Skin, Cutaneous mycobacterial infection, H&E stain

The Mycobacterium genus currently contains more than 170 species, not all of which cause human disease. Mycobacterial skin and soft tissue infections include those caused by M. tuberculosis (TB), M. leprae as well as some atypical mycobacteria (ATM). Diagnosis can be challenging, as disease can present in localized or disseminated forms, the latter of which is much more common in immunosuppressed patients.

Cutaneous TB is not common (4% of extra-pulmonary cases), with the majority of cases due to re-activation. Leprosy affects the skin and nerves, and patients will likely have a personal history of the disease or a history of travel to leprosy endemic regions. ATM skin and soft tissue infections (SSTIs) are generally due to environmental exposure (soil, water) and spread through defects in the skin barrier with the most common causes including M. marrinum, M. ulcerans, M. chelonae, M. fortuitum, and M. abscessus. The clinical presentation is quite varied (from erythematous papules/plaques to nodules/abcesses/ulcers). The number of reported ATM infections has increased over the last few decades and in addition to SSTls, ATM can also cause infections of the pulmonary and lymphatic systems.

For all suspected cutaneous mycobacterial infections, diagnostic work-up includes tissue biopsy with acid-fast staining, culture, and if positive, ultimately PCR for species determination. The histology is largely non-specific (granulomatous tissue), but clumps of acid fast acilli are usually visible. One caveat to this is M. leprae, which may stain falsely negative due to its sensitivity to the acid·alcohol decolourization phase of the Ziehl·Neelsen stain. In such cases, Fite staining can be used.

This slide shows H&E stain. See related content for Acid-fast stain.