An In Vitro Perspective on What Individual Antimicrobials Add to Mycobacterium avium Complex Therapies

The role of rifampicin within the treatment of Mycobacterium avium pulmonary disease

Rifampicin is recommended for the treatment of Mycobacterium avium complex pulmonary disease alongside azithromycin and ethambutol. We evaluated the azithromycin-ethambutol backbone with and without rifampicin in an intracellular hollow fiber model and performed RNA sequencing to study the differences in adaptation. In an in vitro hollow fiber experiment, we simulated epithelial lining fluid pharmacokinetic profiles of the recommended 3-drug (rifampicin, ethambutol, and azithromycin) or a 2-drug (ethambutol and azithromycin) treatment. THP-1 cells infected with M. avium ATCC700898 were exposed to these regimens for 21 days. We determined intra- and extra-cellular bacterial load- and THP-1 cell densities on days 0, 3, 7, 14, and 21, alongside RNA sequencing. The emergence of macrolide resistance was studied by inoculating intra- and extra-cellular fractions of azithromycin-containing Middlebrook 7H10 agar plates. Complete pharmacokinetic profiles were determined at days 0 and 21. Both therapies maintained stasis of both intra- and extra-cellular bacterial populations for 3 days, whilst regrowth coinciding with the emergence of a macrolide-resistant subpopulation was seen after 7 days. THP-1 cell density remained static. Similar transcriptional profiles were observed for both therapies that were minimally influenced by exposure duration. Transcriptional response was slightly larger during 2-drug treatment. Rifampicin did not add to the antimycobacterial effect to the 2-drug therapy or suppression of emergence resistance. RNA transcription was not greatly altered by the addition of rifampicin, which may be due to strong transcriptional influence of azithromycin and host cells. This questions the role of rifampicin in the currently recommended therapy. These findings should be confirmed in clinical trials.

Differences in Drug-Susceptibility Patterns between Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium chimaera Clinical Isolates: Prospective 8.5-Year Analysis by Three Laboratories

Background: It has been suggested that Mycobacterium avium, Mycobacterium intracellulare, and M. chimaera have differential drug susceptibility patterns. We prospectively analyzed and compared the drug susceptibility patterns among these species over an 8.5-year period. Methods: A microdilution method (Slomyco^®) was performed for drug susceptibility testing of 402 M. avium, 273 M. intracellulare, and 139 M. chimaera clinical isolates. Results: M. avium showed significantly higher resistance to moxifloxacin, ciprofloxacin, rifampicin, ethambutol, streptomycin, linezolid, cotrimoxazole, and clarithromycin. M. avium also showed higher minimum inhibitory concentrations (MIC) than M. intracellulare and M. chimaera against all drugs except ethionamide, to which M. intracellulare and M. chimaera showed greater resistance. Conclusions: Our series demonstrated differential drug resistance patterns among the most frequent M. avium complex species. M. avium was more resistant than M. intracellulare and M. chimaera versus eight antibiotics and showed greater MIC values to most of the antibiotics studied. These data suggest that knowledge of the local distribution and susceptibility profiles of these pathogens is essential for adequate clinical management.

RNA-sequencing elucidates drug-specific mechanisms of antibiotic tolerance and resistance in M. abscessus

Mycobacterium abscessus is an opportunistic pathogen notorious for its resistance to most classes of antibiotics and low cure rates. M. abscessus carries an array of mostly unexplored defense mechanisms. A deeper understanding of antibiotic resistance and tolerance mechanisms is pivotal in development of targeted therapeutic regimens. We provide the first description of all major transcriptional mechanisms of tolerance to all antibiotics recommended in current guidelines, using RNA sequencing-guided experiments. M. abscessus ATCC 19977 bacteria were subjected to subinhibitory concentrations of clarithromycin (CLR), amikacin (AMK), tigecycline (TIG), cefoxitin (FOX), and clofazimine (CFZ) for 4 and 24 h, followed by RNA sequencing. To confirm key mechanisms of tolerance suggested by transcriptomic responses, we performed time-kill kinetic analysis using bacteria after preexposure to CLR, AMK, or TIG for 24 h and constructed isogenic knockout and knockdown strains. To assess strain specificity, pan-genome analysis of 35 strains from all three subspecies was performed. Mycobacterium abscessus shows both drug-specific and common transcriptomic responses to antibiotic exposure. Ribosome-targeting antibiotics CLR, AMK, and TIG elicit a common response characterized by upregulation of ribosome structural genes, the WhiB7 regulon and transferases, accompanied by downregulation of respiration through NuoA-N. Exposure to any of these drugs decreases susceptibility to ribosome-targeting drugs from multiple classes. The cytochrome bd-type quinol oxidase contributes to CFZ tolerance in M. abscessus, and the sigma factor sigH but not antisigma factor MAB_3542c is involved in TIG resistance. The observed transcriptomic responses are not strain-specific, as all genes involved in tolerance, except erm(41), are found in all included strains.