Published online May 14, 2024.
https://doi.org/10.3346/jkms.2024.39.e167
Clinical Characteristics and Treatment Outcomes of Pulmonary Diseases Caused by Coinfections With Multiple Nontuberculous Mycobacterial Species
,1
A La Woo,1
Seung Hyun Yong,1
Ah Young Leem,1
Su Hwan Lee,1
Sang Hoon Lee,1
Song Yee Kim,1
Kyungsoo Chung,1
Eun Young Kim,1
Ji Ye Jung,1
Young Ae Kang,1,2
Moo Suk Park,1
Young Sam Kim,1
and Youngmok Park1,3,*
Abstract
Background
Coinfections with multiple nontuberculous mycobacterial (NTM) species have not been widely studied. We aimed to evaluate the clinical characteristics and treatment outcomes in patients with NTM-pulmonary disease (PD) caused by coinfection with multiple NTM species.
Methods
We retrospectively reviewed patients with NTM-PD at a tertiary referral hospital in Korea between March 2012 and December 2018. Coinfection was defined as two or more species of NTM pathogens isolated from the same respiratory specimen or different specimens within three months.
Results
Among 1,009 patients with NTM-PD, 147 (14.6%) NTM coinfections were observed (average age 64.7 years, 69.4% women). NTM species were identified more frequently (median 6 vs. 3 times, P < 0.001) in the coinfection group than in the single species group, and follow-up duration was also longer in the coinfection group (median 44.9 vs. 27.1 months, P < 0.001). Mycobacterium avium complex (MAC) and M. abscessus and M. massiliense (MAB) were the dominant combinations (n = 71, 48.3%). For patients treated for over six months in the MAC plus MAB group (n = 31), sputum culture conversion and microbiological cure were achieved in 67.7% and 41.9% of patients, respectively. We divided the MAC plus MAB coinfection group into three subgroups according to the target mycobacteria; however, no statistical differences were found in the treatment outcomes.
Conclusion
In NTM-PD cases, a significant number of multiple NTM species coinfections occurred. Proper identification of all cultured NTM species through follow-up is necessary to detect multispecies coinfections. Further research is needed to understand the nature of NTM-PD in such cases.
Graphical Abstract
INTRODUCTION
Only a few decades have passed since nontuberculous mycobacteria (NTM) have gained attention as emerging pathogens; however, their incidence and prevalence are rising worldwide.1 Although a few clinical guidelines have been published for NTM pulmonary disease (NTM-PD),2, 3 its treatment remains challenging for many clinicians.
The regimen for NTM-PD treatment varies among causative species.3, 4 Mycobacterium avium complex (MAC) PD is treated with a three-drug regimen that includes macrolides, ethambutol, and rifampicin. However, since M. abscessus and M. massiliense (MAB) is resistant to ethambutol and rifampicin, the use of a macrolide-containing multidrug regimen, combined with intravenous antibiotics in the initial phase, is recommended.5 Therefore, identifying the causative species of NTM-PD is critical for deciding the treatment regimen.
NTM-PD associated with two or more causative species poses additional challenges for clinicians. A few studies have reported cases of NTM-PD caused by coinfection by multiple species.6, 7, 8, 9, 10, 11, 12, 13 The reported proportion of the coinfection varies between studies, ranging from 7–30%.14, 15 However, we lack standardized definitions and treatment guidelines for NTM-PD by multiple species. Therefore, we aimed to estimate the burden of NTM-PD caused by multiple species coinfection and evaluate the clinical characteristics and treatment outcomes in affected patients.
METHODS
Study design and data collection
We retrospectively analyzed patients with NTM-PD at Severance Hospital, a tertiary referral hospital in South Korea. The enrolled patients were diagnosed with NTM-PD and followed up between March 2012 and December 2018. NTM-PD was defined according to the diagnostic criteria of the guidelines.3 Data on demographics, laboratory results, comorbidities, and antibiotic treatment were obtained from electronic medical records. Patients with a follow-up period of less than six months were excluded.
