Netilmicin (Sch 20569), a semisynthetic aminoglycoside related to gentamicin C 1a , was evaluated in vitro in agar dilution testing against 224 different clinical isolates of gentamicin-resistant Enterobacteriaceae and Pseudomonas aeruginosa in parallel with amikacin, gentamicin, sisomicin, and tobramycin. Netilmicin showed a very high degree of activity against gentamicin-resistant organisms, but amikacin was more active in vitro, particularly against Providencia stuartii and P. aeruginosa . Sisomicin and tobramycin were consistently less active than either netilmicin or amikacin. Netilmicin was bactericidal in broth testing against P. aeruginosa . Netilmicin showed a greater difference between results with agar and broth dilution testing than did amikacin.

The in vitro antibacterial activity of four 18-lactam antibiotics (cefatrizine