Nontuberculous mycobacteria in women, young and old

Nontuberculous Mycobacteria, Mucociliary Clearance, and Bronchiectasis

Nontuberculous mycobacteria (NTM) are environmental and ubiquitous, but only a few species are associated with disease, often presented as nodular/bronchiectatic or cavitary pulmonary forms. Bronchiectasis, airways dilatations characterized by chronic productive cough, is the main presentation of NTM pulmonary disease. The current Cole's vicious circle model for bronchiectasis proposes that it progresses from a damaging insult, such as pneumonia, that affects the respiratory epithelium and compromises mucociliary clearance mechanisms, allowing microorganisms to colonize the airways. An important bronchiectasis risk factor is primary ciliary dyskinesia, but other ciliopathies, such as those associated with connective tissue diseases, also seem to facilitate bronchiectasis, as may occur in Lady Windermere syndrome, caused by M. avium infection. Inhaled NTM may become part of the lung microbiome. If the dose is too large, they may grow excessively as a biofilm and lead to disease. The incidence of NTM pulmonary disease has increased in the last two decades, which may have influenced the parallel increase in bronchiectasis incidence. We propose that ciliary dyskinesia is the main promoter of bronchiectasis, and that the bacteria most frequently involved are NTM. Restoration of ciliary function and impairment of mycobacterial biofilm formation may provide effective therapeutic alternatives to antibiotics.

Screening and Drug Resistance Analysis of Non-Tuberculous Mycobacteria in Patients with Suspected Pulmonary Tuberculosis on the Hainan Island, China

Background

China has seen a drastic increase in the incidence of non-tuberculous mycobacteria (NTM) infection, which is a notable public health issue. Due to a lack of reliable epidemiological surveillance information, there is a need to gather accurate epidemiological and surveillance data, which can help clinicians effectively treat NTM patients. Moreover, drug susceptibility testing for NTM is not frequently performed in China. This retrospective study, therefore, determined the prevalence and resistance characteristics of NTM to provide a reference to control the NTM epidemic.

Methods

Sputum, alveolar lavage fluid, and other respiratory specimens were collected from 3025 patients with suspected pulmonary tuberculosis attending The Second Affiliated Hospital of Hainan Medical University from January 2014 to December 2021. Strain identification and species distribution of NTM were performed by DNA chip technology and gene sequencing, and the drug resistance of NTM isolates was evaluated by calculating the minimum inhibitory concentration through antimicrobial susceptibility testing for NTM.

Results

From 2014 to 2021, 373 strains of NTM were isolated and identified from respiratory specimens of 3025 suspected tuberculosis patients. Except in 2014, NTM-infected patients accounted for more than 10% of suspected tuberculosis patients in other years. The median age of patients with NTM infection was 62.0 years (53.0, 71.0), and the male-to-female ratio among these patients was 0.79:1. Among culture-positive strains, 12.3% (373/3040; 95% CI 11.1-13.4%) were identified as NTM comprising forty species of NTM. The forty species of NTM included 23 slow-growing mycobacteria (SGM) and 17 rapidly-growing mycobacteria (RGM). Among the NTM isolates, 58.7% (219/373; 95% CI 53.7-63.7%) were SGM and 41.3% (154/373; 95% CI 36.3-46.3%) were RGM. M.avium complex(MAC)(41.3%; 95% CI 36.3-46.3%) and M.abscessus complex (MABC)(33.2%; 95% CI 28.4-38.0%) were the most frequently detected species, followed by M.simiae Complex (11.8%; 95% CI 8.5-15.1%), M.fortuitum group (5.1%; 95% CI 2.9-7.3%), and others. Drug sensitivity test results showed that most of the NTM isolates were susceptible to amikacin and clarithromycin with a drug resistance rate of less than 10%. However, clarithromycin could induce drug resistance, followed by linezolid and moxifloxacin, and their drug resistance rate was less than 50%.

Conclusion

During 2014-2021, the number of NTM isolates detected in the respiratory specimens of the study patients in The Second Affiliated Hospital of Hainan Medical University increased year by year. M. intracellulare is the most common pathogenic NTM species, and there is a high incidence of NTM infection on Hainan Island. Our findings might be of great importance for diagnosing and treating this patient population in Hainan.

