Lack of Antimicrobial Bactericidal Activity in Mycobacterium abscessus

Antibiotic therapy of infections caused by the emerging pathogen Mycobacterium abscessus is challenging due to the organism''s natural resistance toward most clinically available antimicrobials. We investigated the bactericidal activity of antibiotics commonly administered in M. abscessus infections in order to better understand the poor therapeutic outcome. Time-kill curves were generated for clinical M. abscessus isolates, Mycobacterium smegmatis, and Escherichia coli by using antibiotics commonly categorized as bactericidal (amikacin and moxifloxacin) or bacteriostatic (tigecycline and linezolid). In addition, the impact of aminoglycoside-modifying enzymes on the mode of action of substrate and nonsubstrate aminoglycosides was studied by using M. smegmatis as a model organism. While amikacin and moxifloxacin were bactericidal against E. coli, none of the tested compounds showed bactericidal activity against M. abscessus. Further mechanistic investigations of the mode of action of aminoglycosides in M. smegmatis revealed that the bactericidal activity of tobramycin and gentamicin was restored by disruption of the chromosomal aac(2′) gene in the mycobacterial genome. The lack of bactericidal antibiotics in currently recommended treatment regimens provides a reasonable explanation for the poor therapeutic outcome in M. abscessus infection. Our findings suggest that chromosomally encoded drug-modifying enzymes play an important role in the lack of aminoglycoside bactericidal activity against rapidly growing mycobacteria.     

In vitro susceptibility patterns for rapidly growing nontuberculous mycobacteria in the United States

Antimicrobial susceptibility testing for rapidly growing mycobacteria (RGM) is uncommon or only performed in large reference laboratories. Here we developed a cumulative antibiogram for 14 RGM using the largest sample size to date (N = 3860). All RGM showed 82% to 100% susceptibility to amikacin. Mycobacterium abscessus showed low percentages of susceptibility to most antimicrobials; of antimicrobials without interpretations, the minimum inhibitory concentration-90 for clofazimine was low (≤0.5mg/L). All three subspecies had ≤2.6% rrl resistance mutations, however intact erm(41) was detected in 70% to100% of M. abscessus abscessus and bolletii. Mycobacterium chelonae had a similar susceptibility pattern to M. abscessus subsp. massiliense and Mycobacterium immunogenum except that it was susceptible to tobramycin (87%). Mycobacterium fortuitum complex and similar organisms showed higher frequency of susceptibility to fluoroquinolones, beta-lactams, linezolid, and trimethoprim/sulfamethoxazole. Although relatively small published RGM antibiograms showed substantial variance, a comprehensive antibiogram can help influence treatment and monitoring patterns of resistance.     

Antimicrobial susceptibility testing of Mycobacteroides (Mycobacterium) abscessus complex,Mycolicibacterium (Mycobacterium) fortuitum,and Mycobacteroides (Mycobacterium) chelonae

The drug susceptibility of rapidly growing mycobacteria (RGM) varies among isolates. Treatment strategies similarly differ depending on the isolate, and for some, no clear strategy has been identified. This complicates clinical management of RGM. Following Clinical and Laboratory Standards Institute standard M24-A2, we assessed the susceptibility of 140 RGM isolates to 14 different antimicrobial drugs by measuring their minimal inhibitory concentrations (MICs). We also investigated the correlation of clarithromycin (CAM) MICs with the erm(41) and rrl gene mutations in the Mycobacteroides (Mycobacterium) abscessus complex, the rrl mutation in Mycobacteroides (Mycobacterium) chelonae, and the erm(39) mutation in Mycolicibacterium (Mycobacterium) fortuitum to determine the contribution of these mutations to CAM susceptibility. The five species and subspecies examined included 48 M. abscessus subsp. abscessus isolates (34.3%), 35 (25.0%) being M. abscessus subsp. massiliense, and two (1.4%) being M. abscessus subsp. bolletii. The M. abscessus complex accounted for 85 isolates (60.7%) in total, whereas 43 isolates (30.7%) were M. fortuitum, and 12 (8.6%) were M. chelonae. Our results demonstrated species-specific susceptibility to antimicrobials. In most cases, susceptibility to CAM could be predicted based on genetic pattern, but since one isolate did not fit that pattern, MIC values needed to be measured. Some isolates also exhibited rates of resistance to other drugs that differed from those previously reported in other locations, indicating that accurate identification of the bacterial isolate and use of the correct method for determining MIC are both important for the diagnosis of RGM.     

