Resistance to gentamicin and related aminoglycosides in Staphylococcus aureus isolated in Brazil

Isolates of Staphylococcus aureus obtained from a Brazilian university hosp ital were characterized in relation to resistance to gentamicin and related aminoglycosides. Thirty-six isolates were susceptible to methicillin (MSSA ) and 14 were resistant (MRSA). All isolates were sensitive to nucleic acid -binding compounds. All MRSA isolates and one MSSA isolate were demonstrate d to be resistant to gentamicin and were coincidentally resistant to amikac in, kanamycin, neomycin and tobramycin. Among the gentamicin sensitive MSSA isolates, five isolates were found to be resistant only to kanamycin/neomy cin. The resistance to gentamicin (and related aminoglycosides: kanamycin a nd tobramycin) must be due to AAC(6')-APH(2 ") activity. As these isolates also showed resistance to neomycin, they must carry an additional genetic e lement, probably the one responsible for APH(3')III activity, which account s for the high level of resistance to kanamycin and to amikacin. The resist ance to kanamycin/neomycin in the gentamicin sensitive isolates could not b e attributed to the AAD(4')(4 ") activity because of the tobramycin sensiti vity, and so could be ascribed to the APH(3')III activity. Curing and trans fer experiments, as well as electrophoresis procedures, indicate that genta micin resistance in Staph. nul-eus strains here studied has, characteristic ally, chromosomal localization.