Risk factors for nontuberculous mycobacterial pulmonary disease (NTM-PD) in Croatia

BACKGROUND

The incidence, geographical distribution and clinical relevance of different nontuberculous mycobacteria (NTM) in Croatia are well described. There are few data on the risk factors for developing NTM pulmonary disease (NTM-PD) in this setting.

METHODS

We conducted a retrospective cohort study on all Croatian residents with NTM isolated from respiratory samples in the period from 2006 to 2015 with follow-up to 2018. The American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) guidelines were used to establish NTM-PD diagnosis. Clinical, radiological and treatment data were collected from hospital records.

RESULTS

Risk analysis calculations were made on the 439 isolation episodes that were classified as definitive NTM-PD (n = 137) or no disease (n = 302). Female gender, presence of bronchiectasis, low BMI and long-term systemic corticosteroid treatment were independent risk factors associated with NTM-PD. Hemoptysis and malaise were presenting symptoms independently associated with NTM-PD. Chronic obstructive pulmonary disease (COPD) and low/moderate dose inhaled corticosteroid (ICS) treatment were not associated with NTM-PD. High dose ICS treatment was a significant risk factor for developing NTM-PD (aOR = 4.73, CI 1.69-13.23 p = 0.003).

CONCLUSION

The NTM-PD patients in Croatia are similar to those in other published cohorts in terms of their characteristics and risk factors. The significant dose-dependent association between ICS use and NTM-PD adds to the body of evidence suggesting that high dose ICS use is associated with NTM-PD.

Nontuberculous mycobacterial lung infections in patients with eating disorders: plausible mechanistic links in a case series

Nontuberculous mycobacteria (NTM) are widely distributed in the environment and are almost always acquired into the lungs by bioaerosol inhalation or aspiration of NTM-contaminated water, biofilms, and soil. NTM are increasingly recognized as causes of lung diseases in immunocompetent hosts, a not insignificant number of whom have a life-long or nearly life-long slender body habitus as well as thoracic cage abnormalities such as scoliosis and pectus excavatum. While several hypotheses have been offered to explain the purported increase in susceptibility to NTM lung disease in such individuals, the precise explanation remains unknown. We described three patients with eating disorders associated with severe malnutrition and either purging behaviors or other risks for aspiration who were diagnosed with NTM lung infections-the largest number of such patients to date in a single report. We discuss the clinical and experimental evidence that low body weight and chronic vomiting with attendant jeopardy for aspiration, as seen in patients with eating disorders, may represent risk factors for NTM lung disease. We also speculate the possibility of occult and undiagnosed eating disorders in some of the slender NTM lung disease patients with no known risk factors for the opportunistic infection other than their low body weight.

Low serum estradiol levels are related to Mycobacterium avium complex lung disease: a cross-sectional study

Background

The risk factors for Mycobacterium avium complex lung disease (MAC-LD) are not well known. We hypothesized that low serum estradiol (E2) levels are related to MAC-LD as most patients with MAC-LD are postmenopausal women.

Methods

This cross-sectional study compared patients with MAC-LD and healthy controls. Study subjects were postmenopausal women aged 65 years or younger. Serum testosterone, dehydroepiandrosterone sulfate (DHEA-S), and E2 levels were measured and categorized as high or low based on median levels. We performed multivariate analysis, receiver operating characteristic (ROC) curve analysis, and age- and body mass index (BMI)-matched subgroup analysis to evaluate the association between low serum E2 levels and MAC-LD.

Additionally, using blood samples obtained for other clinical studies, the levels of sex steroid hormones were compared between age- and BMI-matched MAC-LD and bronchiectasis female patients without non-tuberculosis mycobacterial infections (non-NTM BE).

Results

Forty-two patients with MAC-LD and 91 healthy controls were included. The median E2 (2.20 pg/mL vs. 15.0 pg/mL, p < 0.001), testosterone (0.230 ng/L vs. 0.250 ng/L, p = 0.005), and DHEA-S (82.5 μg/dL vs. 114.0 μg/dL, p < 0.001) levels were lower in the MAC-LD group than in the control group. Multivariate analysis revealed that low serum E2 (adjusted odds ratio = 34.62, 95% confidence interval = 6.02–199.14) was independently related to MAC-LD, whereas low DHEA-S and testosterone were not. ROC analysis illustrated a strong relationship between low serum E2 levels and MAC-LD (area under the curve = 0.947, 95% confidence interval = 0.899–0.995). Even the age- and BMI-matched subgroup analysis of 17 MAC-LD patients and 17 healthy controls showed lower serum E2 in MAC-LD patients than in healthy controls. Additionally, serum E2 levels of 20 MAC-LD patients were lower than plasma E2 levels of 11 matched non-NTM BE patients (1.79 pg/mL vs. 11.0 pg/mL, p < 0.001).

