(Section C: Bacteriology) Lecture 21 Flashcards
What are the 4 forms of Mycobacteria mentioned?
- M. tuberculosis
- M. leprae
- M. bovis
- M. avium
Mycobacterium tuberculosis
- Causes tuberculosis in humans
- “TB” = Tubercle Bacilli
Mycobacterium leprae
Causes Leprosy in humans
Mycobacterium bovis
Causes tuberculosis in cows, rarely in humans
* Humans can be infected via unpasteurized milk
* Leads to extrapulmonary tuberculosis
Mycobacterium avium
Causes tuberculosis-like illness in humans, particularly in patients with AIDS
What are the two types of Tuberculosis infection?
- Latent
- Active
How much of the population has latent TB?
2 billion (1/4 of the world’s population)
TB is contagious and spreads…
Through the air by people with active TB
How many people with latent TB will develop active TB in their lifetime?
~10%
How many people die from TB each year?
1.6 million
Mycobacterium tuberculosis
* Type of pathogen
* Generation time
- Intracellular pathogen (lives within macrophages
- Slow generation time of >15 hr
Can M. tuberculosis be grown in labs?
Yes
* Takes 4-6 weeks to get small colonies
* Must be grown on specialized media
What is unusual about the cell envelope of M. tuberculosis?
High concentrations of mycolic acid
What characteristics does mycolic acid impart on M. tuberculosis?
- ‘Waxy’ layer
- Impermeability to stains and dyes (require acid fast staining)
The unusual cell envelope of M. tuberculosis is associated with resistance to:
- Some antibiotics
- Osmotic lysis via complement desposition (resistance to complement pathway)
- Lethal oxidative stress (allows survival inside of macrophages)
Describe the process of:
Acid Fast Stain
- Stained with carbo-fuchsin dye with slow heating (why)
- Washed with ethanol and HCl
- Counter stained with methylene blue
- Differentiate acid-fast vs non-acid fast
- Slow heating will melt the waxy cell envelope to allow staining
In Acid Fast Stain:
What do acid-fast organisms appear like? Non-acid fast organisms?
Acid-fast organisms: Red
Non-acid fast organisms: Blue
“Acid fastness” is due to the presence of…
Mycolic acid
Spread and progression of tuberculosis:
Stage 1
Transmission
* Inhalation of droplets from infected host (coughing, sneezing)
Coughing and sneezing can generate —- droplet nuclei
* Droplet nuclei can contain — bacteria
- 3000
- <10
How big are droplets from coughing and sneezing? What property do they have?
Small diameter (~5μm)
* Stay airborne for extended time
* Can be directly inhaled into lungs
Spread and progression of tuberculosis:
Stage 2
Phagocytosis of TB cells by lung (alveolar) macrophages
How do TB cells escape phagocytosis and continue infection?
(5 points)
- Blocks acidification of the phagosome
- Inhibits fusion of lysosome to the phagosome
- Multiplies in macrophages
- Macrophages lyse = release TB cells to infect more
- Delays dendritic cell migration to lymph nodes
ESX secretion system
Enable the transport of select bacterial molecules across the thick Mtb cell envelope
How many ESX systems exist in Mtb?
5
What functions do the molecules secreted by ESX secretion systems have?
- Damage to the phagosome membrane
- Other functions that inhibit the immune responses
Multiple functions
Spread and progression of tuberculosis:
Stage 3
Infected macrophages may form granulomas
What are TB granulomas?
“Tubercles” of immune cells that try to destroy invading pathogens
* Typically comprised of macrophages
What do TB granulomas represent?
Latent infection
* A balance between the pathogen and the host
How do granulomas work?
- T cell activated macrophages can kill TB
- Activated T cells secrete cytokines (IFN-gamma) to activate macrophages
- Macrophages at the center of the granuloma ramin harder to activate
Caseous necrosis
Chronic inflammation that causes “cheese-like” necrosis
Spread and progression of tuberculosis:
Stage 4
Active tuberculosis
* Some macrophages remain unactivated and infected, tubercle grows
* Erosion of granuloma into airway leads to transmission
* Deterioration of host immunity results in life-threatening infection
The caseous center of granulomas can liquefy, leading to…
Cavitation
Extrapulmonary tuberculosis
Infection outside the lungs
* Can infect multiple organ systems (bone, joints, liver, spleen, gastrointestinal tract and brain)
Who is more likely to get extrapulmonary tuberculosis?
- Immunocompromised individuals (HIV infected patients etc.)
- Young children
Miliary tuberculosis
Widespread dissemination occurs, almost always fatal
What is the test for tuberculosis?
Tuberculin test
* PPD (purified protein derivative) from M. tuberculosis
Tuberculin test
* Immune response
* Positive result
- T cell-mediated response
- Positive result is a red and swollen circle at 48 hrs
In a TB skin test, how is person determined to be infected?
If they convert from negative to positive on a TB skin test
What could positive and negative results mean on a TB skin test?
