Mycobacterium

General Characteristics

Basic features

Morphology and properties:

• Straight or slightly curved bacilli
• Growth:
  • Slow-growing bacilli
  • Most species can be cultured on simple substrates using amino acids and glycerol.
  • Most species grow on blood agar.
• Nonmotile, non-spore-forming bacilli
• Obligate aerobes (some are microaerophilic)
• Catalase positive
• Facultative intracellular bacilli
• Have a characteristic cell wall (envelope) that confers several unique properties:
  • Presence of a hydrophobic waxy layer composed of mycolic acids
  • Linked to a peptidoglycan layer by a polysaccharide arabinogalactan
  • ↓ Permeability to many drugs and efflux pumps
  • Mycobacteria can invade host cells without presenting with any clinical signs.

Staining properties:

• Acid-fast bacilli:
  • Property conferred by mycolic acid
  • Do not destain by acid alcohol after being stained with aniline dyes
• Gram stain:
  • Usually cannot penetrate the waxy cell wall of the Mycobacterium tuberculosis complex (MTBC)
  • Most commonly produce no stain or variable results

Classification

• Phylum: Actinobacteria
• Family: Mycobacteriaceae
• Genus: Mycobacterium (consists of > 150 species)

Mycobacteria:

• Typical mycobacteria: MTBC species:
  • M. tuberculosis (most important)
  • M. bovis
  • M. africanum
  • M. microti
  • M. canetti
• Atypical, non-tuberculous mycobacteria (NTM): 
  • Classified according to the Runyon system
  • Classification is based on growth rate and pigment production.
  • Groups Ⅰ–Ⅲ are slow growing (> 7 days), and group Ⅳ is considered fast growing (< 7 days):
    • Group I: Photochromogens produce pigment in the presence of light.
    • Group II: Scotochromogens produce pigment in the absence of light.
    • Group III: Non-photochromogens lack pigment (includes Mycobacterium avium complex (MAC) bacteria).
    • Group IV: produce mature colonies in culture media in < 7 days
• Noncultivable: M. leprae complex (M. leprae and M. lepromatosis)
• Saprophytic
• Skin-ulcer types

Most important pathogenic species:

• MTBC causes tuberculosis (TB).
• M. leprae complex causes leprosy.
• M. avium and M. intracellulare (known together as M. avium complex (MAC)) cause: 
  • Pulmonary infections similar to TB
  • Disseminated infections in immunocompromised individuals

Pathogenesis

Reservoirs

• M. tuberculosis: humans are the only natural reservoir.
• M. leprae natural reservoir: armadillos
• M. avium and M. intracellulare are found in water and soil.

Transmission

• M. tuberculosis transmission: respiratory droplets from patients with active disease
• M. leprae transmission: 
  • Direct contact with lesions
  • Inhalation of infectious droplets
• MAC transmission: 
  • Inhalation into the respiratory tract
  • Ingestion into the GI tract

Virulence

• Cell envelope:
  • Major constituent: mycolic acid
  • Mycolic acid is attached to glycolipids.
  • Glycolipids are responsible for “cord formation” on microscopy (grossly corresponds to granuloma formation).
• Catalase-peroxidase: resists host-cell oxidative response
• Sulfatides and trehalose dimycolate: triggers toxicity
• Lipoarabinomannan (LAM): induces cytokines

Host risk factors

• Elderly
• Children
• Immunocompromised patients
• Institutionalized patients
• IV drug use
• HIV or other immunodeficiencies
• Travel to high-risk regions

General mycobacterial pathophysiology

• In humans, mycobacterial infections can affect multiple anatomical sites.
• Bacteria enter through the skin and mucosal barriers.
• Pulmonary and cutaneous infections are the most common.
• Infections with NTM occur most commonly in immunocompromised hosts as opportunistic infections.
• Mycobacteria can colonize their hosts and cause latent infections without any obvious clinical signs.

M. tuberculosis complex

Epidemiology

• MTBC bacteria cause TB.
• TB is an airborne disease that affects the lungs and, sometimes, other organs.
• Causes 1.4 million deaths per year worldwide
• 30% of the global population is infected.
• 10% of infected people will develop active disease.
• Leading cause of deaths in patients with HIV

Pathophysiology

• 1st step is inhalation of aerosol droplets.
• Deposition into the lungs leads to 3 possible outcomes:
  • Clearance of bacteria
  • Primary active disease:
    • Proliferation of bacteria within alveolar macrophages
    • Cytokines produced by macrophages (IL-12 and Il-18) attract other phagocytic cells.
    • A tubercle (granulomatous structure) forms (promoted by TNF-ɑ)
    • Tubercle expands into lung parenchyma → Ghon complex
    • Bacteria can then spread to draining lymph nodes → lymphadenopathy
    • Ghon complex + lymphadenopathy/calcification → Ranke complex
    • If spread is not controlled by immune cells, bacteremia with seeding of other organs may occur → miliary TB
    • When bacteria erode into airways (caseating granulomas), the patient becomes contagious.
    • Infection may progress to a chronic stage with episodes of healing and subsequent scarring of lesions.
    • Spontaneous eradication is rare.
• Latent infection (clinical (secondary) disease may occur many years later):
  • Lifetime risk of reactivation: 5%–10%
  • Immunosuppression is a factor in reactivation.

