MOLECULAR MECHANISMS OF CLARITHROMYCIN RESISTANCE IN MYCOBACTERIUM-AVIUM - OBSERVATION OF MULTIPLE 23S RDNA MUTATIONS IN A CLONAL POPULATION

The peptidyltransferase region of the 23S rRNA gene (the probable targ et site for the macrolides) was investigated in blood isolates of Myco bacterium avium recovered from 38 patients before and after the develo pment of clarithromycin resistance. Point mutations were identified in 100% of the 74 resistant relapse blood isolates but in none of 69 sus ceptible pretreatment isolates. Multiple mutations were identified in isolates from 23 (61%) of 38 patients. Of the 63 identified mutations, 95% involved adenine at bp 2058. Single-colony clones from cultures t hat were mixtures of more than one mutation revealed a single mutation within each clone. Pulsed field gel electrophoresis of genomic DNA re striction fragments revealed that 13 (81%) of 16 multiple mutations id entified in the same patient were derived from a single infecting stra in. In vitro investigation revealed the same point mutations observed in vivo. This study defines the probable mechanism of clarithromycin r esistance in M. avium and provides in vivo evidence that mutational re sistance is random and selection-directed.