Conclusions

Low serum E2 levels were strongly related to MAC-LD in postmenopausal women.

Lady Windermere syndrome in Castile and León

INTRODUCTION

Lady Windermere syndrome (LWS) is a pulmonary disease caused by Mycobacterium avium complex (MAC). The objective of this study is to ascertain its frequency and characteristics in the northern area of the autonomous community of Castile and León.

METHODS

A retrospective study of patients with MAC isolates in respiratory samples from five public hospitals in the autonomous community over a six-year period, following the ATS/IDSA criteria. The MAC strains were identified by GenoType Mycobacterium reverse hybridisation probes or PCR-RFLP analysis of the hsp65 gene.

RESULTS

Of 183 cases of MAC identified, only five women (2.7%) aged 68.8±10.7years met LWS criteria. In three cases, MAC was isolated jointly and intermittently with other pathogens. Only one patient was treated according to ATS/IDSA criteria.

DISCUSSION

LWS remains underestimated, with affected patients representing a significant burden on healthcare resources over long periods of time. As a result, greater microbiological and therapeutic knowledge of the syndrome is needed.

Solitary pulmonary caseating granulomas: A 5-year retrospective single-center analysis

Solitary pulmonary caseating granulomas (SPCGs) are a characteristic type of tuberculomas associated with infection with non-tuberculous mycobacteria (NTM) and other microbes; however, their significance remains unclear. The aim of the present study was to describe the clinical characteristics of patients with SPCGs in terms of diagnosis, presence of lung cancer and treatment status. A retrospective analysis of 17 immunocompetent patients with histopathologically confirmed caseating granulomas after undergoing video-assisted thoracoscopic surgery (VATS) was conducted at our center between 2011 and 2015. The patients comprised 10 men and 7 women with a mean age of 59.1±14.4 years. Of the 17 patients, 14 (82.4%) were asymptomatic and the lesions were discovered incidentally. In 2 patients the SPCGs were accompanied by a small satellite nodule (SPCG mean diameter, 16.2±5.1 mm). Mycobacteria, including Mycobacterium tuberculosis (11.8%), Mycobacterium avium (11.8%) Mycobacterium kansasii (23.5%) and other Mycobacterium spp. (5.9%), were isolated from 9 of the patients (52.9%). Concurrent lung cancer was present in 3 patients (17.6%). When microbial agents could not be isolated, the interferon-γ release assay was useful for diagnosis. Positron emission tomography was not found to be useful for differentiating SPCGs from lung cancer, or for differentiating tuberculomas from NTM pulmonary nodules (NTMPNs). NTMPNs in cases of SPCGs were diagnosed more frequently in men. The findings indicate that a course of observation may be sufficient for patients in whom an SPCG from NTM (NTMPN) is identified by VATS. However, the presence of concurrent lung cancer in certain cases indicates that malignancy should not necessarily be excluded, particularly in NTMPNs, and highlights the necessity of aggressive diagnosis by VATS.

Normal estrogen, but low dehydroepiandrosterone levels, in women with pulmonary Mycobacterium avium complex. A preliminary study

RATIONALE

For unclear reasons, the phenotypical hosts for nontuberculous mycobacterial lung infection are often thin, elderly, white women without underlying lung disease. As these women are usually postmenopausal, we hypothesized that a state of relative hormone deficiency may predispose some women to pulmonary nontuberculous mycobacterial infection.

OBJECTIVES

To conduct a prospective cross-sectional study to assess for alterations in systemic levels of sex hormones in patients with confirmed pulmonary Mycobacterium avium complex infection compared with healthy control subjects.

METHODS

Female patients with pulmonary M. avium complex infection (n = 35) were recruited along with similar-aged control subjects (n = 27) without lung disease from the general population of our institution. Levels of dehydroepiandrosterone-sulfate (DHEA-S), estrone, and ultrasensitive estradiol were measured from sampled blood.