Positive:
* Latent/active TB
* BCG vaccinated
* Previously infected
Negative:
* Not infected
* Immune compromised (e.g. AIDS)
* Not infected long enough
What other diagnosis methods are used to diagnosis M. tuberculosis?
- History
- Chest X-ray (shows upper lobe “shadowing”, indicates lesions)
- Staining of sputum and culturing
- Interferon-gamma response assay
True or False:
Active TB can kill 1 out of 3 people if untreated
False, active TB can kill ~2 out of 3 people if untreated
Why are TB treatments very long?
Due to the slow growth of the bacteria
How is tuberculosis treated?
Generally, multiple types of antibiotics are used
* Rifampin
* Isoniazid
Rifampin
Inhibits RNA polymerase
Isoniazid
Inhibits mycolic acid synthesis
MDR-TB
Multi-drug resistant TB
* Defined as being resistant to the two most effective first-line therapeutic drugs (isoniazid and rifampin)
XDR-TB
Extensively-drug resistant tuberculosis
* Also resistant to the most effective second-line therapeutic drugs used commonly to treat MDR-TB
How prevalent is XDR-TB?
Found in all regions of the world
BCG
“Bacille Calmette-Guerin”
* A living vaccine prepared from attenuated M. bovis
How is the M. bovis attenuated in the BCG vaccine?
- Lacks ESX-1 secretion system
- Shares antigenicity with TB
Why is the BCG vaccine controversial?
Variable efficacy (~80% to much less) for pulmonary TB
True or False:
BCG vaccinated individuals can give a false positive for the tuberculin test
True
What happens as a side effect of BCG vaccine?
Vaccination leaves large scars
* Larger the scar, the more efficient it was
What group of individuals are recommended to get the BCG vaccine?
Individuals with high risk to exposure of TB
Leprosy
Chronic disease cause by M. leprae
* Sometimes called Hansen’s disease
Leprosy
* Progression time
* Damage
* Prevalence
- Very slow progression (incubation period of ~5 years)
- Permanent damage to skin, nerves, limbs and eyes
- Very rare in high-income countries
How many people are permanently disabled by leprosy?
~2 million
* Mainly in tropical developing countries
* May be a low estimate
Mycobacterium leprae
* Gram stain
* Shape
* Envelope
- “Gram positive” acid fast stain
- Rod shaped
- Waxy cell envelope (mycolic acid)
Why is M. leprae less well-studied than M. tuberculosis?
Cannot be cultivated in vitro
What cells does M. leprae infect?
- Macrophages of skin
- Schwann cells in nerves
How can M. leprae be grown?
- Grown in foot pads of mice (low numbers)
- Systemic infection in armadillo (10^10 organisms per gram of infected tissue)
Leprosy vs. Tuberculosis
- Lesions in tuberculosis are “hidden” (internal)
- Lesions in leprosy are “visible” (external)
Leprosy is much less infectious than TB
What has happened to victims of leprosy historically?
Ostracized
* Rejected by family and friends
* Driven out of communities
What are the two major forms of Leprosy?
- Tuberculoid
- Lepromatous
Tuberculoid Leprosy
* Characteristics
* Signs/symptoms
- Cell-mediated immunity present
- Light coloureed lesions with “anesthetic” areas
- Sometimes loss of hair and pigmentation
In Tuberculoid Leprosy:
Do macrophages contain the bacteria?
Yes they can
What result do tuberculoid leprosy patients have on the tuberculin test?
Patients become tuberculin positive
Are bacterial cells recoverable from lesions in Tuberculoid Leprosy?
Generally not
What does it mean that Tuberculoid Leprosy is a self-limiting disease?
It can resolve on its own
Lepromatous Leprosy
* Characteristics
* Signs/symptoms
- Cell mediated immune responses are absent
- Nerve damage, loss of sensation, traumatic lesions
- Loss of eyebrows, thickening and enlarge nares, ears and cheeks
- Lesions can become secondarily infected, eventually resulting bone resorption, disfigurements and mutilation
What are Schwann cells?
Provide myelin insulation to peripheral nerves
Macrophages
* Lepromatous vs Tuberculoid Leprosy
Lepromatous:
* Macrophages are not activated
Tuberculoid:
* Macrophages are activated
What happens to bacteria in macrophages in Lepromatous Leprosy?
M. leprae survives and multiplies in macrophages and Schwann cells
Does the bacteria attack and damage Schwann cells in Lepromatous leprosy?
No, the bacteria guides the macrophages to self-harm the Schwann cells
How is leprosy spread and transmitted?
Not well understood
* Requires close and direct contact for extended periods of time
* Most exposed individuals do not develop disease, thus host genetics likely plays an important role
What has been the treatment for Leprosy?
MDT (multidrug therapy) since 1980s
* Uses 3 antibiotics
* 6 months to 1 year
True or False:
Patients can still transmit leprosy after one dose of MDT
False, patients are thought to no longer transmit the disease after one dose
What is the ultimate goal of MDT?
Elimination of leprosy
