New Mycobacterium Species Named After Lung Infection in Cord Blood Transplant Patient

In a quiet corner of clinical microbiology, a bacterium that had gone nameless for years has finally been given an identity — and the story behind it reads like a detective case. Japanese researchers report the discovery of pulmonary disease caused by a previously unclassified slow-growing mycobacterium in a woman who had undergone an umbilical cord blood transplantation years earlier. Using whole-genome sequencing, the team linked her clinical isolates to a mysterious strain previously recovered from a cat, and in doing so formally proposed a new species: Mycobacterium meguroense. The case, published in the journal New Microbes and New Infections, highlights both the growing challenge of nontuberculous mycobacterial disease and the power of genomic tools to unmask organisms that conventional laboratories simply cannot name.

Nontuberculous mycobacteria, often abbreviated as NTM, are a sprawling group of environmental organisms found in soil and water. Unlike their notorious relatives Mycobacterium tuberculosis and Mycobacterium leprae, most NTM species do not spread from person to person, but a subset of them can cause serious lung disease, particularly in people with underlying respiratory conditions or weakened immune systems. Clinicians have watched with growing concern as the prevalence of NTM pulmonary disease rises worldwide, imposing substantial morbidity on aging populations. At the same time, advances in multilocus sequence analysis and whole-genome sequencing have revealed a hidden layer of diversity within this group: novel species that look unremarkable in routine cultures yet are genetically distinct from anything in the reference databases. The Japanese case is a textbook example of this emerging frontier.

The patient at the center of the report was a 73-year-old woman with a history of acute myeloid leukemia. She had received an umbilical cord blood transplantation at the age of 67, a procedure in which stem cells harvested from umbilical cord blood are used to rebuild a patient’s blood and immune system after intensive chemotherapy. Although her leukemia was in remission, she carried a heavy burden of comorbidities, including a prior myocardial infarction, diabetes mellitus, dyslipidemia, and hypertension. Roughly a year before she was referred for specialist evaluation, she developed a cough and hemoptysis — the coughing up of blood — a symptom combination that immediately signals something is seriously wrong in the airways.

What followed was a microbiological puzzle that stumped the laboratory for months. Sputum cultures tested positive for mycobacteria from the very onset of her symptoms, and acid-fast smears — the classic stain used to detect mycobacterial cells — were repeatedly positive as well. Yet the organism could not be identified. Chest computed tomography painted a characteristic picture of chronic infection: peripheral granular opacities scattered through both lower lobes, accompanied by bronchiectasis, a permanent widening and scarring of the airways, along with thickened bronchial walls and mucus plugging. Two separate mycobacterial isolates, designated KO_240205 and KO_240413 from sputum specimens collected on different dates, resisted identification by MALDI-TOF mass spectrometry, the workhorse technique most clinical laboratories rely on for bacterial identification.

The team then turned to the molecular level, sequencing three classic mycobacterial marker genes — 16S rRNA, hsp65, and rpoB — but even here the results were inconclusive, with no 100 percent match to any known species in BLAST database searches. Taken together, however, the clinical picture, the radiological findings, and the repeated microbiological evidence fulfilled the accepted diagnostic criteria for nontuberculous mycobacterial pulmonary disease. An additional complication emerged: Pseudomonas aeruginosa, another opportunistic pathogen notorious in chronic lung infections, was first detected approximately eleven months after the respiratory symptoms began, while mycobacterial cultures had remained positive throughout. Because the mycobacterial evidence preceded the Pseudomonas isolation by nearly a year, the researchers judged the mycobacterium to be a genuine contributor to the lung disease, although they could not entirely exclude a role for the later-arriving bacterium.

