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.     

Nontuberculous mycobacterial infections in cancer patients in a medical center in Taiwan, 2005-2008

Data on the nontuberculous mycobacterial (NTM) species that cause infection and the characteristics of disease caused by these pathogens in cancer patients are limited, so we perform this study to investigate the species distribution of NTM isolates from various clinical specimens and to elucidate the epidemiologic trends in NTM isolates and diseases among cancer patients. From 2005 through 2008, cancer patients with NTM infections as defined by the American Thoracic Society/Infectious Diseases Society of America criteria were identified at the National Taiwan University Hospital. The medical records of all patients were reviewed. During the study period, a total of 219 cancer patients with NTM infections were identified. Among them, 133 (60.7%) patients were older than 65 years, most of whom were men. Lung cancer was the most common type of cancer, followed by hematologic cancer and gastrointestinal tract cancer. Pulmonary NTM infection was the most common type of infection in 205 (93.6%) patients, followed by skin and soft tissue infections (n = 7, 3.2%), disseminated infections (n = 4, 1.8%), and genitourinary tract infection (n = 3, 1.4%). Disseminated infections occurred exclusively in patients with hematologic cancer. Mycobacterium avium complex (MAC) caused the majority of pulmonary NTM infections in cancer patients; in contrast, M. abscessus was the most common causative pathogen of extrapulmonary NTM diseases, followed by MAC. In conclusion, physicians need to be aware of the possibility of co-existing pulmonary NTM infection in patients with lung cancer. In addition, disseminated NTM infection should be considered in patients with hematologic cancer.     

Inactivation of nontuberculous mycobacteria by gaseous ozone treatment

Nontuberculous mycobacteria (NTM) are environmental bacteria resistant to many common disinfectants and ultraviolet radiation. Inhalation of aerosols generated from NTM-containing water and soil causes NTM lung disease, especially in people with underlying lung diseases and decreased immunity. To prevent healthcare-acquired NTM infections, it is important to eradicate NTM living in hospital environments. Therefore, we evaluated the efficacy of gaseous ozone for the inactivation of NTM, namely Mycobacterium (M.) avium, M. intracellulare, M. kansasii, M. abscessus subsp. abscessus and M. abscessus subsp. massiliense. Gaseous ozone treatment at 1 ppm for 3 h reduced the bacterial number of all strains by more than 97%. Gaseous ozone treatment could be a practical, effective and convenient disinfection method for NTM living in hospital environments.     

Differentiating rapid- and slow-growing mycobacteria by difference in time to growth detection in liquid media

Nontuberculous mycobacteria (NTM) are classified into 2 categories: slow-growing mycobacteria (SGM) and rapid-growing mycobacteria (RGM), based on interval to colony formation by subculture on solid media. However, little is known about the growth rate of NTM in liquid broth media. We evaluated the differences in time to growth detection (TGD) of RGM and SGM in liquid broth media according to acid-fast stain. Among the 696 NTM isolates, 201 were RGM and 495 were SGM. In acid-fast bacilli (AFB)–negative specimens, the mean TGD was 133 h for RGM and 269 h for SGM (P < 0.001). In AFB-positive specimens, the mean TGD was 112 ± 37 h for RGM and 155 ± 125 h for SGM (P = 0.063). In the AFB-negative group, a cut-off value of 6 days was most effective for distinguishing SGM from RGM; however, in the AFB-positive group, an appropriate cut-off value was hard to define with TGD only.     

Simultaneous Detection of Mycobacterium tuberculosis and Atypical Mycobacteria by DNA-Microarray in Egypt

Background and ObjectivesImmunocompromised patients are a high-risk group for developing mycobacterial infections with either pulmonary and/or extrapulmonary diseases. Low-cost/density DNA-microarray is considered an easy and efficient method for the detection of typical and atypical mycobacterial species.Materials and MethodsThirty immunocompromised patients were recruited to provide their clinical specimens (sputum, serum, urine, and lymph node aspirates). Real-time polymerase chain reaction (PCR) and DNA-microarray techniques were performed and compared to the conventional methods of Ziehl-Neelsen staining and Lowenstein Jensen culturing.ResultsMycobacterium tuberculosis complex was detected in all 30 clinical specimens (100% sensitivity) by real-time PCR and DNA-microarray. Additionally, coinfection with 4 atypical species belonging to nontuberculous mycobacteria was identified in 7 sputum specimens. These atypical mycobacterial species were identified as M. kansasii 10% (n = 3), M. avium complex 6.6% (n = 2), M. gordanae 3.3% (n = 1), and M. peregrinum 3.3% (n = 1).ConclusionThis study documents the presence of certain species of atypical mycobacteria among immunocompromised patients in Egypt.     

