Mycobacteria: Mycobacterium Leprae and Nontuberculous Mycobacteria

Tropism. A striking and defining property is that M. leprae preferentially infects peripheral nerves, specifically Schwann cells, and skin. It has a particular predilection for cooler body sites: skin, superficial peripheral nerves, the nasal mucosa, the anterior chamber of the eye, and the testes. This directly reflects the organism's optimal growth temperature, which is lower than core body temperature.

The organism cannot be cultured on standard artificial laboratory media at all. This is unique among the medically important mycobacteria. It distinguishes M. leprae sharply from M. tuberculosis and the nontuberculous mycobacteria, all of which can be cultured, albeit sometimes slowly.

It has historically been studied using the mouse footpad model and the armadillo, a naturally susceptible animal host given its low core body temperature. Given these practical limitations, molecular methods are increasingly used.

Transmission. Believed to occur predominantly via prolonged close contact with respiratory secretions from an untreated individual with high bacterial burden (multibacillary disease). This requires substantially more prolonged and intimate exposure than M. tuberculosis transmission.

The precise mechanism remains incompletely understood. The great majority of exposed individuals never develop clinical disease. Host genetic and immune factors play a strong role in determining susceptibility and, critically, the clinical pattern of disease that develops.

Host immune response. The type and strength of the host's cell-mediated immune response to the organism fundamentally determine the clinical spectrum of leprosy. Disease ranges between two clinical poles rather than presenting uniformly. This is a defining concept for this organism.

A strong, effective cell-mediated (Th1-predominant) immune response contains the organism to localised sites. This produces the paucibacillary, tuberculoid pole of disease.

A weak or absent cell-mediated response, with a relatively Th2-skewed pattern instead, fails to contain the organism. This produces the multibacillary, lepromatous pole.

Tuberculoid disease (paucibacillary) occurs in patients with a strong cell-mediated immune response. It produces well-localised skin lesions that are few in number, hence paucibacillary. Typically there is a small number of hypopigmented, anaesthetic (sensory-loss) plaques with well-defined, raised borders.

Peripheral nerves near the lesion are palpably enlarged, given the organism's neural tropism and the vigorous local granulomatous immune response.

This form has a low bacterial burden and low infectiousness to others. Skin biopsy typically shows well-formed granulomas with few or no detectable organisms on staining, reflecting effective immune containment.

Lepromatous disease (multibacillary) occurs in patients with a weak or absent effective cell-mediated response. It produces widespread, numerous skin lesions distributed diffusely and symmetrically. Early in the disease course these often lack the sharply defined borders and anaesthesia of tuberculoid lesions. Extensive nerve involvement and sensory loss develop over time as disease progresses.

This form has a high bacterial burden, with numerous organisms readily visible on skin biopsy staining. It is the most infectious form of the disease. It is associated with:

• More extensive, symmetric peripheral nerve damage.
• Characteristic facial changes: the classic leonine facies from diffuse facial skin infiltration, and saddle-nose deformity from nasal cartilage destruction. These are now less commonly seen given earlier treatment.
• The potential for extensive, disfiguring, and disabling nerve damage if untreated, given the direct organism burden.

Acute inflammatory nerve damage can also occur during treatment-associated immune reactions. This is a recognised complication requiring specific management during therapy.

The clinical spectrum between these two poles comprises borderline forms, with features and immune responses intermediate between tuberculoid and lepromatous. This reflects the continuous rather than strictly binary nature of the underlying host immune spectrum.

Nontuberculous mycobacteria are a broad, taxonomically diverse group of mycobacterial species other than M. tuberculosis complex and M. leprae. They are typically environmental in origin (soil, water, including household plumbing biofilms) rather than obligately human-associated.

They cause disease predominantly as opportunists in patients with structural lung disease, immunosuppression, or other predisposing local or systemic factors. They are not primary pathogens in an otherwise healthy host. This is a fundamental distinction from M. tuberculosis.

Critically, NTM are not transmitted person-to-person in the way M. tuberculosis is, reflecting their environmental rather than obligate human reservoir. This is an important infection control distinction. A patient with NTM pulmonary disease does not require the airborne isolation precautions mandated for suspected or confirmed active tuberculosis.

MAC is a group of closely related, slow-growing NTM species, principally M. avium and M. intracellulare, environmental in origin (water, soil).

Two principal, distinct clinical patterns occur.

• Pulmonary disease in patients with underlying structural lung disease (bronchiectasis, prior tuberculosis, chronic obstructive pulmonary disease). It is typically a chronic, slowly progressive process.
• Disseminated disease in severely immunocompromised patients, most classically those with advanced, untreated HIV infection and a very low CD4 count. It presents with fever, weight loss, and disseminated organ involvement.

Before the widespread availability of effective antiretroviral therapy, disseminated MAC was a common and severe opportunistic infection in this population. It reflects the same fundamental dependence on cell-mediated immunity seen with mycobacteria generally.

M. kansasii is a slow-growing NTM, notable among the nontuberculous mycobacteria for producing a clinical and radiographic pulmonary disease pattern that can closely resemble classic pulmonary tuberculosis, including cavitary upper lobe disease, making it a specific and important differential diagnosis to consider when M. tuberculosis culture or PCR results are negative but the clinical or radiological picture remains strongly suggestive of mycobacterial disease.

It predominantly affects patients with underlying structural lung disease, similar in general risk profile to MAC pulmonary disease.

A group of NTM species, principally Mycobacterium abscessus, Mycobacterium fortuitum, and Mycobacterium chelonae, are distinguished by an unusually short generation time relative to other mycobacteria, allowing visible growth on standard culture media within days rather than the weeks typically required for M. tuberculosis or the slow-growing NTM above, a defining and practically useful laboratory feature.

They are environmental, water, soil, and characteristically associated with infection following direct inoculation or exposure to contaminated water or equipment.

This includes skin and soft tissue infection following trauma, surgery, including cosmetic procedures, or tattooing, and catheter-related and prosthetic device infection.

Particularly for M. abscessus, there is chronic pulmonary infection in patients with underlying structural lung disease, notably including cystic fibrosis. There it represents an increasingly recognised and often difficult-to-eradicate pathogen given frequently extensive intrinsic antibiotic resistance.