Microbiologic and radiologic evaluation
All NTM species identified from expectorated sputum or samples obtained from bronchoscopy were analyzed to define multiple species coinfections. Acid-fast bacillus smears and cultures of the respiratory specimens were performed using standard methods.16, 17 Species identification was conducted via polymerase chain reaction-restriction fragment length polymorphism analysis of the rpoB gene and/or the tuf gene.18, 19 The erm(41) gene pattern detection or rrs, hsp65, and rpoB gene sequencing were used to isolate subspecies of MAB.20, 21 MAC is the combination of M. avium and M. intracellulare, and MAB refers to M. abscessus subsp. abscessus (hereafter M. abscessus) and M. abscessus subsp. massiliense (hereafter M. massiliense). M. bolleti, which is uncommon in Korea, was not included in this study.22
Radiographic patterns were categorized into four types based on chest radiography and computed tomography findings.16 The fibrocavitary form was defined as cavitary opacities in the upper lobes, with or without consolidation and pleural thickening. Bilateral bronchiectasis with multiple nodules and tree-in-bud opacities was classified as nodular bronchiectatic (NB) and subdivided into those with and without a cavity. Unspecified forms, such as solitary pulmonary nodules or those lacking evident characteristics of either form, were designated as “others.”
Definitions of multiple NTM species coinfection
Based on the criteria for diagnosis recommended by the current guideline,3 the causative species of NTM were defined as two or more identified species in expectorated sputum or species identified once in bronchial washing, bronchoalveolar lavage, and lung biopsy samples. To distinguish the actual multiple species coinfections from contaminations or changes in NTM species, “coinfection” was defined as two or more species of NTM pathogen isolated from the same respiratory specimen or different specimens within three months. If the minimum culture interval for one species was longer than three months, it was not considered a coinfected species, even if multiple species were discovered in a single specimen. According to this definition, a change from one species to another or flip back and forth over three months were categorized as a single-species infection.
Treatment outcomes
Sputum culture conversion was defined as three consecutive negative cultures. Definitions for microbiologic cure and recurrence referred to the current consensus.23 Kim S and Park Y reviewed the radiologic images independently, and a consensus was arrived at for categorizing the results as “Worsened,” “No change,” or “Improved.” The occurrence of major adverse drug reactions requiring regimen changes was also reviewed. In addition, mortality results were assessed on December 31, 2021, by reviewing the electronic medical records.
Statistical analysis
Differences between groups were analyzed using the Pearson χ2 or Fisher’s exact test for categorical variables and the Mann-Whitney U test for continuous variables. Analysis of variance and Kruskal-Wallis tests were used for the three subgroup analyses. Statistical analyses were performed using R software (version 4.1.2; R Foundation for Statistical Computing, Vienna, Austria). A two-tailed P < 0.05 was considered statistically significant in all statistical analyses.
Ethics statement
The study protocol was reviewed and approved by the Institutional Review Board (IRB) of Severance Hospital (IRB No.4-2022-1527). The requirement for written consent from the patients was waived owing to the retrospective nature of this observational study.
RESULTS
Clinical characteristics of multiple NTM species coinfection
A total of 1,009 patients were diagnosed with NTM-PD, and multiple NTM coinfections were discovered in 147 (14.6%) patients (Fig. 1). The baseline characteristics of individuals with multiple NTM species coinfections were compared to those with single-species infections; the results are presented in Table 1. A higher proportion of female individuals was observed in the coinfection group in comparison to the single-species group (69.4% vs. 60.2%, P = 0.034); however, other clinical features, including age, body mass index, smoking history, symptoms, and comorbidities, were similar in both groups.
Fig. 1
Study population.
NTM = nontuberculous mycobacterial, MAC = Mycobacterium avium complex (M. avium and M. intracellulare), MAB = M. abscessus and M. massiliense.
Table 1
Baseline characteristics of the study population
NTM species were identified more frequently in the coinfection group (median 6 vs. 3 times, P < 0.001), and the follow-up duration was longer (median 44.9 months vs. 27.1 months, P < 0.001) compared to the single species group.
Combination of multiple NTM species coinfection
In the coinfection group, the MAC plus MAB combination was dominant (n = 71, 48.3%), while the rest (n = 76) were present in various species combinations: MAC (n = 39), MAB (n = 4), MAC or MAB plus other species, and coinfection with only a combination of species other than MAC or MAB (n = 33). Other species included M. chelonae, M. genavense, M. gordonae, M. lentiflavum, M. mucogenicum, M. scrofulaceum, M. simiae, and unclassifiable species.
Detailed combinations of species in the MAC plus MAB coinfection group are displayed in Fig. 2. M. avium tended to co-isolate with M. massiliense, whereas M. intracellulare co-isolated with M. abscessus. Three species were mixed in 10 cases; however, no cases of coinfection with four species occurred.
Fig. 2
Combinations of mycobacteria in MAC plus MAB coinfection group (n = 71). Only the nontuberculous mycobacterial species that met the diagnostic criteria are displayed.