Isolation of nontuberculous mycobacteria from patients with pneumoconiosis

This study aimed to confirm the isolation of nontuberculous mycobacteria (NTM) from patients with pneumoconiosis. Monthly sputum examinations in 155 patients were performed from April 1998 to December 2002. When NTM were isolated, species were identified and the frequency of isolation was reviewed. We then identified the patients who satisfied the bacteriologic criteria for the diagnosis of nontuberculous mycobacterial pulmonary disease (NTM pulmonary disease) recommended by the American Thoracic Society (ATS). Symptoms and findings on computed tomography (CT) scans were evaluated. NTM were isolated from 60 patients (39%): common etiologic species defined by the ATS, i.e., Mycobacterium avium, M. intracellulare, M. abscessus, and M. kansasii, were identified in 21 patients; unusual etiologic species, i.e., M. fortuitum, M. simiae, and M. szulgai, were identified in 11 patients; and undefined species, which appeared to be nonpathogenic, were identified in 41 patients. The bacteriologic criteria were satisfied in 8 patients. NTM species isolated in conformity with the bacteriologic criteria were: M. avium in 4 patients, M. intracellulare in 2, a combination of M. intracellulare and M. kansasii in 1, and M. gordonae in 1 patient. Two patients, from whom M. avium were repeatedly isolated, satisfied the ATS diagnostic criteria for NTM pulmonary disease. It is important to note that NTM, including both pathogenic species and nonpathogenic species, were isolated from patients with pneumoconiosis.     

Nontuberculous mycobacteria pulmonary infection in medical intensive care unit: the incidence,patient characteristics,and clinical significance

Background  The clinical significance of nontuberculous mycobacteria (NTM) pulmonary infection in medical intensive care unit (ICU) is still unclear. Materials and methods  We conducted a retrospective study in the medical ICUs of a medical center in Taiwan from January 1999 to June 2007. Patients with NTM isolated from respiratory specimens within 1 month before or during the ICU course were identified. Those who fulfilled the diagnostic criteria of NTM pulmonary infection were identified and compared with patients with NTM colonization and control subjects who were culture-negative for mycobacteria. Results  Among the 5,378 patients admitted to medical ICUs, 2,866 (53.3%) had received mycobacterial culture for respiratory specimens. NTM were isolated from 169 (5.8%) patients. Of them, 47 (27.8%) were considered NTM pulmonary infection. M. avium complex and M. abscessus were the most common pathogens. Within 100 days after ICU admission, significantly more patients with NTM infection died than those with NTM colonization and control subjects (47 vs. 8 vs. 14%, P < 0.001). Twenty-one (49%) patients with NTM pulmonary infection received anti-NTM treatment, with four experiencing adverse effects. Although statistically insignificant, anti-NTM treatment was associated with prolonged survival for those who died in the ICU and shorter ICU stay for those who survived the ICU course. Conclusion  Our findings suggest that NTM pulmonary infection seems to associate with higher mortality in medical ICUs. Anti-NTM treatment is probably associated with a better outcome. Therefore, keeping a high suspicion when NTM is isolated and using careful consideration when starting anti-NTM treatment should be emphasized. Taiwan Anti-Mycobacteria Investigation (TAMI) group: J.-Y. Wang, L.-N. Lee, C.-J. Yu, P.-C. Yang, W.-J. Su, C.-C. Shu, H.-C. Lai, C.-H. Lee and M.-C. Yu.     

Vertebral osteomyelitis caused by Mycobacteroides abscessus subsp. abscessus resulting in spinal cord injury due to vertebral body fractures

Nontuberculous mycobacteria (NTM) rarely cause vertebral osteomyelitis; however, the clinical characteristics of vertebral osteomyelitis caused by NTM are poorly understood due to its rarity. A 74-year-old man with lung cancer was treated with prednisolone for immune checkpoint inhibitor-associated immune-related adverse events. He had been experiencing mild back pain without febrile episodes for five months, and was admitted to the hospital for worsening back pain and progressive paraplegia. Magnetic resonance imaging showed spinal cord compression at T4-5 due to fractures of the T5 and T7 vertebral bodies. The culture of a sample of pus from the T7 vertebral body obtained at the time of spinal fusion surgery yielded the Mycobacteroides abscessus (M. abscessus) complex. The patient was diagnosed with vertebral osteomyelitis caused by M. abscessus complex and treated with clarithromycin, amikacin, and imipenem; clarithromycin was later replaced by sitafloxacin because of inducible macrolide resistance. However, his neurologic deficits were irreversible, and he died due to a deteriorating general condition. The strain was identified up to subspecies level as M. abscessus subsp. abscessus by hsp65 and rpoB sequencing and nucleic acid chromatography. Although vertebral osteomyelitis due to NTM is rare, delayed diagnosis can lead to serious complications or poor outcomes. A prolonged clinical course, less frequent fever, vertebral destruction or spinal deformity, neurological deficits, or immunosuppressed conditions might be suggestive of NTM vertebral osteomyelitis.     