MEASUREMENTS AND MAIN RESULTS

DHEA-S levels of patients with M. avium complex infection were significantly lower than control subjects (mean 33 μg/dl vs. 59 μg/dl, P = 0.001). No significant difference was found in the levels of estrone (mean, 27 pg/ml vs. 28 pg/ml, P = 0.665) or ultrasensitive estradiol (mean, 9 pg/ml vs. 9 pg/ml, P = 0.364). Patients with M. avium complex had a lower body mass index (BMI) than control subjects (mean, 22 vs. 26, P = 0.001). There was no association between levels of DHEA-S, estrone, or estradiol, and BMI or age.

CONCLUSIONS

Women with M. avium complex infection had lower DHEA-S levels, but not lower estrogen levels, compared with control subjects. There was no relationship between BMI and hormone levels in the study population. Further study of these hormonal effects on immune function in nontuberculous mycobacterial infection is warranted.

Nutritional status and eating disorders: neglected risks factor for nontuberculous mycobacterial lung disease?

Nontuberculous mycobacterial lung disease (NTMLD) in immunocompetent patients is an increasingly important epidemiologic concern. However, risk factors associated with susceptibility to NTMLD are not completely known. A prevalence of NTMLD appears to be rising, mainly in some populations such as middle-aged or elderly thin women, (a group including those with Lady Windermere syndrome) with neither remarkable history of respiratory disease nor smoking habit. Right middle lobe (RML) and lingula are often involved. Various predisposing factors and genetic defects have been described as possible causes of development of NTMLD, namely: voluntary suppression of cough, RML anatomical factors, menopause and mutations in cystic fibrosis transmembrane conductance regulator (CFTR). Malnutrition is also an important and common risk factor associated with other mycobacterial disease like tuberculosis (TB) and its probable association with NTMLD as have been pointed out for some authors. However, a real description of all nutritional aspects and eating habits of patients prior to NTMLD diagnosis is lacking. We hypothesized that malnutrition and eating disorders like anorexia nervosa could be risk factors that may promoting NTMLD. From a clinical viewpoint, if this hypothesis proves to be correct, eating habits and nutritional aspects should be taken into account in the diagnosis process of suspected NTMLD, since they are easily identifiable and treatable conditions.

Slender, older women appear to be more susceptible to nontuberculous mycobacterial lung disease

BACKGROUND

Nontuberculous mycobacteria (NTM) are environmental microbes that are associated with a variety of human diseases, particularly chronic lung infections. Over the past several decades, NTM lung disease has been increasingly seen in postmenopausal women with slender body habitus.

OBJECTIVE

This article reviewed the clinical and experimental evidence that supports the observation that thin older women (aged 50-80 years) are predisposed to NTM lung disease. We posited 3 potential pathways for this predisposition: relative estrogen deficiency, abnormal levels of adipokines that alter immune responses, and abnormal expression of transforming growth factor-beta (TGF-beta) related to fibrillin anomalies similar to Marfan syndrome (MFS).

METHODS

Using the PubMed database, a literature search was performed (all publications up to July 2009) by pairing the key phrase nontuberculous mycobacteria with weight, malnutrition, female gender, body habitus, leptin, adipokines, estrogen, menopause, postmenopausal, or body mass index. Non-English-language articles were included if their abstracts were in English. Relevant articles were also identified from the abstracts.

RESULTS

Published case reports and series indicate that in the past 20 years, NTM lung disease has been recognized in disproportionately increased numbers in postmenopausal women. Among these patients, slender body habitus and thoracic cage abnormalities, such as pectus excavatum and scoliosis, are commonly described. Notably, no long-term prospective clinical studies exist to corroborate that low weight is an independent risk factor for NTM lung disease. However, based on the findings of a limited number of experimental studies, we hypothesize that decreased leptin, increased adiponectin, and/or decreased estrogen in older women with slender body habitus may account for their increased susceptibility to NTM infections. We further speculate that in some patients with features mindful of MPS (slender, scoliosis, pectus excavatum, or mitral valve prolapse), there may be anomalies of fibrillin, similar to MFS, that lead to the expression of the immunosuppressive cytokine TGP-beta further increasing their susceptibility to NTM.

CONCLUSIONS

It is likely that both sufficient environmental exposure and host susceptibility are required for the establishment of NTM lung disease. The observation that NTM lung infections are more common in slender, older women without any overt immune defects suggests that abnormal expression of adipokines, sex hormones, and/or TGF-beta may play an important role in their susceptibility.

