tuberculosis Flashcards
annual deaths on tb
So currently annual deaths are estimated at 1.5 million, so it’s still a huge problem worldwide.
- So there’s approximately 10 million new cases of TB each year
In terms of prevalence worldwide,
- although tb’s present across the globe. It’s higher prevalence in certain countries. It’s not evenly distributed and it is very much a disease of poverty.
at the moment It’s a particular problem in Africa and Southeast Asia and Western Pacific.
mycobacterium tuberculosis
the organism that causes TB is an intracellular pathogen.
- So it’s able to survive and replicate in macrophages.
So the very cells that are supposed to destroy it , it can manipulate and it can replicate in those cells.
biggest risk factor for developing TB
one of their Biggest risk factors for developing, the disease TB is being hiv-positive.
issues with the disease - being a carrier
one of the issues about this disease is once you’re infected with it You might not develop symptoms, but the organism can persist for decades within your tissues and then it can re-emerge and cause an infection later on in life
the current estimates are that a third of the world’s population are currently latent
- infected with mycobacterium tuberculosis, and they’ve got the potential to have the disease in the future.
epidemiology
it’s spread by aerosols.
- infectious dose estimated at 1 bacterium
usually it requires prolonged contact with someone that’s infected.
So those people that get infected with TB, actually only five to ten percent of those will go on to develop the actual disease unless they have got a predisposing factor
- and if it’s untreated its fatal in about 50% of patients.
susceptible individuals
- immune status/ immunocompromised
- HIV co-infection, poverty, unemployment, homelessness, alcoholism, drug abuse, steroid use, young and elderly
- living in congregate settings
- genetic predisposition?
- strain virulence?
symptoms
- primary TB is often asymptomatic
-fever, coughing and it’s often coughing with a blood Sputum - also as a disease progresses you start to see weight weight loss, loss of appetite loss of energy, night sweats
- and as it progresses further you start to see lung damage and then that can lead to systemic disease.
- So once the bacteria of got out of the lungs and it’s become systemic they can pretty much spread Anywhere,
- once it becomes systemic, it’s pretty much always fatal
mycobacterium tuberculosis structure and features
- actinobacteria (phylum), family Mycobacteriaceae
- gram-positive (so has a thick layer of peptidoglycan on the outside), rod-shaped
- non spore forming
- obligate aerobe (It has to have oxygen to be able to grow)
- acid-fast
- the cell wall has a thick layer of glycolipids on the outside
- really slow-growing organism - generation time (18-25 hours)
how do you stain mycobacterium tuberculosis
- the cell wall has got a really thick layer of glyco lipids on the outside. So it’s like a layer of wax on the outside. So if you try and stain it the stain can’t get through this layer of glycolipids.
- So what you have to do is treat the stain with phenol and that allows the stain to go in
- any other bacteria you stain you can then decolorize with acid alcohol, but with the actinobacteria if you add the acid alcohol, they stay stained so they called acid fast because you can’t decolorize them with acid alcohol
mycobacterium cell wall
- glycolipid outer layer that forms a lipid shell so it’s essentially gram positive.
-it’s got a cytoplasmic membrane here and then you’ve got a layer of peptidoglycan on the outside. - But scattered amongst that peptididaglycan you’ve got these unusual lipids:
- Pim that helps anchor it to the cell membrane.
all of this make it really resistant to detergents disinfectants antibiotics as things can’t pass through that cell wall.
it means it can survive for a long time in the environment and its really resistant to desiccation so it can hang around for a while on surfaces
diagnosis of tb
this classic test is a skin test called the tuberculin test
- just on your wrist you get injected with purified protein derivative from mycobacterium tuberculosis. And it just gets injected Under the Skin then it’s left for a few days.
- If you’ve been infected with the organism, you’ll then get these raised lumps where you’ve had that injection.
-if somebody test positive they then get followed up with other tests. So that could be that they look for acid-fast rods in the sputum
○ Sometimes they try and cultivate the organism but that is really slow
testing for tb - interferon-gamma release assay
- a much more rapid test
- useful because the tuberculin test can’t discriminate between someone that’s been vaccinated and someone that’s been infected.
- what they do here is they take your white blood cells and they incubate your lymphocytes with a protein from mycobacterium tuberculosis, and it’s a protein not present in the vaccine strain
- and then they look for Interferon gamma release
- if you get interferon-gamma release, the result means you’ve been exposed to TB long enough for your adaptive immunity to kick in
○ it means you have been infected with it
The only problem with this test is it has to be done with fresh white blood cells and it’s costly so it’s not something that’s done routinely and certainly not done across the world.
