Severe pneumonia and bloodstream dissemination caused by Mycobacterium houstonense in a patient with achalasia: clinical importance of genospecies typing

Infections caused by members of the Mycobacterium fortuitum complex are generally mild and confined to skin and soft tissues, while severe disease in immunocompetent individuals is uncommon. Within the Mycobacterium fortuitum complex, the unnamed third biovariant complex comprises several closely related species that cannot be reliably distinguished using routine molecular diagnostic methods. Importantly, species within this complex show marked interspecies variability in antimicrobial susceptibility, including predictable intrinsic resistance patterns, with direct consequences for empiric treatment selection in severe infections. Because antimicrobial susceptibility testing for rapidly growing mycobacteria typically requires at least one week, rapid and accurate species-level identification is essential to guide effective early therapy. We report a case of life-threatening pneumonia with bloodstream dissemination caused by a third-biovariant M. fortuitum complex species in an otherwise immunocompetent 44-year-old woman with previously undiagnosed achalasia. Routine diagnostics identified the isolate as belonging to the M. fortuitum complex but were insufficient for species-level identification. Whole-genome sequencing was performed and identified the pathogen as Mycobacterium houstonense, a species historically considered rare in human disease. The patient developed severe respiratory failure and sepsis, requiring prolonged intensive care treatment. Targeted antimicrobial therapy guided by species identification, combined with treatment of the underlying achalasia, led to gradual clinical improvement, although prolonged antimicrobial therapy was required to achieve culture conversion. This case represents, to our knowledge, the first reported case in which M. houstonense was isolated from blood cultures in association with invasive disease. It demonstrates that infections caused by species within the unnamed third biovariant M. fortuitum complex can be severe and that reliance on complex-level identification alone may delay appropriate empiric therapy. Rapid genospecies identification is therefore crucial in severe infections caused by species within the unnamed third biovariant M. fortuitum complex, particularly when antimicrobial susceptibility is known to vary markedly between closely related species. Rapidly growing mycobacteria are increasingly detected in clinical specimens, but many species cannot be reliably distinguished using routine diagnostic methods. We describe a severe and life-threatening infection caused by Mycobacterium houstonense, a rarely identified member of the unnamed third biovariant group within the Mycobacterium fortuitum complex, in an immunocompetent patient with achalasia. Accurate species identification was only achieved using whole-genome sequencing and directly influenced empiric antimicrobial treatment. Recent case reports of invasive infections caused by third biovariant M. fortuitum complex species suggest that some members of this group may be more pathogenic than previously assumed. This case illustrates how limited species-level resolution can delay appropriate therapy and supports the use of advanced molecular identification in severe infections caused by this group of mycobacteria.