Gallium Compounds Exhibit Potential as New Therapeutic Agents against Mycobacterium abscessus

The rapidly growing nontuberculous mycobacterial species Mycobacterium abscessus has recently emerged as an important pathogen in patients with cystic fibrosis (CF). Treatment options are limited because of the organism''s innate resistance to standard antituberculous antibiotics, as well as other currently available antibiotics. New antibiotic approaches to the treatment of M. abscessus are urgently needed. The goal of the present study was to assess the growth-inhibitory activity of different Ga compounds against an American Type Culture Collection (ATCC) strain and clinical isolates of M. abscessus obtained from CF and other patients. In our results, using Ga(NO₃)₃ and all of the other Ga compounds tested inhibited the growth of ATCC 19977 and clinical isolates of M. abscessus. Inhibition was mediated by disrupting iron uptake, as the addition of exogenous iron (Fe) restored basal growth. There were modest differences in inhibition among the isolates for the same Ga chelates, and for most Ga chelates there was only a slight difference in potency from Ga(NO₃)₃. In contrast, Ga-protoporphyrin completely and significantly inhibited the ATCC strain and clinical isolates of M. abscessus at much lower concentrations than Ga(NO₃)₃. In in vitro broth culture, Ga-protoporphyrin was more potent than Ga(NO₃)₃. When M. abscessus growth inside the human macrophage THP-1 cell line was assessed, Ga-protoporphyrin was >20 times more active than Ga(NO₃)₃. The present work suggests that Ga exhibits potent growth-inhibitory capacity against the ATCC strain, as well as against antibiotic-resistant clinical isolates of M. abscessus, including the highly antibiotic-resistant strain MC2638. Ga-based therapy offers the potential for further development as a novel therapy against M. abscessus.     

Molecular Cloning and Functional Analysis of a Novel Tetracycline Resistance Determinant, tet(V), from Mycobacterium smegmatis

The nucleotide sequence and mechanism of action of a tetracycline resistance gene from Mycobacterium smegmatis were determined. Analysis of a 2.2-kb sequence fragment showed the presence of one open reading frame, designated tet(V), encoding a 419-amino-acid protein (molecular weight, 44,610) with at least 10 transmembrane domains. A database search showed that the gene is homologous to membrane-associated antibiotic efflux pump proteins but not to any known tetracycline efflux pumps. The steady-state accumulation level of tetracycline by M. smegmatis harboring a plasmid carrying the tet(V) gene was about fourfold lower than that of the parental strain. Furthermore, the energy uncoupler carbonyl cyanide m-chlorophenylhydrazone blocked tetracycline efflux in deenergized cells. These results suggest that the tet(V) gene codes for a drug antiporter which uses the proton motive force for the active efflux of tetracycline. By primer-specific amplification the gene appears to be restricted to M. smegmatis and M. fortuitum.     

Pulmonary infection caused by nontuberculous mycobacteria in a medical center in Taiwan, 2005-2008

The aim of this study was to investigate the epidemiology and clinical characteristics of pulmonary infections caused by nontuberculous mycobacteria (NTM) in a university hospital in Taiwan from 2005 to 2008. During the study period, a total of 312 patients with NTM pulmonary infection were identified. Most patients with NTM pulmonary infection had preexisting pulmonary diseases or malignancies. The incidence (per 100,000 inpatients and outpatients) of patients with NTM isolations (6.67 in 2005 and 9.28 in 2008, P < .0001) from respiratory specimens and the incidence of patients with NTM pulmonary infection (3.54 in 2005 and 4.45 in 2008, P < .0141) increased significantly annually. The most common pathogens in patients with NTM-associated pulmonary infections were Mycobacterium avium complex (n = 110, 35.3%), followed by M. abscessus (n = 66, 21.2%). Incidence (per 100,000 inpatients and outpatients) of patients with pulmonary infections caused by rapidly growing mycobacteria (RGM) also increased significantly (1.06 in 2005 and 2.00 in 2008, P = .008). In conclusion, RGM, especially M. abscessus, had an increasingly important role in NTM pulmonary infections.     