BCG vaccine for TB
- Attenuated strain of the mycobacteria that causes TB in cows - mycobacterium bovis
- Live vaccine
- lots of debate as to whether it does have any efficacy. It looks like it depends on the population.
treatment for tb - antibiotics
and the standard short course treatment is two months with four different antibiotics all targeting the cell wall (isoniazid, rifampicin, pyrazinamide and ethambutol) and then followed by four months with just two antibiotics (isoniazid, rifampicin)
- the rationale for this is that it’s very unlikely that you’re going to get 4 spontaneous mutations all at the same time that will give resistance to all four antibiotics
multi drug resistant tb
one problem is that some strains are developing resistance and we have multidrug-resistant TB
multi-drug means it’s at least resistant to the two main antibiotics
why are most antibiotics useless for tb
in terms of treatment, most antibiotics are absolutely useless as they can’t get through the thick cell wall.
The problem is it grows so slowly that it can develop resistance due to mutations.
- So the current treatment relies on very specialized drugs that Target that unusual cell wall.
factors that contribute to the success of M.tuberculosis
once it gets phagocytosed, it stops a phagosome developing and so it prevents its own destruction within macrophages.
initiating the formation of well organised granulomas
- What that does do is put the bacteria in a confined environment and that also can help with the disease because the bacteria can be within that environment and they’re kept separate from drugs
finally, it’s the fact that it can enter this stage of dormancy
- it’s not replicating but it’s still alive it just sits there.
- So again drug treatment host defenses can’t attack it. It just sits there latent
So in terms of the disease, it’s a chronic inflammatory condition: rendering itself extremely resistant to host defenses and drug treatments
the pathogenic life cycle of M.tuberculosis
in terms of the pathogenic life cycle, if you’ve got someone that’s infected, They’ve got TB in their lungs Within These granulomas and they spread the Disease by coughing and then it gets transferred to a new host.
- And so the bacteria go down into the lungs
- macrophages obviously come to try and get rid of the infection and the bacteria get taken up by the macrophages, but the macrophages can’t kill them, So the bacteria continue to replicate
- more macrophages come along to try and get rid of the infection and that just allows the infection to spread
-those macrophages go to lymph nodes to try and Prime adaptive immunity but you start attracting more and more immune cells and form a granuloma.
- sometimes that’s successful and that granuloma will calcify and heal. So someone will test positive for TB. But they’ve managed to contain it and don’t go on to develop symptoms.
But if those bacteria continue to replicate the granuloma ruptures, then the bacteria can spread within the lung and someone becomes contagious and infectious.
granuloma maturation
the granuloma starts to mature. You see that the blood vessels start to disappear and you start to get a fibrous cuff around the outside.
this fibrous cuff is due to Supergenic cytokines being released and then that can start to calcify and heal and can try and control the infection.
foamy macrophages
full of lipids
what happens once the granuloma has formed - in a healthy immune system
once this granuloma forms, if your immune system is able to activate your macrophages, then those activated macrophages have got increased killing potential and they can destroy the ingested bacteria.
○ the lesion heals, calcifies and the individual becomes hypersensitive to TB
- that means they’ll test positive on a test, but they could either have completely got rid of the bacteria, So now they’ve got no infection or they may be latently infected
- in this case the infection is asymptomatic
what happens once the granuloma has formed - in a susceptible individual
For a susceptible individual the activation of the macrophages is ineffective.
- And so because you don’t have the increased killing potential of the macrophages the replication continues and the tubercle continues to grow and you get continuous stimulation of T cells and macrophages
- and this inflammatory response causes lung damage and that lung damage can help the bacteria spread.
-so you’ve got this constant replication at the center of the granuloma. You’ve got inflammation occurring and this causes necrosis of the granuloma.
-ruptures and bursts open and that releases the bacteria and then the person becomes infectious
epithelioid cells
derived from activated macrophages by chronic stimulation with cytokines
giant cells
epithelioid cells fuse to form multinucleate giant cells
- these have little endoplasmic reticulum
- mitochondria and lysozymes are degenerating
- terminal macrophage differentiation
granuloma
- core of epithelioid cells, macrophages and some giant cells usually surrounded by lymphocytes
- zone of necrosis may develop through destruction of cell structures
fibrosis
deposition of collagen fibres
- core surrounded by lymphocytes
foamy macrophage
a macrophage loaded with lipid droplets
- often observed in tissue with proinflammatory stimulus
TNF and granuloma formation
- mouse injected intravenously with BCG
- No further treatment
- mouse develops granulomas in liver, spleen and lungs
- mouse injected intravenously with BCG
- mouse injected intravenously with anti-TNF antibody
- no granulomas develop
different outcomes of infections by mycobacterium tuberculosis
-infection eliminated with innate immune response or with acquired immune response: not infectious, no symptoms, no treatment
- latent TB infection: not infectious, no symptoms, preventative therapy treatment
- subclinical TB disease: sporadically infectious, mild symptoms, multi-drug therapy treatment
- active TB disease: infectious, mild to severe treatment, multidrug therapy treatment
