones obtained are c. 10-fold higher than MICs found by Weiss et al. against non-selected S. maltophilia clinical strains. The MIC50 of ofloxacin (4 mg/L) is two-fold lower than the MIC50 published by Weiss et al. for ciprofloxacin, but the MIC90 is significantly higher. Similar results are seen for most of the fluoroquinolones tested. MIC50s of gatifloxacin, moxifloxacin and trovafloxacin (4 mg/L) were fourto eight-fold higher than MICs given by Weiss et al., but the MIC90 increase ranged between eight-fold (gatifloxacin) and 32-fold (moxifloxacin). Two drugs that are not included in the study by Weiss et al. showed a different behaviour. Grepafloxacin had an MIC50 similar to those of other fluoroquinolones tested (4 mg/L), but the MIC90 was lower (16 mg/L). Sitafloxacin showed the best activity against these strains, with MIC50 and MIC90 fourto eight-fold lower than those of trovafloxacin. According to these results, most newer fluoroquinolones are not significantly better than ofloxacin or other older fluoroquinolones against -lactam multiresistant strains of S. maltophilia. No common mechanisms of resistance between -lactams and fluoroquinolones have been described in S. maltophilia. The efflux mechanisms that pump fluoroquinolones described in S. maltophilia do not affect -lactams. These higher MICs among -lactam multiresistant strains of S. maltophilia might occur as a result of epidemiological factors, such as the frequent use of fluoroquinolones in patients with -lactam multiCorrespondence Journal of Antimicrobial Chemotherapy (2000) 46, 323–342