Chromosomally mediated high-level gentamicin resistance in Streptococcus mitis.

Four blood culture isolates of Streptococcus mitis were found to be resistant to penicillin (MIC, 16 to 32 micrograms/ml) and gentamicin (MIC, 128 or 1,000 micrograms/ml), and the two antibiotics demonstrated a lack of in vitro synergy. As shown by polymerase chain reaction assays, the structural gene known to encode high-level gentamicin resistance in Enterococcus faecalis, Enterococcus faecium, and Streptococcus agalactiae was also present in all four S. mitis strains. Attempts to isolate plasmids were unsuccessful, but an oligonucleotide probe derived from the gentamicin resistance gene hybridized to distinct restriction fragments of genomic DNA, suggesting that the resistance genes in these strains are integrated into the bacterial chromosome.