Clinical presentation

• Pulmonary manifestations associated with systemic symptoms are most common in primary and secondary TB.
• Miliary (disseminated) TB occurs in 15%–20% of cases (usually in children or immunocompromised individuals).

Identification

Sputum:

• 3 specimens, at least 1 early morning
• Acid-fast bacillus (AFB) smear
• Mycobacterial culture
• Nuclear acid amplification test

Blood or urine mycobacterial culture: in patients with HIV or immunocompromised individuals

Tuberculin skin test (TST; PPD (purified protein derivative) or Mantoux test): 

• Intradermal injection of tuberculin antigen
• Can detect active or latent infection
• Measure induration area after 48–72 hours; positive if: 
  • > 5 mm in patients with HIV or immunosuppression, or recent contact with an individual with TB
  • > 10 mm in patients from high-risk countries, IV drug users, medical and lab workers
  • > 15 mm in patients with no known risk factors for TB

lnterferon-γ release assay (IGRA): no distinction between active and inactive TB

M. leprae complex

Epidemiology

• M. leprae complex bacteria cause leprosy.
• Prevalence is < 1 in 10,000.
• Highest incidence in India, Brazil, Indonesia, Bangladesh, and Nigeria
• Risk factors for acquisition:
  • Close contact with patients with leprosy
  • Armadillo exposure (natural reservoir of M. leprae)
  • Elderly
  • Genetic factors
  • Immunosuppression

Pathophysiology

• Bacteria are inhaled or transmitted through direct skin contact.
• Bacteria are taken up by alveolar macrophages and disseminated by blood.
• M. leprae spreads to the nerves and skin, where it proliferates (especially in Schwann cells).
• Leprosy can manifest in different forms and reflects the spectrum of the host immune response.
• Ridley-Jopling classification of leprosy:
  • Tuberculoid (TT)
  • Borderline tuberculoid (BT)
  • Mid-borderline (BB)
  • Borderline lepromatous (BL)
  • Lepromatous (LL)
  • Indeterminate (I)
• Tuberculoid and lepromatous forms are at the 2 ends of the spectrum:
  • Tuberculoid leprosy:
    • Good cellular immune response (TH1 response)
    • Delayed-type hypersensitivity with interferon-γ and macrophage activation
    • Involves the skin (localized) and peripheral nerves
  • Lepromatous leprosy:
    • Minimal cellular immune response
    • Humoral (antibody) response (activated by TH2 cells)
    • Extensive skin involvement
    • Nerve involvement tends to be symmetric

Clinical presentation

• Hypopigmented or reddish skin patches associated with loss of sensation
• Paresthesia
• Painless wounds in the extremities
• Facial or ear lumps
• Enlarged and tender peripheral nerves
• Ophthalmic injury (corneal abrasions/ulcerations, lagophthalmos)
• Late findings may include facial paralysis, collapsed nose, clawed fingers, foot drop, lack of eyelashes/eyebrows.
• Immunological reactions (systemic inflammatory complications) may occur.

Identification

The organism is identified by skin biopsy:

• Histopathological examination
• PCR

M. avium complex

Epidemiology

• Most commonly acquired from environment (water sources)
• No documented human-to-human transmission
• Commonly affects people with pre-existing lung disease and immunocompromised patients (AIDS)

Pathophysiology

• Usually presents as pulmonary infection in immunocompetent individuals
• Can present as disseminated disease or localized lymphadenitis in patients with AIDS:
  • Risk increases as CD4 count falls below 50
  • MAC infection is one of the AIDS-defining conditions.
  • MAC bacteria enter through the GI mucosa and are phagocytosed by macrophages in the lamina propria.
  • Lymphatic drainage transports mycobacteria to the abdominal lymph nodes from where they can enter the bloodstream.
  • Spread can occur to many sites: spleen, bone marrow, and liver are most common
  • Most signs and symptoms are due to the elaboration of cytokines.
  • Rarely the direct cause of death but ↑ the risk of superinfection
  • Localized lymphadenitis:
    • Usually after the initiation of antiretroviral therapy
    • Results from the immune reconstitution inflammatory syndrome (IRIS)

Clinical presentation

• Immunocompetent patients:
  • Older males with chronic obstructive pulmonary disease:
    • Cough, fever, upper-lobe infiltrates/cavities
    • Resembles TB, but less severe
  • Women > 50 years of age with no preexisting history: often presents as bronchiectasis
• Individuals with AIDS:
  • Disseminated infection:
    • Fever, night sweats
    • Abdominal pain
    • Diarrhea
  • Lymphadenitis:
    • Localized lymph node enlargement (abdominal, mediastinal, cervical)
    • Fever

Identification

• Mycobacterial blood culture
• Histopathology and culture of the involved sites (bone marrow, lymph nodes)