Accumulation of Norfloxacin by Mycobacterium aurum and Mycobacterium smegmatis

The modified fluorescence method was used to determine the accumulation of norfloxacin by Mycobacterium aurum A+ and Mycobacterium smegmatis mc²155. By using an exogenous norfloxacin concentration of 10 μg/ml, a steady-state concentration (SSC) of 160 to 180 ng of norfloxacin/mg of cells was obtained for M. aurum, and an SSC of 120 to 140 ng of norfloxacin/mg of cells obtained for M. smegmatis. For both species of mycobacteria, the SSC was achieved within 5 min. The silicon oil method was investigated and gave higher SSCs than the modified fluorescence method. Further studies on the mechanism of norfloxacin accumulation by M. aurum were performed. An increase in the pH of the wash buffer from 7.0 to 9.0 did not significantly affect the final SSC obtained. Accumulation was nonsaturated over a norfloxacin concentration range of 0 to 100 μg/ml, and the proton motive force inhibitor 2,4-dinitrophenol (1 and 2 mM), whether it was added before or after norfloxacin was added, had no effect on the final SSC obtained. 2,4-Dinitrophenol also had no effect on norfloxacin accumulation by M. smegmatis. Furthermore, norfloxacin accumulation by M. aurum was unaffected by the presence of either Tween 80 or subinhibitory concentrations of ethambutol in the growth medium. Therefore, it is proposed that norfloxacin accumulation by mycobacteria occurs by simple, energy-independent diffusion.     

Impact of β-Lactamase Inhibition on the Activity of Ceftaroline against Mycobacterium tuberculosis and Mycobacterium abscessus

The production of β-lactamases Bla_Mab and BlaC contributes to β-lactam resistance in Mycobacterium abscessus and Mycobacterium tuberculosis, respectively. Ceftaroline was efficiently hydrolyzed by these enzymes. Inhibition of M. tuberculosis BlaC by clavulanate decreased the ceftaroline MIC from ≥256 to 16 to 64 μg/ml, but these values are clinically irrelevant. In contrast, the ceftaroline-avibactam combination should be evaluated against M. abscessus since it inhibited growth at lower and potentially achievable drug concentrations.     

Clofazimine Prevents the Regrowth of Mycobacterium abscessus and Mycobacterium avium Type Strains Exposed to Amikacin and Clarithromycin

Multidrug therapy is a standard practice when treating infections by nontuberculous mycobacteria (NTM), but few treatment options exist. We conducted this study to define the drug-drug interaction between clofazimine and both amikacin and clarithromycin and its contribution to NTM treatment. Mycobacterium abscessus and Mycobacterium avium type strains were used. Time-kill assays for clofazimine alone and combined with amikacin or clarithromycin were performed at concentrations of 0.25× to 2× MIC. Pharmacodynamic interactions were assessed by response surface model of Bliss independence (RSBI) and isobolographic analysis of Loewe additivity (ISLA), calculating the percentage of statistically significant Bliss interactions and interaction indices (I), respectively. Monte Carlo simulations with predicted human lung concentrations were used to calculate target attainment rates for combination and monotherapy regimens. Clofazimine alone was bacteriostatic for both NTM. Clofazimine-amikacin was synergistic against M. abscessus (I = 0.41; 95% confidence interval [CI], 0.29 to 0.55) and M. avium (I = 0.027; 95% CI, 0.007 to 0.048). Based on RSBI analysis, synergistic interactions of 28.4 to 29.0% and 23.2 to 56.7% were observed at 1× to 2× MIC and 0.25× to 2× MIC for M. abscessus and M. avium, respectively. Clofazimine-clarithromycin was also synergistic against M. abscessus (I = 0.53; 95% CI, 0.35 to 0.72) and M. avium (I = 0.16; 95% CI, 0.04 to 0.35), RSBI analysis showed 23.5% and 23.3 to 53.3% at 2× MIC and 0.25× to 0.5× MIC for M. abscessus and M. avium, respectively. Clofazimine prevented the regrowth observed with amikacin or clarithromycin alone. Target attainment rates of combination regimens were >60% higher than those of monotherapy regimens for M. abscessus and M. avium. The combination of clofazimine with amikacin or clarithromycin was synergistic in vitro. This suggests a potential role for clofazimine in treatment regimens that warrants further evaluation.     

