Mycobacterium
The Mycobacterium groups is well known for tuberculosis and leprosy causing pathogens. The group are Gram-positive, aerobic rods which are non-motile and non-spore forming. Although they occasionally form filaments.
Mycobacterium’s taxonomic classification follows the Figure, all species that I talk about in this section will follow this taxonomy but have their species name included.
Taxonomic structure
Kingdom = Bacteria
Phylum = Actinobacteria
Class = Actinobacteria
Sub-Class = Actinobacteria
Order = Actinomycetales
Sub-Order = Corynebacterineae
Family = Mycobacteriaceae
Genus = Mycobacterium
Mycolic acid is the main component of an acid-fast cell wall structure, present among the Mycobacterium group. The Mycolic acid can be 60-90 carbons in length, this slows mycobacterium growth due to the slow mycolic acid synthesis. This allows for a unique pathogenicity because the slow growth limits antibiotic effectivity, as metabolic activity is required for antibiotics to work. Slowed growth results in insufficient antibiotic absorbtion, leading to a number of issues;
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Antibiotics have to be administered for long periods of time, sometimes up to years in order to be effective. This is because the body clears them before all mycobacteria are killed.
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This limits the antibiotics available to use as many stronger antibiotics can be toxic and cause organ damage.
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Antibiotic resistance can emerge as the cells receive low doses of antibiotics over time. Multiple antibiotics have to be used to decrease the chances of this happening.
Mycolic acid can also prevent the action of disinfectants, leading to control issues with mycobacterial pathogens.
Mycobacterium Pathogens
Mycobacterium leprae
M. leprae causes leprosy, a disease which can result in disfigurement of the limbs and face. This is because M. leprae has an optimal growth temperature of 30°c, so prefers extremities of the body.
M. leprae cannot be cultured in the lab, therefore it is cultured within 7-banded armadillos whose body temperature is 30°c.
It is estimated that around 21900 new cases of leprosy were detected in 2011, with 83% of cases from; India (58%), Brazil (16%), and Indonesia (9%). M. leprae transmission occurs from human to human transfer between close living individuals much like TB.
Mycobacterium ulcerans
M. ulcerans is the third most common mycobacterial disease. Affecting soft tissues of the skin M. ulcerans produces a cytotoxic toxin called mycolactone, this leads to ulceration of the skin and necrosis.
M. ulcerans is known to affect mostly children under 15, with 5000-6000 cases annually. Thought to be transmitted by aquatic insects, most cases occur around aqueous environments such as wetlands in rural tropical regions, typically African countries but also around the equator. An infection is only suspected when skin ulceration occurs, a positive Ziehl-Neelsen stain can suggest an infection in the field but PCR detection is required to confirm the diagnosis.
M. ulcerans is not transmitted between humans, the exact method is unknown which limits disease control techniques. No vaccine exists and treatment consists of trial and error combination of antibiotics, like M. avium-intracellulare.
Mycobacterium avium-intracellulare
M. avium and M. intracellulare are opportunistic pathogens found in soil and water, there lacks evidence for human to human transmission, infection is from environmental reservoirs.
M. avium-intracellulare rarely causes disease in healthy people, it mostly effects end-stage AIDS patients. First recognised after the AIDS epidemic, infections are systemic and asymptomatic until multiple organ failure occurs.
M. avium-intracellulare can be cultured onto Middlebrook 7H11 selective agar much like M. tuberculosis.
Mycobacterium tuberculosis
M. tuberculosis is the most common cause of tuberculosis in humans, this disease primarily affects the lungs. The pathogen causes asymptomatic tubercle cavities to form in the lungs, decreasing oxygen intake and cause permanent debilitation as a result of oxygen deprivation.
M. tuberculosis can be cultured on Middlebrook 7H11 selective agar which is incubated for 8 weeks.
The WHO estimate that in 2013, 9 million people developed TB and 1.5 million people died. 3.5% of reported cases were multi drug resistant strains (MDR-TB).
56% of TB cases in 2013 occurred In SE-Asia with 25% in Africa. TB is a large killer of HIV positive people in these regions and is transferred from human to human via aerosols.
M. tuberculosis lesions on pig lung tissue.
M. leprae's affects on the hands.
Ulceration caused by M. ulcerans.