Comparison of in vitro activity of Sch 21420, a gentamicin B derivative, with those of amikacin, gentamicin, netilmicin, sisomicin, and tobramycin.

Sch 21420 is a new aminoglycoside synthesized from gentamicin B. Susceptibility tests with Sch 21420, amikacin, gentamicin, netilmicin, sisomicin, and tobramycin were performed on a variety of bacterial species including 44 with known mechanisms of resistance to aminoglycosides. Sch 21420 and amikacin had similar effects on all except Haemophilus influenzae and Neisseria species, which were more susceptible to amikacin. Except with some strains of Serratia marcescens, the drugs used were bactericidal. Sch 21420 and amikacin were more stable than the other four aminoglycosides in the presence of the inactivating enzymes produced by some strains. Strains which were very resistant to Sch 21420 and emikacin either were permeability mutants or produced AAC (6')-I inactivating enzyme. The effect of cations on the susceptibilities of these strains to Sch 21420 and amikacin was seen mostly with Pseudomonas aeruginosa and to Sch 21420 with Acinetobacter. Cations did not affect the susceptibilities of other Pseudomonas species, Enterobacteriaceae, Staphylococcus aureus, or Streptococcus faecalis to Sch 21420 or amikacin.