Mycobacterium abscessus subsp. abscessus empyema complicated with subcutaneous abscess

There have been no case reports of thoracic subcutaneous abscess after surgery for Mycobacterium abscessus complex associated empyema. We herein report a case of Mycobacterium abscessus subsp. abscessus (M. abscessus subsp. abscessus) induced subcutaneous abscesses following surgical treatment for concurrent M. abscessus subsp. abscessus -associated empyema and pneumothorax. A 75-year-old woman had M. abscessus subsp. abscessus -associated empyema and pneumothorax. She underwent surgical treatment of decortication and fistulectomy and suffered from M. abscessus subsp. abscessus -associated subcutaneous abscesses after thoracentesis/drainage. A multidisciplinary approach combined with surgical care, thermal therapy, and multidrug chemotherapy contributed to a successful result. An early multidisciplinary approach is believed to be important in cases of M. abscessus subsp. abscessus -associated empyema and subcutaneous abscess.     

Rapid identification of strains belonging to the Mycobacterium abscessus group through erm(41) gene pyrosequencing

Mycobacterium abscessus and Mycobacterium massiliense lung infections have different clarithromycin susceptibilities, making proper identification important; however, standard multi-gene sequencing in clinical laboratories is laborious and time consuming. We developed a pyrosequencing-based method for rapid identification of strains belonging to the M. abscessus group by targeting erm(41). We examined 55 isolates from new pulmonary M. abscessus infections and identified 28 M. abscessus, 25 M. massiliense, and 2 Mycobacterium bolletii isolates. Multi-gene sequencing of 16S rRNA, hsp65, rpoB, and the 16S–23S ITS region was concordant with the results of erm(41) pyrosequencing; thus, the M. abscessus group can be identified by single-nucleotide polymorphisms in erm(41). The method also enables rapid identification of polymorphic, inducible clarithromycin-resistant sequevars (T28 or C28). Pyrosequencing of erm(41) is a rapid, reliable, high-throughput alternative method for identifying and characterizing M. abscessus species. Further testing of a diverse collection of isolates is necessary to demonstrate the discriminatory power of erm(41) sequencing to differentiating species with this highly divergent group.     

Utility of high-performance liquid chromatography analysis of mycolic acids and partial 16S rRNA gene sequencing for routine identification of Mycobacterium spp. in a national reference laboratory

High-performance liquid chromatography analysis of mycolic acids and partial gene sequencing for the first 500-bp 5′ end of the 16S rRNA gene were used singularly and in combination to evaluate the final identification of species. Examination of 200 cultures revealed 100 strains of slowly growing mycobacteria (SGM), 91 strains of rapidly growing mycobacteria (RGM), and 9 strains of other genera. SGM were discriminated in complexes with both methods for 56 strains, composed primarily of the Mycobacterium spp.: Mycobacterium avium, Mycobacterium terrae, and Mycobacterium simiae–Mycobacterium lentiflavum. For RGM, 73 strains were associated with complexes designated as Mycobacterium abscessus–Mycobacterium chelonae, Mycobacterium fortuitum–Mycobacterium peregrinum, and Mycobacterium mucogenicum–Mycobacterium phocaicum. Consistent identification of all the isolates differentiated to single species within the Mycobacterium genus was not possible with either test method. Sequencing results often distinguished complexes containing fewer species, and combining the results from each method increased the confidence of identifying the correct species.     

Standardization of multilocus sequence typing scheme for Mycobacterium abscessus and Mycobacterium massiliense

This study aims to develop a multilocus sequence typing (MLST) scheme for Mycobacterium abscessus complex for the typing of stains within each species. A total of 89 clinical isolates of M. abscessus complex from 71 patients of 2 tertiary care hospitals in South Korea were included. Forty-two isolates were identified as M. abscessus, and 29, as Mycobacterium massiliense through sequencing of 8 housekeeping genes and rpoB. The MLST scheme identified 26 different sequence types(STs) and 13 different clonal complexes (CCs) in M. abscessus and 12 different STs and 6 different CCs in M. massiliense. The MLST data showed high concordance with the XbaI-macrorestriction patterns of pulsed-field gel electrophoresis in the duplicated isolates. Our MLST schemes could identify different strains of M. abscessus and M. massiliense, and the schemes also showed a reliable reproducibility. Therefore, our MLST schemes may be useful in studying the epidemiology of M. abscessus and M. massiliense infections.     

