Acid fast Bacilli

The Mycobacteria

Characteristics of Genus Mycobacterium
· slim rod shaped organisms (1-10 µm long)
· Nonmotile, nonsporing, slow growing obligate aerobes
· cell envelope with a high lipid content, mycolic acids, complex long chain fatty acids
· Acid-fast with Ziehl-Neelson stain
Mycobacterium tuberculosis
· slim (1-4µm), un-encapsulated, strongly acid-fast rod, cigar shaped
· prefers 370 C, 5-10% CO2, pH 6.0-7.6
· obligate aerobe, slow growth - doubling time 18 h
· requires complex media such as Löwenstein Jensen medium for culture.
· resistant to drying and most disinfectants except for formaldehyde and glutaraldehyde.
· also resistant to highly polar inorganic acids and alkaline solvents.
· UV and heat sensitive. (2 h in the sunlight will kill it. If it is in sputum, 20-30 h are needed)
· multiplies intracellularly in phagosome
· Cell Wall of M. tuberculosis
· The cell envelope also contains peptidoglycan and has a complex glycolipid structure forming the outermost layer.
· Other important components include: lipoarabinomannan, sulfatides, mycolic acid, trehalose 6,6 dimycolate and waxes.
· Virulence Factors
· Cell wall rich in waxes and lipids, especially mycolic acids - serves as LPS.
· Mycolic acid glycolipids and trehalose 6,6 dimycolate (cord factor), - cause granuloma formation in animal tissue.
· Catalase, peroxidase and lipoarabinomannan - help resist the host cell oxidative response.
· Lipoarabinomannan also induces cytokine production.
· Sulfatides and trehalose dimycolate, are toxic to animal models.
Natural History of TB
· M. tuberculosis is acquired through the inhalation of droplets
· If a particle is less than 5µm, it can escape the first line of respiratory defenses
· inhaled droplets containing the organism are engulfed by macrophages.
· Bacteria not initially killed by the macrophage - multiply within.
· When the organisms reach sufficient numbers inside the macrophage is killed, - released organisms are ingested by other macrophages.
· Infected macrophages produce cytokines and chemokines - attract other phagocytic cells -formation of a nodular granuloma - the tubercle.
· Eventual dissemination of bacteria to the lymph and bloodstream
· Immunity is caused by T cell, and not by B cell proliferation.
· Cells involved include: CD4, CD8 T cells and NK cells.
· CMI develops 2-6 weeks after infection and in 90% of people, the infection is aborted without ever having symptoms of disease.
· Less than 10% go on to develop progressive disease.
· Formation of Th1 (type 1) granulomas composed of T cells and macrophages. Macrophages in the granuloma fuse creating Langerhans giant cells.
· Cytokines are produced during this process and include IL-1, IL-12, TNF-alpha and IFN-gamma.
· Hosts that cannot raise a Th1 response will develop miliary tuberculosis.
· Increased risk of disease after primary infection is found in:
· young children,
· HIV infected,
· intravenous drug abusers (IVDA).
· prison inmates, the homeless,
· the elderly
The Tuberculin Test
· The tuberculin test uses the Mantoux method (intradermal injection), and measures DTHR to PPD.
· Positive reaction leads to 10 mm induration after 48-72 h in in a normal individual.
· Positive reaction indicates previous exposure to M. tuberculosis or nontuberculous mycobacteria, including BCG vaccine - tuberculin conversion.
· Negative reaction indicates non-exposure, pre-hypersensitive state, or loss of TB sensitivity from primary infection.
Diagnosis of Tuberculosis
· Ziehl-Neelson stain- showing acid fast bacilli.
· Culture – LJ medium
· Nucleic Acid Assays (NAA)
· Chest x-ray
After a positive tuberculin test consists of INH for 6 to 9 months or 4 to 9 months of rifampin.
Bacillus of Calmette and Guerin (BCG)
· 70% effective in preventing infection
· Attenuated Mycobacterium bovis of low virulence is inoculated as vaccine.
· Six months of therapy with INH and rifampin plus two other drugs for 6 weeks cures infection in 95% of cases
· First line antituberculosis drugs: INH, rifampin, pyrazinamide, streptomycin, ethambutol
· Second line drugs: para-aminosalicylic acid (PAS), ethionamide, cycloserine, kanamycin, rifabutin, fluoroquinolones
· Combined therapy is employed to avert resistance formation – MDRMT (multi-drug resistant M. tb)
Nontuberculous Mycobacteria (NTM):
General Characteristics
· Found in soil and water
· Non-communicable; no human to human spread.
· Disease from NTMs usually develops in trauma, or immunosuppression.
· acid-fast, INH resistance is common
The Runyon Classification
· classifies NTMs based on pigmentation produced under light and dark conditions and their rate of growth
Photochromogens – R1
