Resistance mechanisms and drug susceptibility testing of nontuberculous mycobacteria
Introduction
Nontuberculous mycobacteria (NTM) are increasingly recognized as causative agents of opportunistic infections in humans. Pulmonary infections are most frequent and tend to affect patients with pre-existing pulmonary diseases. Cervicofacial lymphadenitis is the second most common and relatively benign manifestation and affects immunocompetent children. In severely immunocompromised patients, NTM can cause localized extrapulmonary or disseminated disease. NTM can be acquired by inhalation, ingestion or direct inoculation in the skin (Griffith et al., 2007).
Treatment of infections caused by NTM is either by surgery, drug therapy, or both. Drug therapy of NTM disease is long, costly, and often associated with drug-related toxicities. The treatment regimens differ by species with the most important distinction being that between slow and rapid growing NTM (see Table 1). For most slow growers, the recommended regimen consists of rifampicin (or rifabutin), ethambutol and a macrolide antibiotic and is given for 18–24 months; amikacin or streptomycin can be added in the initial 3–6 months in cases of severe disease. For the rapid growers, regimens are primarily based on in vitro drug susceptibility test (DST) results. For Mycobacterium abscessus, the most notorious causative agent of disease among rapid growers, these regimens often include a macrolide antibiotic, amikacin and either cefoxitin, imipenem or tigecycline (Jarand et al., 2011). Treatment for NTM disease, in particular pulmonary NTM disease, can be disappointing and clinical improvement and prolonged culture conversion is not achievable for all patients. Cure rates of pulmonary NTM disease differ by species, ranging from 30–50% in M. abscessus disease, to 50–70% in Mycobacterium avium complex and 80–90% in Mycobacterium malmoense and Mycobacterium kansasii disease (Hoefsloot et al., 2009, Jarand et al., 2011, van Ingen et al., 2009).
The exact role of DST remains subject of debate, in particular for slow growers. One of the reason is that there are important discrepancies between drug susceptibility measured in vitro and the activity of the drug observed in vivo (Griffith et al., 2007, Society, 2001). In part, these discrepancies derive from laboratory technical difficulties of DST, standardization of methods and a lack of clinical validation. Yet, with the increasing clinical importance of NTM disease, DST of NTM is again in the spotlight. In this review we provide a comprehensive overview of the mechanisms of drug resistance in NTM, phenotypic methods in past and current use for DST of NTM, as well as molecular approaches to assess drug resistance. We also discuss the usefulness of these tests in clinical practice. This review is limited to susceptibility and susceptibility testing to antimicrobial compounds in past or current clinical use; susceptibility to disinfectants or non-medical substances falls outside the scope of this review.
Section snippets
Literature search strategy
Relevant publications for review were identified using the PubMed database. The following Medical Subject Heading (MeSH) terms were used alone and in combination: “Mycobacteria, Atypical” or “Mycobacterium” and “Drug Resistance, Bacterial”. We restricted the search to English-, German-, French- and Dutch-language papers only. The search was last performed on March 31st, 2011.
The search based on “Mycobacteria, Atypical” and “Drug Resistance, Bacterial” yielded 221 results. The use of
Drug resistance mechanisms in nontuberculous mycobacteria
DST measures the result of a highly complex interplay between natural resistance, inducible resistance and mutational resistance acquired during suboptimal drug exposure and selection. The role of these three determinants of drug susceptibility differs for the various drugs used to treat NTM disease. Knowledge of their relative importance is essential for the selection and optimization of drug treatment regimens. A simplified overview of the various determinants of resistance is presented in
Phenotypic drug susceptibility testing of mycobacteria
Assessments of drug susceptibility of NTM at first were not a clinical tool. In the early 1950s, DST of NTM started as a laboratory diagnostic tool to discern the more resistant nontuberculous mycobacteria from the more susceptible M. tuberculosis. An assessment of susceptibility to the newly developed antituberculosis drugs isoniazid, streptomycin and para-aminosalicylic acid (PAS) was combined with phenotypic features including pigmentation, biochemical properties and pace of growth to
Molecular methods for assessment of drug susceptibility
Thus far, no commercial assays exist for the detection of resistance-conferring mutations in NTM. Nonetheless, species-specific methods have been developed. Most of these are based on sequencing of the target gene and comparisons with wild type strains of the same species and include rpoB gene mutation analysis to assess rifamycin susceptibility in M. kansasii, 23S rRNA and erm gene mutation analysis to assess macrolide susceptibility in MAC and rapid growers and 16S rRNA gene mutation analysis
Ongoing controversy – in vitro drug susceptibility versus in vivo outcome of treatment
In this review, we have summarized the various methods that have been used to determine drug susceptibility of NTM, for mostly clinical purposes. Yet, clinical validation of these methods, i.e. relating MICs and breakpoint concentrations for resistance to pharmacokinetic data and treatment outcome, has hardly been done.
The role of synergy in current drug regimens
The current CLSI recommendations all involve testing of single drugs. There is evidence of synergy, in vitro, between the various drugs used to treat NTM disease. Three drugs are particularly known for their synergistic effects, in vitro: ethambutol, clarithromycin and clofazimine.
For ethambutol, the best known synergistic activity is with rifampicin. The combination of rifampicin and ethambutol has synergistic activity against SGM, particularly MAC, M. malmoense and M. xenopi. The synergistic
Perspective
Despite the fact that DST of NTM is now an established tool in the laboratory, its clinical value has been not been sufficiently studied. Natural drug resistance determines a large part of the multidrug-resistance that is commonplace in NTM. This multidrug-resistance, in turn, is a likely explanation of the limited efficacy of current treatment regimens for NTM disease. Testing the susceptibility of individual clinical isolates is of limited value for drugs to which natural resistance is to be
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