Microbiology Flashcards
Give the 2 main species of mycobacteria
M. Tuberculosis (1/4th leading infection that results in death. - wasting, fever & bloody cough)
M. Leprae (affects skin, mucous membranes & nerves, can lead to disformaties and disfigurement in severe cases)
How do you stain for mycobacteria?
Ziehl-Neelsen stain
Their cell wall has a thick lipid layer which is pale staining in gram stains due to the mycolic acids in the wall.
Need acid fast stain
Ziehl-Neelsen: carbon fuchsin, acid alcohol, methylene blue.
AFB are resistant to destaining so stay pink/red
Explain how the immune system tries to prevent an infection due to a mycobacterium
Mycobacterium (AFB) phagocytosed by macrophages & enter a phagolysosome.
Host aims to degrade AFB and display the antigens for T-cells.
CD4 cells generate IFN-gamma for macrophages for intracellular killing. IL-12 stimulates T Helper and IFN-gamma.
Explain how a mycobacterium can lead to in infection
Signalling pathways are required to attract macrophages for killing.
Problems in the signalling pathways can lead to infection.
- genetic defects in IL-12/INF-g/signaling
- CD4 (HIV)
- TNF inhibitors (rheumatoid arthritis, inflammatory bowel disease)
- Immunosuppresed patients
Explain how granulomas form in mycobacterial infections
Lesions that try and contain mycobacteria
Macrophages:
- become epitheliod cells
- M0 fuse to form giant multinucleated cells ‘langhans giant cells’
Central part may necrose = caseating granuloma
Granuloma? Rule out TB
Which of the following is true?
a) Mycobacteria are not immunogenic
b) Mycobacteria are highly immunogenic
Mycobacteria are highly immunogenic
Due to mycobacterial lipids
How can you test exposure to TB?
Which is the most useful test and why?
Tuberculin skin test - intradermal injection of purified proteins, difficult to tell if immunitiy to BCG/TB
Interferon gamma release assays IGRAs - ELLISPOT/TSOT use antigens specific for M. tuberculosis. Demonstrate exposure but not active infection. - more useful
For the following give the main reason for the tissue damage
a) Tuberculoid leprosy
b) Lepromatous leprosy
a) Tuberculoid leprosy - too much immune response -tissue hypersensitivity and granulomata
b) Lepromatous leprosy - Too little immune response - tissue damage to uncontrolled bacilli & poorly formed granulomata
What are the principles of mycobacterial treatment?
Prolonged treatment as slow growing bacteria
Can grow in different locations - intra/extracellular
Combination of drugs
Resistance (big problem) - need to target all populaions and mutants - MDR & XDR
Compliance is essential
In primary tuberculosis where are the bacilli most likely to settle and why?
a) Carina of trachea
b) Apex of lung
c) Lower legft and right lobes
d) Right main broncus
Bacilli settle in the apex of the lung and form granuloma.
This is because the apex has more air and less blood supply (fewer defending WBC)
With TB which doesn’t makes up the primary complex?
a) granuloma
b) lymphatics
c) lymphnodes
d) capillary beds
Primary Complex = Granuloma + Lymphatics + Lymph nodes
Describe how laten TB can become active
Latent TB = no clinical disease
Vigorous T cell control
Detectable cell mediated immunity of IGRA/tuberculin test
May become active if there is a compromise to the immune system - aging, HIV, malnutrition, diabetes
Control lost by the T cells
Which bacteria is being described?
Purple on gram stain. Negative catalase test with chains of cocci. Appears yellow on a haemolysis test.
a) Beta haemolytic stretococcus
b) Streptococcus pneumoniae
c) Staphlococcus pneumonia
d) Staphlococcus aureus
Beta haemolytic stretococcus
Purple on gram stain = +ve
Negative catalase test with chains of cocci = Streptococcus
Appears yellow on a haemolysis test = beta haemolytic
Which of the following are being described?
Purple on gram stain. Clusters of cocci that are catalase +ve and coagulase +ve.
a) Beta haemolytic stretococcus
b) Streptococcus pneumoniae
c) Staphlococcus pneumonia
d) Staphlococcus aureus
Staphlococcus aureus
Purple on gram stain = gram postive
Clusters of cocci that are catalase +ve = Staphlococcus
Coagulase +ve = S. aureus
Which of the following are being described?
Purple on gram stain. Catalase test -ve. Green haemolysis test. Optochin sensative.
a) Beta haemolytic stretococcus
b) Streptococcus pneumoniae
c) Staphlococcus pneumonia
d) Staphlococcus aureus
Streptococcus pneumoniae
Purple on gram stain = +ve
Catalase test -ve = Streptococcus
Green haemolysis test = alpha haemolysis
Optochin sensative = S.pneumoniae
Explain the life cycle of malaria
Malaria bites an infected human - takes up gametocytes
Gamaetocytes to sporozoites in mosquito salivary gland
Mosquito bites human
Human liver into merozites - released into blood stream
Merozites invade RBC, divide and destroy them