MAC = Mycobacterium avium complex (M. avium and M. intracellulare), MAB = M. abscessus and M. massiliense.
Comparison between the MAC plus MAB coinfection group and the “other” coinfections group
We compared the MAC plus MAB coinfection group (n = 71) with the “other” combination group (n = 76) (Table 2). The MAC plus MAB group had a lower body mass index (20.2 vs. 21.4 kg/m2,P = 0.020) than the “other” combination group; however, the other clinical characteristics were similar between the two groups. The antibiotics and target species of patients who underwent NTM treatment for at least six months are presented in Tables 3 and 4.
Table 2
Baseline characteristics of patients with NTM-PD and multiple NTM species coinfection
Table 3
Treatment regimens of patients treated for nontuberculous mycobacterial pulmonary disease for at least 6 months (N = 56)
Table 4
Target species in patients treated for nontuberculous mycobacterial pulmonary disease for at least 6 months (N = 56)
Treatment regimen and outcomes in the MAC plus MAB coinfection group
Of the 71 patients with MAC plus MAB coinfection, 45 were treated with antibiotics, of whom 32 received antibiotics for over six months (Fig. 1). Table 5 displays the treatment regimen and outcomes for 31 patients, excluding one receiving medication during the assessment period. Fourteen patients were treated only for MAC, and four patients only for MAB, without changes to their treatment regimens. Of the 13 patients treated for both pathogens, 10 (76.9%) were initially treated for both species as they were considered primary pathogens from the start. The remaining 3 (23.1%) patients, although both NTM species met the diagnostic criteria for PD from the start, were initially treated for one species considered the primary pathogen. After repeated cultures of the other species were these regimens adjusted to treat both species.
Table 5
Treatment regimen and outcomes in the MAC plus MAB coinfection group
The median treatment duration was 18.2 months, and sputum culture conversion and microbiological cure were achieved in 67.7% and 41.9% of the patients, respectively. Chest radiology findings improved in 61.3% of the patients, and major adverse drug reactions occurred in 45.2%. Subsequently, we divided the patients into three subgroups according to the target mycobacteria by antibiotic regimen; however, no differences were observed in treatment outcomes.
DISCUSSION
Coinfection with NTM constitutes a considerable proportion of cases in real-world practice. Therefore, recognizing this gap and defining multi-species coinfection should be the first step in future research. In this study, we examined the clinical characteristics and treatment outcomes of NTM-PD caused by coinfection with multiple NTM species. Coinfection with NTM accounted for 14.6% of patients with NTM-PD, and women tended to be coinfected with multiple NTM species. Moreover, the coinfection group underwent a higher frequency of NTM identification tests and had a longer follow-up duration. MAC plus MAB coinfection was the most common combination, which could be challenging for clinicians. However, the treatment outcomes did not differ between the subgroups according to the target mycobacterial species.
Currently, guidelines do not provide definitions for multispecies NTM coinfection.2, 3 Therefore, the incidence, prevalence, clinical features, and optimal treatment regimen for NTM coinfection remain areas that have not been extensively studied. Nonetheless, it is noteworthy that we observed a significant proportion of multispecies coinfection even after imposing a strict definition of coinfection, which involved identifying multispecies pathogens in the same or adjacent samples within three months. As an illustration, Kim et al.24 reported a retrospective review of over 21,000 patients in Korea, demonstrating that 8.5% of the clinical isolates had mixed NTM infections. In addition to this, other studies have also reported a significant proportion of NTM coinfection (7.5–30.1%).14, 15
Coinfection with NTM is still reported when different diagnostic methods are used. A two-step multiplex polymerase chain reaction assay or clone library analysis targeting mycobacterial 16S rRNA genes can identify NTM species with reported coinfection rates of 2.2% and 13.5%, respectively.25, 26 Naito et al.25 reported that ordinary mycobacterial culture methods might underestimate NTM coinfection. As mentioned above, coinfection with NTM constitutes a considerable proportion of cases in real-world practice. Therefore, recognizing this gap and defining multi-species coinfection should be the first step in future research.
