Abstract
One hundred isolates of Pseudomonas and Enterobacteriaceae, of which 85 were chosen because of their resistance to gentamicin or amikacin, were tested for susceptibility to netilmicin (SCH 20569), a new semisynthetic derivative of sisomicin, and to four other aminoglycosides. Tests were performed in Mueller-Hinton agar and, with 43 of these isolates, also in Mueller-Hinton broth. Most isolates of Escherichia coli, Klebsiella, Enterobacter, Citrobacter, and Serratia that were gentamicin resistant proved to be susceptible to netilmicin and amikacin. Tests of representative isolates of this group showed that they owed their resistance to the production of aminoglycoside-adenylylating enzymes. Four isolates of Serratia, detected by their resistance to amikacin, were also highly resistant to netilmicin but were susceptible to gentamicin. These isolates produced aminoglycoside-acetylating enzymes. Gentamicin-resistant Proteus and Providencia were, in general, highly resistant to netilmicin but were susceptible to amikacin. These isolates also produced aminoglycoside-acetylating enzymes. Most gentamicin-resistant strains of Pseudomonas were resistant to netilmicin, either by enzymatic aminoglycoside modification or by other undefined mechanisms. Thus, like amikacin, netilmicin extends the aminoglycoside susceptibility pattern of Enterobacteriaceae to include gentamicin-resistant isolates that produce aminoglycoside-adenylylating enzymes. It is ineffective against strains, some of them susceptible to amikacin, gentamicin, or tobramycin, that produce aminoglycoside-acetylating enzymes.