Subinhibitory Doses of Aminoglycoside Antibiotics Induce Changes in the Phenotype of Mycobacterium abscessus

Subinhibitory doses of antibiotics have been shown to cause changes in bacterial morphology, adherence ability, and resistance to antibiotics. In this study, the effects of subinhibitory doses of aminoglycoside antibiotics on Mycobacterium abscessus were investigated. The treatment of M. abscessus cells with subinhibitory doses of amikacin was found to change their colony from a smooth to a rough morphotype and increase their ability to adhere to a polyvinylchloride plate, aggregate in culture, and resist phagocytosis and killing by macrophages. M. abscessus cells treated with a subinhibitory dose of amikacin also became more potent in Toll-like receptor 2 (TLR-2) stimulation, leading to increased tumor necrosis factor alpha (TNF-α) production by macrophages. The MAB_3508c gene was shown to play a role in mediating these phenotypic changes, as its expression in M. abscessus cells was increased when they were treated with a subinhibitory dose of amikacin. In addition, overexpression of MAB_3508c in M. abscessus cells caused changes similar to those induced by subinhibitory doses of amikacin, including a switch from smooth to rough colony morphology, increased ability to aggregate in liquid culture, decreased motility, and increased resistance to killing by macrophages. These findings suggest the importance of using sufficient doses of antibiotics for the treatment of M. abscessus infections.     

Clinical characteristics of extrapulmonary nontuberculous mycobacteria infections in comparison with pulmonary infections: A single-center,retrospective study in Japan

IntroductionThe prevalence of nontuberculous mycobacteria (NTM) infections is increasing worldwide. Although NTM can affect extrapulmonary organs, studies on the clinical characteristics of extrapulmonary NTM are rare.MethodsWe retrospectively analyzed patients who were newly diagnosed with NTM infections at Hiroshima University Hospital between 2001 and 2021 to investigate species distribution, infected sites, and risk factors of extrapulmonary NTM compared to pulmonary NTM.ResultsOf the 261 NTM infections, 9.6% and 90.4% had extrapulmonary and pulmonary NTM, respectively. The mean ages of patients with extrapulmonary and pulmonary NTM were 53.4 and 69.3 years, 64.0% and 42.8% were male, 36.0% and 9.3% received corticosteroids, 20.0% and 0% had acquired immune deficiency syndrome (AIDS), and 56.0% and 16.1% had any immunosuppressive conditions, respectively. Younger age, corticosteroid use, and AIDS were associated with extrapulmonary NTM. In pulmonary NTM, Mycobacterium avium complex (MAC) accounted for 86.4% of NTM species, followed by M. abscessus complex (4.2%), whereas in extrapulmonary NTM, M. abscessus complex, MAC, M. chelonae, and M. fortuitum accounted for 36.0%, 28.0%, 12.0%, and 8.0%, respectively. Compared to pulmonary NTM, extrapulmonary NTM were significantly more likely to be rapid-growing mycobacteria (RGM) (56.0% vs. 5.5%). The most common sites of infection were the skin and soft tissues (44.0%), followed by the blood (20.0%), tenosynovium, and lymph nodes (12.0%).ConclusionYounger age and immunosuppressive conditions are associated with extrapulmonary NTM, with a higher prevalence of RGM in extrapulmonary NTM than in pulmonary NTM. These results provide a better understanding of extrapulmonary NTM.     

Mycobacterium abscessus and massiliense lung infection during macrolide treatment for bronchiolitis obliterans after allogeneic hematopoietic stem cell transplantation

In patients undergoing allogeneic hematopoietic stem cell transplantation (allo-SCT), post-transplant lung infection is critical for their prognosis. Mycobacterium abscessus complex is not fully recognized as a nontuberculous mycobacteria (NTM) pathogen of post-SCT lung infection. Here, we present three post-allogeneic SCT patients who developed pulmonary infection caused by M. abscessus complex including M. abscessus and M. massiliense. In all three cases, macrolide antibiotics had been administered for bronchiolitis obliterans syndrome (BOS) before the confirmation of their infection, and macrolide resistance was noted in the M. abscessus isolates, one of which resulted in an unfavorable treatment outcome. It is important to consider M. abscessus lung infection as well as other NTM in patients receiving allo-SCT, particularly those receiving macrolide therapy for BOS.     