In our study, we could not identify distinct clinical characteristics to distinguish between single- and multi-species NTM infections other than sex. To the best of our knowledge, no study has compared the clinical features of the two groups. Moreover, the clinical characteristics reported in previous studies were inconsistent. For instance, chronic obstructive PD and age ≥ 65 years were associated with the isolation of multiple NTM species in a Singapore study.15 Asaoka et al.27 observed 55 (26%) cases of multiple NTM species isolated from 213 non-MAC-PD patients, in which co-identification was more prevalent in women, never-smokers, and those with fewer cavitary presentations. Meanwhile, another study reported single and mixed infections in 120 patients with MAC; however, mixed NTM infections were associated with male sex and a history of asthma.28
In our study, the number of NTM species identified was higher and the follow-up duration was longer in the multispecies coinfection group than in the single-species group. Current guidelines recommend identification of all NTMs isolated from respiratory specimens.2 However, real-world practice has not entirely addressed this recommendation. Lim et al.10 reported that, among 532 patients with pathogenic NTM isolates, only 25% were identified twice or more. Kim et al.24 also reported that the identification rate of NTM was 28.5% among 2,521 isolates from 646 patients. As single- and multispecies infections cannot be differentiated using clinical characteristics, guideline-directed practice with sufficient follow-up could help detect multispecies coinfections.
MAC and MAB were isolated most frequently in this study, a result that is consistent with previous reports.10, 24, 29 Kim et al.24 reported that a combination of MAC plus MAB was the most common clinical isolate in Korea (44.7%). Shin et al.13 reported the clinical characteristics and treatment outcomes of MAC plus MAB coinfection, emphasizing that the NB form accounted for 96% of the 71 cases. Interestingly, the number of cases of MAC plus M. massiliense was nearly double that of MAC plus M. abscessus (47 vs. 24 cases). Moreover, the sputum culture conversion rates after 12 months of treatment were much lower in patients infected with MAC plus M. abscessus than in those infected with MAC plus M. massiliense (25% vs. 61%, P < 0.033).13 Our results also showed a high prevalence of the NB form, though the species combinations differed; MAC plus M. abscessus and MAC plus M. massiliense were both prominent (36 vs. 30 cases). Unfortunately, we could not compare the treatment outcomes of MAC plus M. abscessus and M. massiliense owing to the study’s small sample size.
The long-term use of multiple antibiotics targeting MAC and MAB species is challenging for patients and clinicians due to the variation in treatment regimens depending on the NTM species. In this context, our study aimed to determine the treatment outcomes depending on which species of MAC and MAB coinfections were targeted for antibiotic treatment; however, no significant difference was observed. Therefore, clinicians might have no choice but to constitute treatment regimens targeting the clinician-decided primary pathogen until a revised guideline is available.
Our study has some limitations. First, it was a retrospective study conducted at a single hospital. Potential reporting bias might skew the reported incidence of mixed infections. Therefore, cautious interpretation and generalization are required. Cross-verifying with other data sources would minimize the bias. Prospective studies in the future would also address the possibility of under-diagnosis. Second, no detailed information was available regarding the long-term outcomes of medication use or follow-up. Third, mixed NTM infections were subjectively referred to as coinfections. Therefore, switches to other NTM species should be investigated in future studies. Finally, various mixed NTM species combinations other than MAC plus MAB were analyzed as the “other coinfections” group, which could oversimplify the nature of the disease.
In summary, multiple NTM species coinfections comprised a considerable proportion of NTM-PD cases. A combination of two major NTM pathogens, MAC and MAB, was predominant. Given that the clinical characteristics were similar between the single and coinfection groups, the guideline-based practice of identifying every cultured NTM species and employing sufficient follow-up would aid in the detection and management of multiple species coinfection. Therefore, further prospective research is required to define and classify multiple species of NTM infections, serving as the initial step toward establishing an optimal strategy to treat this complicated infection.
Disclosure:The authors have no potential conflicts of interest to disclose.
Author Contributions:
Conceptualization: Kang YA, Park Y.
Data curation: Park Y, Kim S.
Formal analysis: Park Y, Kim S.
Methodology: Kang YA, Park Y.
Supervision: Woo AL, Yong SH, Leem AY, Lee SH, Lee SH, Kim SY, Chung K, Kim EY, Jung JY, Kang YA, Park MS, Kim YS, Park Y.
Writing - original draft: Kim S.
Writing - review & editing: Woo AL, Yong SH, Leem AY, Lee SH, Lee SH, Kim SY, Chung K, Kim EY, Jung JY, Kang YA, Park MS, Kim YS, Park Y.
ACKNOWLEDGMENTS
We appreciate the Medical Illustration & Design (MID) team, a member of Medical Research Support Services of Yonsei University College of Medicine, for their excellent support with medical illustration.
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