Mobile rRNA methylase genes coding for erythromycin resistance in Actinobacillus actinomycetemcomitans.

We found that 17 (68%) of 25 Actinobacillus actinomycetemcomitans isolated from periodontally diseased sites had MICs of erythromycin > or = 8 mg/L. The isolates were hybridized with five rRNA methylase genes and gene mobility was determined. Twenty-four (96%) of 25 isolates hybridized with one or more rRNA methylase genes. Representative isolates were able to transfer erythromycin resistance to Haemophilus influenzae and Enterococcus faecalis recipients. The transconjugants were shown to carry rRNA methylase genes by DNA probe hybridization and had MICs of erythromycin 32 to 256 mg/L.