slow growing, and produce a yellow-orange pigment when exposed to light.
Ex: M. kansasii
causes lung disease clinically resembling TB, especially in those with preexisting lung conditions such as COPD
antigenically similar to M. tuberculosis.
long, banded, beaded, "barber pole", yellow bacillus due to beta carotene crystals.
M. marinum
causes swimming pool granulomas, and abscesses (fish fancier’s finger).
infects sites of abrasions because the organism likes a lower temperature.
Inhabits both fresh and salt water. It infects a lot of marine organisms.
Scotochromogens- R2
slow growing, and produce a yellow-orange pigment in light or in the dark.
Ex: M. scrofulaceum
Found in water, Enters through oropharynx and draining lymph nodes
produces scrofula, a granulomatous cervical adenitis, usually in children.
Nonchromogenic- R3
slow growing, and do not produce pigment
M. avium intracellulare complex (MAC)
Pleomorphic, at least 30 types, widespread in the environment, but only a few cause disease
Strains that infect immunocompromised hosts - M. avium
Strains that infect immunocompetent hosts - M. intracellulare.
MAC binds and invades oropharyngeal and gastrointestinal cells causing cough, weight loss, upper lobe x-ray changes, and cavities - all similar to TB.
Scrofula, cervical lymphadenitis in children
commonest NTM seen in late AIDS
Rapid Growers- R4
rapid growing mycobacteria (colonies in 5 days). They do not produce pigment.
Ex: M. chelonae and M. fortuitum
Infections occur as complications of surgical procedures - prosthetic implants, mechanical heart valves, contaminated operating equipment, mammoplasty etc.
present as soft tissue infections, lung infections, bone infections, CNS infections, or keratitis in the eye.
highly drug resistant
Mycobacteria not in the Runyon Grouping
Mycobacterium leprae
grown in armadillos (a natural host) or in mouse footpads (12.5 day doubling time).
Optimal temperature for growth is lower than core body temperature, so it grows on skin and superficial nerves.
lives in macrophages and Schwann cells.
Cell wall contains lipoarabinomannan (LAM) similar to M. tuberculosis, but there is a unique M. leprae-specific phenolic glycolipid (PGL-1).
Causes leprosy, which is also known as Hansen’s Disease.
The incubation period for leprosy is 5-7 years.
Clinically, there are two ends to the spectrum of disease, - tuberculoid and lepromatous leprosy, and gradations in-between.
Tuberculoid leprosy
Granulomas with very few organisms can be seen
Activated CD4+ cells are abundant in the core of the tuberculoid granuloma - produce interferon γ,stimulating macrophages to contain M. leprae growth.
CD4+ cells also secrete IL-2, which results in expansion of the CD4+ cell population.
On the periphery of the granuloma CD8+ cells are present, inhibiting expansion of the granuloma and extensive tissue destruction.
Clinically, tuberculoid leprosy is characterized by anesthetic plaques, and asymmetric peripheral nerve trunk involvement.
Lepromatous leprosy
poor CMI response with large number of organisms.
CD4 and CD8 cells are found throughout the lesion. CD8 cells secrete cytokines that enhances the growth of M. leprae and suppresses CD4 function and expansion.
Granulomas are poorly formed, and macrophages are full of M. leprae. CD4+ cells are present but in much fewer numbers compared to tuberculoid leprosy.
IL-4 appears to be an important cytokine that suppresses CD4+ cell function and proliferation.
Clinically, characterized by symmetric skin nodules, plaques, leonine (i.e., lion-like) facies, loss of eyelashes and body hair, and testicular dysfunction
Mycobacterium ulcerans
Causes Buruli ulcers, the third most common mycobacterial infection
Typically, a single lesion begins as small, painless, subcutaneous nodule - most often on a limb.
In 1 - 2 months, the nodule ulcerates, and enlarges to 15 or more centimeters.
There is no pus, and a biopsy of an ulcerated lesion shows low inflammatory infiltrate.
Skin adjacent to the lesion, and sometimes the whole limb, can become edematous. Ulcer can widen and deepen, destroying adjacent structures and bone.
Treatment: surgical debridement to stop the spreading ulcer
Clinical significance
Pigmentation Growth
M Tuberculosis , M bovis
M ulcerans
Strict pathogens
No No
M leprae
Strict pathogen
- -
Runyon Group 1
M marinum , kansasii
Runyon Group 2
Usually pathogenic
Photochromogens slow
M scrofulaceum
Rarely pathogenic
Scotochromogens slow
Runyon Group 3
M avium intracellulare
Pathogenic in immunocompromised
No slow
Runyon Group 4
M fortuitum, M chelonae
Rarely pathogenic
No ‘rapid’
Next Post »
Thanks for your comment

Popular Posts


This content is not yet available over encrypted connections.