Host immune response to rapidly growing mycobacteria, an emerging cause of chronic lung disease

Rapidly growing mycobacteria (RGM) are environmental organisms classified under the broader category of nontuberculous mycobacteria. The most common RGM to cause human diseases are Mycobacterium abscessus, Mycobacterium chelonae, Mycobacterium fortuitum, and Mycobacterium massiliense. Infections due to the RGM are an emerging health problem in the United States. Chronic pulmonary disease and skin/soft-tissue infections are the two most common disorders due to these organisms. Clinical outcomes in the treatment of M. abscessus infections are generally disappointing. Because less is known about the nature of the immune response to M. abscessus than for tuberculosis, we herein highlight the major clinical features associated with infections due to M. abscessus and other RGM, and review the known host immune response to RGM, drawing from experimental animal and clinical studies. Based on in vitro and in vivo murine models, Toll-like receptor 2, dectin-1, tumor necrosis factor (TNF)-α, IFN-γ, leptin, T cells, and possibly neutrophils are important components in the host defense against RGM infections. However, excessive induction of TNF-α by the R morphotype of M. abscessus may allow it to be more pathogenic than the S morphotype. Clinical observations and/or genetic studies in humans corroborate many of the findings in animals in that those with cell-mediated immunodeficiency, genetic defects in IFN-γ-IL-12 axis, and those individuals on TNF-α blockers are at increased risk for nontuberculous mycobacteria infections, including the RGM. However, much remains to be discovered on why seemingly healthy individuals, particularly slender postmenopausal women with thoracic cage anomalies, appear to be at increased risk.

Acute kidney injury in a patient with nontuberculous mycobacterial infections: a case report

Nontuberculous mycobacterial infections are an increasingly recognized cause of chronic lung disease in both immunocompromised and immunocompetent patients. Pre-existing lung disease, alcohol abuse, diabetes mellitus, malignancy, and smoking have been identified as important risk factors in nontuberculous mycobacterial infections, with only few cases of Nontuberculous mycobacterial infection in renal failure patients, mostly on peritoneal dialysis. However, acute kidney injury associated with atypical mycobacterial infection is a very rare clinical event. To our knowledge, the present patient is the first case of acute kidney injury in a patient with documented nontuberculous mycobacterial infection. Our case is also a first report of Mycobacteria avium complex and Mycobacteria gordonae isolated simultaneously from individual patient with nontuberculous mycobacterial disease.

Mycobacterium chelonae empyema in an immunocompetent patient

Thoracic empyema caused by rapidly growing mycobacteria (RGM) and complicated with bronchopleural fistula is rarely reported, especially in immunocompetent patients. A 53-year-old healthy woman presented initially with a productive cough and intermittent fever. The patient received a complete treatment course following an initial diagnosis of pulmonary tuberculosis. After the anti-tuberculosis agents were discontinued, a right thoracic empyema with bronchopleural fistula occurred, and the pathogens from both pus and sputum were identified as Mycobacterium chelonae. Thoracotomy with decortication and wedge resection of the right middle lung was performed, followed by clarithromycin plus ciprofloxacin therapy for 36 months. This patient has not suffered a relapse in the last 3 years. In addition to the experience of successful treatment, this case indicates that RGM such as M. chelonae can emerge as causative pathogens of thoracic empyema, even in healthy persons.

Extracellular-regulated kinase activation regulates replication of Mycobacterium avium intracellularly in primary human monocytes

Mycobacterium avium-intracellulare (MAI) is a ubiquitous environmental pathogen that causes disseminated infection in immunocompromised patients, such as those with human immunodeficiency virus, interleukin-12 deficiency, or interferon-gamma receptor mutation. Colony morphotypes are associated with MAI pathogenicity. Our previous studies have reported that smooth-transparent (SmT) morphotypes are more virulent and induce less cytokine (interleukin-1beta and tumor necrosis factor-alpha) production by human monocytes than the smooth-domed (SmD) morphotypes. Mitogen-activated protein (MAP) kinases such as extracellular-regulated kinase (ERK) are activated by the phagocytosis of particle antigens in macrophages, and this ERK activation subsequently influences cytokine expression and the control of intracellular pathogen growth. The influence of MAP kinase activation on MAI replication in human monocytes was examined. Peripheral blood monocytes isolated from healthy subjects by Ficoll-Hypaque sedimentation were infected with virulent SmT or avirulent SmD MAI without or with MAP kinase inhibitors. MAP kinase activities were determined by in vitro kinase assay, intracellular MAI growth by CFU assay, and cytokines by enzyme-linked immunosorbent assay. MAI infection induced ERK and p38 activation. Inhibition of ERK by PD98059, but not p38, significantly increased intracellular MAI growth. Tumor necrosis factor-alpha release and interleukin-1beta production in response to MAI were reduced by MAP kinase inhibition. p38 inhibition tended to reduce cytokine production more substantially. These data suggest that ERK activation limits intra-monocytic MAI replication and enhances monocytic cytokine release, whereas p38 activation influences only cytokine release. The effect of MAP kinases on MAI growth might thus be mediated by the modulation of cytokine production.

