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mycobacteria
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immunology notes for bioscience students
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Mycobacteria. Mycobacteria: Unicellular rod-shaped micro-organisms; obligate aerobes; non-
motile; non-capsulated; non-sporulating; 2-4 m length X 0.2-0.5 m width
Obligate aerobes- this means that they require oxygen to grow but they will survive
anaerobically, but they will not grow anaerobically.
There are loads of saprophytic mycobacteria- saprophytic means they live in soil.
Lots of mycobacteria live in the soil.
Only a few of them are able to cause disease in humans. They have a saprophytic
origin it means that they are very good at persisting in the environment.
They come from the phylum, which is actinobacteria because they are filamentous,
we see them in filaments called serpentine cords.
They are from the family mycobacteriaceae and the genus is mycobacterium. The
species is tuberculosis.
Many important species in terms of their infectivity are slow growing. Mycobacteria
can take roughly 15-20 hours to double. This means working with them is a
challenge.
The cell wall is comprised of a very high lipid content (unlike gram positive and gram
negative bacteria) they have a huge waxy cell wall which is a major virulence factor.
Gram negative and gram positive cell wall structure.
Gram positive cell wall- we have a cytoplasmic membrane and a thick peptidoglycan
layer and lipoteichoic acid and naked peptidoglycan on the outside of the cell wall.
Whereas gram-negative cell wall we have the cytoplasmic membrane, a small
peptidoglycan layer and then we have an outer membrane, and these are leafleted
with lipo-polysaccharide
Mycobacteria: unique cell wall structure.
Mycobacteria cell wall:
There is an inner mycobacterial membrane, which comprises of a periplasma
involving lipoarabinomannan and then there is a thin peptidoglycan layer the other
side of the lipoarabinomannan. This then leads onto the mycolic acid and there are
free lipids as well, these tend to be immunomodulatory.
The major component of the cell wall is mycolic acid, there are other acids as well
such as LAM and PIM.
There is also cord factor (TDM), this is the lipid that is responsible for the bacteria
being able to form those serpentine cords. It is very hydrophobic, so it allows the
bacteria to form serpentine cords.
TDM is a major virulence factor, because it is the virulence factor that is able to stop
the lysosome fusing with the phagosome once the bacteria have been taken up by
an alveolar macrophage. It is the thing that stops the lysosome fusing with the
phagosome and destroying the bacteria, this allows them to cause infection. It allows
them to persist.
So TDM is important.
The cell wall benefits intracellular survival.
It confers resistance to loads of antibiotics, to heat, to desecration and also it makes
it very difficult for us to stain them as well.
, Mycobacteria are referred to as acid fast bacteria, this is because we have to use an
acid-fast stain in order to be able to visualize them.
Ziehl-Neelson stain for acid fast bacteria.
The main acid-fast stain that is used and is also used clinically is the ziehl-neelson
stain.
The ziehl- nelson stain first involves staining with hot concentrated carbol fuchsin at
this point all the bacterial cells stain pink.
Then you need to de-stain with a 1% acid alcohol or with a 20% sulphuric acid.
This is the differential step where the mycobacterial cells remain pink, they are acid-
fast, so upon acid decolorization they remain pink.
Whereas the others go colorless and then a counterstain of methylene blue can be
used so the mycobacterial cells stay pink whereas all the other bacterial cells will
stain blue.
This is the process by which we differentiate mycobacteria.
Infections caused by mycobacteria.
Majority of tuberculosis infections are latent tuberculosis, this is asymptomatic .
In active tuberculosis you get pneumonia like presentations. ( the rest is explained
on the slide.)
It is a wasting disease; you get significant weight loss and if it is left untreated it is
fatal.
Mycobacterium bovis, the animal vector for this is a cow. It can be acquired from
the consumption of unpasteurized milk. People who are immunocompromised are at
particular risk, but you can get bovis if you are immunocompetent and it can cause a
respiratory infection just like normal tuberculosis. (rest of info on slide).
Mycobacterium avium intracellulare causes disseminated TB particularly people
who are highly immunocompromised such as people with HIV and AIDS. This causes
lung infections, spreads to bones and joints and meninges. (rest of info on slide).
Mycobacterium Leprae, this causes leprosy where you get lesions that form on the
face and the body. Causes significant damage to peripheral parts of the body, often
causes limb loss and significant disfiguration.
Only one way in which it can be grown, and it has to be grown in an organism, which
is a banded armadillo. So it is hard to research on it.
Factors of TB infection
Close contact with large populations of people, i.e., schools, nursing homes,
dormitories, prisons, etc
Poor nutrition
iv drug use
Alcoholism
HIV infection is the MAIN predisposing factor for TB infection. 10 percent of all HIV-
positive individuals have TB (400-times the rate associated with the general public)
TB infection: droplet nuclei
How do we get infected with tuberculosis.
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