A Novel Gene,erm(41), Confers Inducible Macrolide Resistance to Clinical Isolates ofMycobacterium abscessusbut Is Absent fromMycobacterium chelonae

Abstract

Mycobacterium abscessus infections tend to respond poorly to macrolide-based chemotherapy, even though the organisms appear to be susceptible to clarithromycin. Circumstantial evidence suggested that at least some M. abscessus isolates might be inducibly resistant to macrolides. Thus, the purpose of this study was to investigate the macrolide phenotype of M. abscessus clinical isolates. Inducible resistance to clarithromycin (MIC > 32 g/ml) was found for 7 of 10 clinical isolates of M. abscessus previously considered susceptible; the remaining 3 isolates were deemed to be susceptible (MIC < 0.5 g/ml). Inducible resistance was conferred by a novel erm gene, erm(41), which was present in all 10 isolates and in an isolate of Mycobacterium bolletii (M. abscessus type II). However, the erm(41) alleles were nonfunctional in the three susceptible M. abscessus isolates. No evidence of erm(41) was found in Mycobacterium chelonae, and an isolate of Mycobacterium massiliense appeared to be an erm(41) deletion mutant. Expression of erm(41) in M. abscessus conferred resistance to clarithromycin and erythromycin and the ketolide HMR3004. However, this species was found to be intrinsically resistant, independent of erm (41) The rapidly growing mycobacteria (RGM) are significant human pathogens, with approximately 90% of RGM infections being caused by Mycobacterium abscessus, Mycobacterium chelonae, and Mycobacterium fortuitum (11,12). In particular, M. abscessus is being recognized as an emerging pathogen following trauma and surgery and in patients with cystic fibrosis (15,35,36). Although there appears to be a decline in the incidence of infections caused by some species of slowly growing nontuberculous mycobacteria, infections caused by RGM infections may be increasing (15,43). Many pathogenic RGM species are considered susceptible to the newer macrolides (e.g., clarithromycin and azithromycin), and these agents are important components in the treatment of RGM infections (11,12,17,19,43). Paradoxically, however, M. abscessus infections of the lung are often intractable to macrolide-based chemotherapy, although high doses of clarithromycin may lead to clinical improvement in some patients (17). The lack of efficacy of macrolides, such as clarithromycin, is puzzling, as pretreatment isolates are usually reported as susceptible to clarithromycin when CLSI procedures are used (34). Intriguingly, Brown et al. (10) reported that the clarithromycin MIC increased for some M. abscessus isolates if the incubations for the susceptibility assays were extended, although these isolates would still be deemed macrolide susceptible if an incubation period based on CLSI guidelines were used (34). This phenomenon was not observed for M. chelonae isolates (10). An explanation for the instability of clarithromycin MIC is that M. abscessus might have inducible resistance to macrolides. If so, this could explain the lack of efficacy of macrolide-based chemotherapy against M. abscessus infections.Clinically acquired macrolide resistance in mycoba...

Preliminary Results of Bedaquiline as Salvage Therapy for Patients With Nontuberculous Mycobacterial Lung Disease

“…If the isolate for Mab had a nonfunctional erm gene and in vitro susceptibility to macrolide/azalide, azithromycin was given at doses of 250 to 500 mg po daily. [19][20][21] For both species, drug regimens were given for a minimum of 24 weeks. Pretreatment chest radiograph and high-resolution CT scan, sputum for AFB smear and culture, CBC count, complete metabolic panel, serum magnesium and phosphorous levels, and ECG were obtained.…”

Azithromycin blocks autophagy and may predispose cystic fibrosis patients to mycobacterial infection

“…Although frequently sensitive to azithromycin in vitro, M. abscessus becomes highly macrolide resistant in vivo ( Figure 6E) through inducible expression of the ribosomal methyltransferase erm(41) (22), equivalent to erm(37) found in M. tuberculosis, as well as through formation of a protective biofilm (23). To examine the effect of azithromycin treatment on the host response to mycobacteria in vivo, we infected mice via aerosol challenge with azithromycin-resistant M. abscessus ( Figure 7).…”

Evaluation of the GenoType® NTM DR for Subspecies Identification and Determination of Drug Resistance in Clinical M. abscessus Isolates

“…The minimal inhibitory concentration (MIC) was defined as the lowest drug concentration at which no visual growth was observed. Clarithromycin resistance was defined as a MIC of ≥ 8 μg/ml, susceptibility as a MIC ≤ 2 μg/ml, and Amikacin resistance was defined as a MIC of ≥ 64 μg/ml, susceptibility ≤ 16 μg/ml [9,11]. Inducible resistance for Clarithromycin was defined as MIC<8 ug/ml before day 5 and MIC ≥ 8 μg/ml after 7 or 14 days of incubation, respectively.…”

“…The primary innate mechanism of macrolide resistance in M. abscessus is the inducible expression of an erythromycin ribosomal methylase, Erm(41). Erm(41) methylates the adenine at position 2058 of the 23S ribosomal RNA leading to reduced binding of macrolides to their target site in the 50S ribosomal subunit [9]. M. abscessus subsp.…”

“…M. abscessus subsp. massiliense has a dysfunctional erm(41) gene due to a 2 bp deletion of nucleotides 64-65 and a 274 bp deletion of nucleotides 159-432 leading to a macrolide susceptible phenotype [9]. In addition, acquired high-level resistance to macrolides is due to point mutations at positions 2058 and 2059 in the 23S rRNA (rrl) gene [11].…”