Myobacterium Flashcards
What are the two groups of the genus myobacteria?
Rapid growing mycobacteria - M.smegmatis and M.fortuitum
Slow growing – M. tuberculosis, M. avium and M.leprae.
Describe clinical features of myobacteria
Non-motile rod-shaped (bacilli) bacterium that often occur in chains or bunches.
Obligate aerobes
Weakly gram positive - they don’t have the chemical characteristics of either – complex cell wall, which does have peptidoglycan. If a Gram stain is performed on MTB, it stains very weakly Gram-positive or not at all
Classified as acid-fast bacteria due to their impermeability by certain dyes and stains – due to the waxy lipids (myolic acid) in their cell wall).
Despite this, once stained, acid-fast bacteria will retain dyes when treated with acidified organic compounds – resist destaining.
fastidious organisms (have complex nutritional requirements) which make them hard to grow in culture.
What features does the waxy lipid on cell wall give the myobacteria
Their high lipid content makes them extremely hydrophobic.
This allows them to
Survive in harsh environments – acidic, alkaline and oxidative burst.
Resist phagocytosis, complement lysis and antibiotics due to their impermeable cell wall.
This makes them very hard to treat.
Describe the Ziehl-Neelsen stain
Used to identify myobacteria, based on acid-fast feature. Mechanism -
The MTB smear is fixed, stained with carbol-fuchsin (a pink dye), and decolorized with acid-alcohol.
The smear is counterstained with methylene-blue or certain other dyes.
Acid-fast bacilli appear pink in a contrasting background as they resist destaining.
Describe genomics of myobacteria
they have high GC content in their genes, with a large number of genes being involved in lipid metabolism.
The M. tuberculosis genome consists of single circular chromosome.
Complex gene regulatory system – it has to switch between aerobic and microaerophilic conditions.
Describe tuberculosis - epidemiology, transmission and infection
- caused by M. tuberculosis in humans.
- mainly a disease of lung, but in 20% of cases it can infect lymph nodes, brain, bone and kidneys which is very dangerous.
- Epidemiology
TB was once a very common disease but it fell in developed countries as standards improved. - However it’s estimated that one third of global pop. is infected with TB today and it has increased due to the expanding AIDs epidemic. More prevalent today than before.
Transmission
- via spread of droplet nuclei of the bacterium from one individual to another by coughing and sneezing.
- one droplet nuclei contains no more than 3 bacilli.
Infection
- After droplet nuclei are inhaled, the bacteria are non-specifically taken up by alveolar macrophages.
- Initial infection of TB results in an asymptomatic lung infection – the immune response will control this infection but not eliminate it entirely - latency.
- Much later in life, the latent TB infection may be activated (usually if immunosupression occurs) leading to chronic illness
Describe multi drug resistant (MDR) and extensively drug resistant TB (XDR)
Multidrug-resistant tuberculosis (MDR TB) is TB that is resistant to at least two of the best anti-TB drugs, isoniazid and rifampicin. These drugs are considered first-line drugs and are used to treat all persons with TB disease.
It’s generally treatable but expensive.
Two stages: primary infection with the drug resistance MTB and secondary where poor treatment will cause development of this MTB.
Potentially major public health disaster.
Extensively drug resistant TB (XDR TB) is a relatively rare type of MDR TB, when TB which is resistant to isoniazid and rifampin, plus resistant to any fluoroquinolone and at least one of three injectable second-line drugs (i.e., amikacin, kanamycin, or capreomycin).
Because XDR TB is resistant to first-line and second-line drugs, patients are left with less effective treatment options, and cases often have worse treatment outcomes. Serious threat!
What is myobacterium leprae?
Causative agent of leprosy, also known as Hansen’s disease - an ancient disease appearing throughout history.
M. leprae is slowly growing bacteria that prefers cooler temperatures of the body’s extremities
What are the two types of leprosy and how are they caused?
the pathogen that infects peripheral nerves and causes their destruction by chronic inflammation.
Tuberculoid leprosy –> if cell-mediated immunity successfully stops the proliferating bacteria. Rarely transmit to others.
Lepromatous leprosy –> if cell-mediated immunity fails to develop or is supressed. The nose and throat will contain high numbers of M. leprae and can transmit to others.
What are the symptoms of leprosy
Early signs include changes in skin pigmentation and deformities.
Characterised by disfigurement, loss of limbs and blindness due to actions on skin and peripheral nerves.
What are the differences between M.tuberculosis and M.leprae?
M. leprae is much lower in infectivity compared to MTB – incubation period of about 5 years which makes eradication difficult as disease is undetectable during that time.
Treatment for M.leprae is much more effective (i.e. less resistance) than MTB –> multi-drug therapy is used - rifampicin, clofazimine, dapsone for 6-12 months. Highly effective
Describe the three environmental myobacterium?
M. ulcerans
o Slow-growing mycobacterium that infects the skin and subcutaneous tissues, giving rise to painless ulcerated lesions.
Mainly in school-age children.
o Referred to as Buruli ulcer as it was region in Uganda where the disease was studied.
o Endemic in wetlands of tropical or subtropical countries, koalas and possums are naturally infected animals in Australia.
Might reside in aquatic animals.
o Treatment: drugs (rifampicin and streptomycin) and surgery
M. marinum
o Free-living bacterium, which causes opportunistic infections in humans.
o M. marinum sometimes causes a rare disease known as aquarium granuloma, which typically affects individuals who work with fish or keep home aquariums.
M.avium complex
- myobacterium avium complex are a group of genetically related bacteria - they include M. avium and M.intracellulare.
- includes ubiquitous atypical bacteria that can infect HIV+ - MAC causes disseminated disease in up to 40% of HIV patients - producing fever,sweats, weight loss and anemia.
- in immune-competent children it causes swelling of lymph nodes - lymphadenitis.