Evaluation of DNA Primase DnaG as a Potential Target for Antibiotics

Mycobacteria contain genes for several DNA-dependent RNA primases, including dnaG, which encodes an essential replication enzyme that has been proposed as a target for antituberculosis compounds. An in silico analysis revealed that mycobacteria also possess archaeo-eukaryotic superfamily primases (AEPs) of unknown function. Using a homologous recombination system, we obtained direct evidence that wild-type dnaG cannot be deleted from the chromosome of Mycobacterium smegmatis without disrupting viability, even in backgrounds in which mycobacterial AEPs are overexpressed. In contrast, single-deletion AEP mutants or mutants defective for all four identified M. smegmatis AEP genes did not exhibit growth defects under standard laboratory conditions. Deletion of native dnaG in M. smegmatis was tolerated only after the integration of an extra intact copy of the M. smegmatis or Mycobacterium tuberculosis dnaG gene, under the control of chemically inducible promoters, into the attB site of the chromosome. M. tuberculosis and M. smegmatis DnaG proteins were overproduced and purified, and their primase activities were confirmed using radioactive RNA synthesis assays. The enzymes appeared to be sensitive to known inhibitors (suramin and doxorubicin) of DnaG. Notably, M. smegmatis bacilli appeared to be sensitive to doxorubicin and resistant to suramin. The growth and survival of conditional mutant mycobacterial strains in which DnaG was significantly depleted were only slightly affected under standard laboratory conditions. Thus, although DnaG is essential for mycobacterial viability, only low levels of protein are required for growth. This suggests that very efficient inhibition of enzyme activity would be required for mycobacterial DnaG to be useful as an antibiotic target.     

Successful treatment with chemotherapy and corticosteroids of pulmonary Mycobacterium abscessus infection accompanied by pleural effusion

We describe a 50-year-old woman with rapidly progressive pulmonary Mycobacterium abscessus (M. abscessus) infection accompanied by pleural effusion and organizing pneumonia (OP). CT scan showed consolidation, centrilobular shadows, ground-glass opacity (GGO), and cavities. A transbronchial lung biopsy showed nonnecrotizing granuloma surrounded by infiltrative lymphocyte-dominant inflammatory cells, and lymphocytes in bronchoalveolar lavage fluid (BALF) were increased. We considered OP occurred secondary to M. abscessus infection because clarithromycin, amikacin, and imipenem/cilastatin administration resulted in partial improvement. We added corticosteroids to the regimen, which resulted in a remarkable improvement. We report a case of pulmonary M. abscessus infection involving pleural effusion that responded favorably to medical therapy including corticosteroids.     

Moxifloxacin's Limited Efficacy in the Hollow-Fiber Model of Mycobacterium abscessus Disease

Current regimens used to treat pulmonary Mycobacterium abscessus disease have limited efficacy. There is an urgent need for new drugs and optimized combinations and doses. We performed hollow-fiber-system studies in which M. abscessus was exposed to moxifloxacin lung concentration-time profiles similar to human doses of between 0 and 800 mg/day. The minimum bactericidal concentration and MIC were 8 and 2 mg/liter, respectively, in our M. abscessus strain, suggesting bactericidal activity. Measurement of the moxifloxacin concentrations in each hollow-fiber system revealed an elimination rate constant (k_el) of 0.11 ± 0.05 h⁻¹ (mean ± standard deviation) (half-life of 9.8 h). Inhibitory sigmoid maximal effect (E_max) modeling revealed that the highest E_max was 3.15 ± 1.84 log₁₀ CFU/ml on day 3, and the exposure mediating 50% of E_max (EC₅₀) was a 0- to 24-h area under the concentration time curve (AUC_0–24)-to-MIC ratio of 41.99 ± 31.78 (r² = 0.99). The EC₈₀ was an AUC_0–24/MIC ratio of 102.11. However, no moxifloxacin concentration killed the bacteria to burdens below the starting inoculum. There was regrowth beyond day 3 in all doses, with replacement by a resistant subpopulation that had an MIC of >32 mg/liter by the end of the experiment. A quadratic function best described the relationship between the AUC_0–24/MIC ratio and the moxifloxacin-resistant subpopulation. Monte Carlo simulations of 10,000 patients revealed that the 400- to 800-mg/day doses would achieve or exceed the EC₈₀ in ≤12.5% of patients. The moxifloxacin susceptibility breakpoint was 0.25 mg/liter, which means that almost all M. abscessus clinical strains are moxifloxacin resistant by these criteria. While moxifloxacin''s efficacy against M. abscessus was poor, formal combination therapy studies with moxifloxacin are still recommended.