Amikacin, an aminoglycoside with marked activity against antibiotic-resistant clinical isolates

A total of 319 clinical isolates known to be resistant to one or more aminoglycoside antibiotics were tested for their susceptibility to 10 aminoglycosides. The percentages of isolates found by an agar dilution method to be susceptible were: amikacin, 83.7%; tobramycin, 41.4%; butirosin A, 33.2%; dideoxykanamycin B, 32.6%; gentamicin C, 27.3%; lividomycin A, 17.6%; neomycin B, 10.7%; paromomycin, 10.3%; kanamycin A, 10.0%; and ribostamycin, 7.2%. The effectiveness of the antibiotics was related to their degree of resistance to bacterial enzymes; e.g., of the nine enzymes known to inactivate antibiotics containing 2-deoxystreptamine, amikacin was affected by one enzyme, tobramycin by five, and gentamicin and kanamycin by six. Examination of cell-free extracts from the 52 strains resistant to amikacin revealed that only four contained the amikacin-inactivating enzyme aminoglycoside-6'-acetyltransferase, a finding indicating that this mechanism of resistance is rare. Other experiments suggest that most amikacin-resistant strains, which are almost invariably resistant to all aminoglycosides, lack the ability to accumulate effectively either amikacin or presumably the other antibiotics intracellularly.