The decisive step came from whole-genome sequencing. The researchers assembled the genomes of both clinical isolates and compared them against a reference panel of 227 mycobacterial genomes in a phylogenomic analysis. The result was striking: the two clinical isolates clustered together with a previously unnamed organism known as Mycobacterium sp. strain MFM001. The two isolates from the patient shared 100.00 percent average nucleotide identity with each other, confirming they were the same organism recovered on two occasions. Each isolate showed 99.98 percent average nucleotide identity and 99.8 percent digital DNA–DNA hybridization with strain MFM001 — values well above the thresholds microbiologists use to assign organisms to the same species. By contrast, comparisons with the closest named relatives, M. branderi, M. celatum, and M. kyorinense, yielded average nucleotide identities of only 90.08 to 90.64 percent and hybridization values of 37.9 to 39.7 percent, leaving no doubt that this was a species apart.

The history of strain MFM001 adds an intriguing zoological twist. The strain was previously reported by other researchers in a case of disseminated infection with granulomatous gastroenterocolitis in a cat receiving immunosuppressive treatment. That same team also noted partial 16S rRNA gene sequence identity between MFM001 and a strain isolated from a human pulmonary infection in Germany, hinting at a connection to human disease — but without whole-genome comparison, species identity could not be confirmed. The new genomic analysis now firmly links the Japanese clinical isolates to the feline strain, and on that basis the authors propose the name Mycobacterium meguroense for the previously unnamed species. Together, the feline report and the present human case suggest this organism can cause disease in different mammalian hosts, though its environmental reservoir and the routes by which people acquire it remain unknown.

Treatment posed its own questions. Published clinical information on this species is so limited that no established regimen exists, so the researchers looked to reports of disease caused by its close relatives for context. In those accounts, pulmonary infections with M. branderi, M. celatum, and M. kyorinense were all managed with multidrug regimens, though the combinations and durations varied considerably, and some cases required surgical resection of damaged lung tissue. For the present patient, broth microdilution testing yielded low clarithromycin minimum inhibitory concentrations of 0.125 and 0.25 micrograms per milliliter, providing microbiological support for including a macrolide in the regimen. Five months after referral, she began a three-drug combination of clarithromycin, ethambutol, and levofloxacin, and her condition improved. The last positive sputum cultures for both the mycobacterium and Pseudomonas aeruginosa were obtained in the month treatment began, and neither organism was recovered afterward. She has now continued treatment for a full year without relapse.

The authors are careful to frame this success appropriately. Low clarithromycin minimum inhibitory concentrations supported the choice of a macrolide, but phylogenetic relatedness alone does not guarantee that related species share antimicrobial susceptibility patterns or respond similarly to therapy. The improvement seen in this patient remains a single clinical observation rather than evidence of a preferred regimen for M. meguroense, and much more work is needed to define the species’ full clinical spectrum, the relationship between laboratory susceptibility values and treatment outcomes, and the optimal drug combination and duration of therapy. What the case does demonstrate, emphatically, is the essential role of accurate molecular identification and careful assessment of competing causes of lung disease when clinicians encounter rare or unclassified mycobacteria. As whole-genome sequencing becomes more accessible, organisms like M. meguroense — invisible to conventional methods and hiding in plain sight in laboratories around the world — are likely to keep emerging from the shadows, one painstaking case at a time.

Subject of Research: A newly identified slow-growing mycobacterium, Mycobacterium meguroense, causing pulmonary disease after umbilical cord blood transplantation

Article Title: Pulmonary disease caused by Mycobacterium meguroense , a newly identified slow-growing mycobacterium, after umbilical cord blood transplantation: A case report from Japan and a mini-review of the literature

Article References: Pulmonary disease caused by Mycobacterium meguroense , a newly identified slow-growing mycobacterium, after umbilical cord blood transplantation: A case report from Japan and a mini-review of the literature. (n.d.). Original publication

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Keywords: Mycobacterium meguroense, nontuberculous mycobacteria, whole-genome sequencing, umbilical cord blood transplantation, pulmonary disease, NTM-PD, MALDI-TOF, clarithromycin, average nucleotide identity, emerging pathogens, clinical microbiology, case report