High-level aminoglycoside resistance mediated by aminoglycoside-modifying enzymes among viridans streptococci: implications for the therapy for endocarditis.

Two strains of Streptococcus mitis with high-level resistance to streptomycin and kanamycin were identified from a collection of multiple-resistant streptococci. The strains were resistant to penicillin-streptomycin and penicillin-kanamycin synergy but were synergistically killed by penicillin-gentamicin. Both strains adenylylated streptomycin at the 3'' position. Neither strain could be cured of its streptomycin resistance by novobiocin, acridine orange, or ethidium bromide. Attempts to pass the resistance into two recipients were unsuccessful. Agarose gel electrophoresis and cesium chloride gradients failed to reveal plasmid bands. This is the first report of high-level streptomycin resistance mediated by modifying enzymes among viridans streptococci. It is possible that endemic streptomycin resistance among viridans streptococci will develop.