Non-tuberculous mycobacteria pulmonary infection: Management and follow-up of 31 infected patients

BACKGROUND

The global number of non-tuberculous mycobacteria (NTM) pulmonary infections is increasing. Treatment decisions and management are difficult and the prognosis of these infections has been rarely evaluated.

METHODS

The aim of this study was to evaluate management and prognosis of patients with NTM pulmonary infection in a French teaching hospital. In this study, we evaluated management of patients satisfying the ATS (American Thoracic Society) criteria for NTM pulmonary infection in Amiens hospital from 1992 to 2002 and retrospectively compared this management to ATS guidelines. Short-term and long-term survival was also described.

RESULTS

Thirty-one patients satisfying the ATS criteria for NTM pulmonary infection were included: 15 patients were infected with Mycobacterium xenopi, nine with Mycobacterium avium intracellulare, four with Mycobacterium kansasii and three patients were infected with rapidly growing mycobacteria. Twenty-seven patients had past or concomitant diseases responsible for local or systemic immunosuppression. Eleven patients were not treated. In the 20 treated patients, 13 different antibiotic combinations were used, often comprising three or more drugs. The median survival was 15 months. Twenty-one patients (67%) died before 5 years of follow-up. Thirty per cent of deaths were attributed to NTM pulmonary infection. Adjusting treatment to the results of susceptibility tests or ATS guidelines was not associated with any significant difference in survival.

CONCLUSION

As the high mortality rate may be related to concomitant diseases, management of NTM pulmonary infection also depends on comorbidities, and should be defined according to the severity of underlying diseases.

Non-tuberculous mycobacteria in end stage cystic fibrosis: implications for lung transplantation

BACKGROUND

WC and NS contributed equally. Non-tuberculous mycobacteria (NTM) frequently colonise patients with end stage cystic fibrosis (CF), but its impact on the course of the disease following lung transplantation is unknown.

METHODS

Lung transplant recipients with CF who underwent lung transplantation at our institution between January 1990 and May 2003 (n=146) and CF patients awaiting lung transplantation in May 2003 (n=31) were studied retrospectively.

RESULTS

The prevalence rate of NTM isolated from respiratory cultures in patients with end stage CF referred for lung transplantation was 19.7%, compared with a prevalence rate of 13.7% for NTM isolates in CF lung transplant recipients. The overall prevalence of invasive NTM disease after lung transplantation was low (3.4%) and was predicted most strongly by pre-transplant NTM isolation (p=0.001, Fisher's exact test, odds ratio (OR) 6.13, 95% CI 3.2 to 11.4). This association was restricted to Mycobacterium abscessus (p = 0.005, Fisher's exact test, OR 7.45, 95% CI 2.9 to 16.9). While NTM disease caused significant morbidity in a small number of patients after transplantation, it was successfully treated and did not influence the post-transplant course of the disease.

CONCLUSION

The isolation of NTM before transplantation in CF patients should not be an exclusion criterion for lung transplantation, but it may alert the clinician to patients at risk of recurrence following transplantation.

Mycobacterium avium Complex Pulmonary Disease in Immunocompetent Patients

Mycobacterium avium complex is becoming increasingly recognized as one of the most common mycobacterial pathogens in humans. It is rapidly becoming a significant cause of pulmonary disease even in those with an intact immunity. In 1997, the American Thoracic Society published recommendations for the diagnosis and treatment of nontuberculous mycobacteria. On the basis of the authors' clinical experience of the myriad presentations of pulmonary Mycobacterium avium complex disease in an immunocompetent host, a clinical classification is proposed. The current data are summarized, and a practical approach to management of the various pulmonary forms of the disease is provided.