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.     

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.     

Clinicopathologic characteristics of nontuberculous mycobacterial lung disease in Taiwan

Taiwan is an endemic area for tuberculosis (TB), and the incidence of pulmonary infection caused by nontuberculous mycobacteria (NTMs) is also increasing. This study aims to investigate the clinicopathologic characteristics of patients with NTM lung disease during 1998 to 2007 at a medical center in Taiwan. The medical records of patients with confirmed NTM pulmonary infections who underwent open lung surgery in a medical center were reviewed. Twenty-four patients with confirmed NTM pulmonary infections were identified. These patients were histologically classified into 4 types: fibrocavitary/tuberculoid (n = 10), nodular bronchiectatic (n = 4), sarcoidal (n = 6), and other (n = 4). The fibrocavitary/tuberculoid type usually (90%) develops in the upper lobes of old patients with preexisting lung disease. Pulmonary TB (n = 7, 70%) was the major underlying disease before 2003. Nodular bronchiectatic type occurred mainly in the middle lobe of middle-aged women without preexisting lung disease. Sarcoidal type was usually associated with Mycobacterium avium complex infection and develops in middle-aged women. Immunoreactive bacilli were detected in 21 patients (87 %) by immunohistochemical staining using a polyclonal antibody against Mycobacterium tuberculosis and other mycobacterial species (M. avium-intracellulare, Mycobacterium phlei, and Mycobacterium parafortuitum), whereas conventional acid-fast staining was positive in only 21% of patients. In conclusion, TB was the major underlying disease in patients with NTM lung disease in Taiwan. The different histologic types of pulmonary NTM infection suggest each had a distinct pathogenesis.     

Recent increase in non-tuberculous mycobacterial infection in patients with connective tissue diseases in Japan

BackgroundNon-tuberculous mycobacterial (NTM) infection is currently a growing health concern due to the increasing incidence and the need for prolonged therapy. In patients with connective tissue diseases, use of immunosuppressants may lead to an increased risk of NTM infection. However, few studies have examined the recent incidence of NTM infection among connective tissue diseases patients. This study investigated recent trends in NTM infection among connective tissue diseases patients.MethodsWe included adult patients from whose cultures NTM were isolated between January 2009 and October 2017 in our hospital. By reviewing their medical records, connective tissue diseases patients were identified. Types of connective tissue disease, NTM species, and treatment of NTM infection were extracted.ResultsNTM was isolated from 657 patients during the period. Among these, 24 patients had connective tissue diseases. The number and rate of NTM isolates from connective tissue diseases patients increased during the period, with 4 patients 2009 to 2012 (1.9%), and 20 patients from 2013 to 2017 (3.3%; P = 0.04). The proportion of Mycobacterium avium complex (MAC) to total NTM tended to be lower among connective tissue diseases patients (58.3%) than among non-connective tissue disease-patients (72.8%), but the difference was not significant (P = 0.20). Mycobacterium xenopi was significantly more frequent in connective tissue disease patients than in non-connective tissue diseases patients (P < 0.01).ConclusionThe recent increase in the incidence of NTM infections in connective tissue diseases patients was larger than that in the total population. NTM species other than MAC were isolated from connective tissue diseases patients.     

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.     

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.     

Drug susceptibility patterns of Mycobacterium abscessus and Mycobacterium massiliense isolated from respiratory specimens

In this study, we aimed to retrospectively investigate and compare the drug susceptibility patterns of two major Mycobacterium abscessus complex (MABC) species; M. abscessus and M. massiliense. A total of 546?MABC respiratory isolates (277?M. abscessus and 269?M. massiliense) from 2011 to 2016 were analyzed in this study. We estimated minimum inhibitory concentrations (MICs) using the broth microdilution method, and we calculated MIC₅₀ and MIC₉₀ values from the MIC distribution. Both M. abscessus and M. massiliense were highly susceptible to amikacin and linezolid. For M. abscessus, the proportions of inducible and acquired resistance to clarithromycin were 68.6% and 12.3%, respectively. Only 15.2% of M. abscessus remained susceptible at day 14. On the other hand, none of the M. massiliense showed inducible resistance and 6.3% showed acquired resistance to clarithromycin. A total of 92.6% of the M. massiliense remained susceptible at day 14. The resistance rate of M. abscessus to moxifloxacin (90.3%) was significantly higher than that of M. massiliense (83.3%; p?=?0.016). These susceptibility differences may explain the divergent treatment outcomes between patients with pulmonary disease caused by these two species.     

