Activities of tobramycin and six other antibiotics against Pseudomonas aeruginosa isolates from patients with cystic fibrosis

The in vitro activity of tobramycin was compared with those of six other an timicrobial agents against 1,240 Pseudomonas aeruginosa isolates collected from 508 patients with cystic fibrosis during pretreatment visits as part o f the phase III clinical trials of tobramycin solution for inhalation. The tobramycin MIC at which 50% of isolates are inhibited (MIC50) and MIC90 wer e 1 and 8 mu g/ml, respectively, Tobramycin was the most active drug tested and also showed good activity against isolates resistant to multiple antib iotics. The isolates were less frequently resistant to tobramycin (5.4%) th an to ceftazidime (11.1%), aztreonam (11.9%), amikacin (13.1%), ticarcillin (16.7%), gentamicin (19.3%), or ciprofloxacin (20.7%), For all antibiotics tested, nonmucoid isolates were more resistant than mucoid isolates. Of 56 isolates for which the tobramycin MIC was greater than or equal to 16 mu g /ml and that were investigated for resistance mechanisms, only 7 (12.5%) we re shown to possess known aminoglycoside-modifying enzymes; the remaining w ere presumably resistant by an incompletely understood mechanism often refe rred to as "impermeability".