Absence of a Functional erm Gene in Isolates of Mycobacterium immunogenum and the Mycobacterium mucogenicum Group, Based onIn VitroClarithromycin Susceptibility: TABLE 1

Abstract

Macrolide resistance has been linked to the presence of a functional erythromycin ribosomal methylase (erm) gene in most species of pathogenic rapidly growing mycobacteria (RGM). For theseMycobacteriumisolates, extended incubation in clarithromycin is necessary to determine macrolide susceptibility. In contrast, the absence of a detectableermgene in isolates ofM. chelonae,M. senegalense, andM. peregrinumand a nonfunctionalermgene inM. abscessussubsp.massilienseand 15% to 20% ofM. abscessussubsp.abscessusisolates renders these species intrinsically macrolide susceptible. Not all RGM species have been screened for the presence of anermgene, including theMycobacterium mucogenicumgroup (M. mucogenicum,M. phocaicum, andM. aubagnense) andMycobacterium immunogenum. A total of 356 isolates of these two pathogenic RGM taxa from two reference laboratories (A.R.U.P. Reference Laboratories and the Mycobacteria/Nocardia Laboratory at the University of Texas Health Science Center at Tyler) underwent clarithromycin susceptibility testing with readings at 3 to 5 days and 14 days. Only 13 of the 356 isolates had resistant clarithromycin MICs at initial extended MIC readings, and repeat values on all available isolates were ≤2 μg/ml. These studies suggest that these two additional RGM groups do not harbor functionalermgenes and, likeM. chelonae, do not require extended clarithromycin susceptibility testing. We propose to the Clinical Laboratory and Standards Institute that isolates belonging to these above-mentioned six rapidly growing mycobacterial groups based on molecular identification with no known functionalermgenes undergo only 3 to 5 days of susceptibility testing (to exclude mutational resistance).