Differences in drug susceptibility pattern between Mycobacterium avium and Mycobacterium intracellulare isolated in respiratory specimens

Mycobacterium avium complex (MAC) is the most common etiologic organisms of nontuberculous mycobacteria (NTM) lung disease. In this study, we aimed to retrospectively investigate the differences in drug susceptibility patterns of two major MAC species; Mycobacterium avium and Mycobacterium intracellulare. A total of 1883 major two MAC isolates (1060 M. avium and 823 M. intracellulare) from respiratory specimens were included in this study during the period 2011─2016. The minimum inhibitory concentrations (MICs) were determined by broth microdilution method and MIC₅₀/MIC₉₀ values were derived from MIC distribution. M. intracellulare had generally low susceptible rates than M. avium for almost all tested antimicrobials except ethambutol and amikacin. The susceptible rate to clarithromycin was >94% of the MAC without significant differences between the two species. The MIC₅₀ values of ciprofloxacin, clarithromycin, linezolid, moxifloxacin, and rifampicin were higher in M. intracellulare than in M. avium, contrary to the results of ethambutol with a higher MIC₅₀ in M. avium. In general, M. intracellulare showed a higher resistance rate and higher MIC₅₀ values than M. avium. Differences between this study and previous reports suggest regional differences in drug susceptibility profile of MAC species.     

Reduced phagocytic activity of human alveolar macrophages infected with Mycobacterium avium complex

IntroductionCo-infection of nontuberculous mycobacteria (NTM) with other bacteria is associated with increased frequency of hospitalization and reduced quality of life. However, the clinical significance of co-infection with NTM and other bacteria remains unclear. Here, we investigated the distribution of alveolar macrophage populations, characterized their phagocytic function in bronchoalveolar lavage fluid (BALF), and assessed the bactericidal function of macrophages infected with NTM using cell lines.MethodsBALF samples were prospectively obtained from 30 patients with suspected NTM lung disease to evaluate phagocytic activities of macrophages using immunostaining. Bactericidal activities of Staphylococcus aureus (S. aureus) and Mycobacterium intracellulare (M. intracellulare)-infected or -non-infected macrophages were evaluated using macrophage cell lines.ResultsEleven patients with Mycobacterium avium complex (MAC) infection and 19 patients with chronic lower respiratory tract infections except for NTM infection (controls) were enrolled. The percentage of non-polarized (HLA-DR⁺, CD40^?, and CD163^?) macrophages in patients infected with MAC was significantly higher than that in controls; non-polarized macrophages demonstrated an impaired ability to phagocytose S. aureus. In vitro experiments revealed higher intracellular S. aureus colony-forming unit counts and proinflammatory cytokine levels in M. intracellulare-infected macrophages than in non-NTM-infected macrophages. Electron microscopy showed morphologically damaged macrophages and M. intracellulare and S. aureus growing in the same phagosome.ConclusionThe proportion of alveolar macrophages (HLA-DR⁺, CD40^?, and CD163^?) with impaired phagocytosis increased in MAC-infected individuals. M. intracellulare-infected macrophages reduced bactericidal activity in vitro. Dysfunction of alveolar macrophages may contribute to persistent infection by other bacteria, leading to MAC lung disease progression.     

Characteristics of patients with bronchoscopy-diagnosed pulmonary Mycobacterium avium complex infection

Background

We occasionally treat patients with clinically suspected pulmonary Mycobacterium avium complex (MAC) infection and negative MAC culture on bronchoscopy.

In Vitro Synergy between Clofazimine and Amikacin in Treatment of Nontuberculous Mycobacterial Disease

Disease caused by nontuberculous mycobacteria (NTM) is increasing in frequency. The outcome of treatment for NTM lung disease is poor, particularly lung disease caused by Mycobacterium simiae and M. abscessus. Exploring synergy between active available drugs is a sensible way forward given the lack of new active drugs. We tested for synergy between amikacin and clofazimine, using standardized methods, in 564 consecutive clinical isolates identified as 21 species of rapidly growing mycobacteria, 16 clinical M. avium complex isolates, and 10 M. simiae isolates. Clofazimine and amikacin are each active in vitro against NTM; 97% (n = 548) of the rapid growers revealed MICs of clofazimine of ≤1 μg/ml, and 93% (n = 524) proved susceptible to amikacin. The combination showed significant synergistic activity in 56 of 68 (82%) eligible M. abscessus isolates, 4 of 5 M. chelonae isolates, and 1 M. fortuitum and 1 M. cosmeticum isolate, with 4- to 8-fold decreases in MICs to both drugs. Significant synergy could also be demonstrated against all M. avium complex and M. simiae isolates, with fractional inhibitory concentrations of <0.5. Clofazimine and amikacin show significant synergistic activity against both rapidly and slowly growing nontuberculous mycobacteria. The safety and tolerability of adding clofazimine to amikacin-containing regimens should be tested in clinical trials, and the results of susceptibility tests for these two compounds and their combination merit clinical validation. Synergy between clofazimine and other antibiotics with intracellular targets should be explored.