B1 R&C Lec2 : Mycobacterium tuberculosis Flashcards
Important properties of M. tuberculosis
- grows slowly
- it has a doubling time of 18 hours (in contrast to most bacteria, which can double in number in 1 hour or less)
Why cultures of clinical specimens must be held for 6 to 8 weeks before being recorded as negative?
*Because growth is so slow.
• Media used for its growth (e.g., Löwenstein-Jensen medium).
• M. tuberculosis is an obligate aerobe; this explains its predilection for causing disease in highly oxygenated tissue
What does the special media of M. tuberculosis contain ?
- Complex nutrients (e.g., egg yolk)
- Dyes (e.g., malachite green) to inhibit the unwanted normal flora in sputum samples
Mycobacteria is a gram positive or negative?
It’s neither , because they are stained poorly by the dyes used in Gram stain
What is an acid base organism?
organism’s ability to retain the carbolfuchsin stain despite subsequent treatment with an ethanolhydrochloric acid mixture
Why is M.tuberculosis an acid-fast property ?
attributed to high lipid content of their cell wall in the form of long-chain fatty acids called mycolic acids
What is cord factor in M.tuberculosis?
a glycolipid in the cell wall of M. tuberculosis that inhibits phagosome-lysosome fusion and prevents lysis of phagocytosed mycobacteria.
Virulent strains grow in a characteristic “serpentine” cordlike pattern, whereas avirulent strains do not
Which proteins cause delayed hypersensitivity?
antigens in the purified protein derivative (PPD) skin test (tuberculin skin test)
What is M. tuberculosis resistant to ?
- Acids and alkalis
- Dehydration
- Drug resistance
- Mutations in a gene for mycolic acid synthesis
- mutations in a gene for catalase-peroxidase
What is the function of catalase peroxidase?
an enzyme required to activate isoniazid within the bacterium
How is M.tuberculosis transmitted?
from person to person by respiratory aerosols mostly generated by the coughing
What is the initial site of M.tuberculosis?
Lung
Who is the natural reservoir of M.tuberculosis ?
Humans
Which other organism causes tuberculosis in developing countries ?
Mycobacterium bovis
Where is M.bovis found ?
found in cow’s milk, which, unless pasteurized, can cause gastrointestinal tuberculosis in humans
Who is in a risk of M.bovis infection?
highest among socioeconomically disadvantaged people, who have poor housing and poor nutrition
M.tuberculosis preferably infects which cells?
- infects macrophages (alveolar macrophages lung)
- other reticuloendothelial cells.
Which type of hypersensitivity is M.tuberculosis?
Type IV hypersensitivity - Delayed T cell-mediated reactions
M. tuberculosis produces no ____________ and does not contain__________ in its cell wall.
no exotoxins ; endotoxin
Explain the pathogenesis of M.tuberculosis
1.Mycobacteria are phagocytosed, survives and multiplies in alveolar macrophages within a cellular vacuole called a phagosome
- CD4+ T cells (T helper cells) recognize mycobacterial antigens that are presented by macrophages
- CD4+ T cells then differentiate into Th1 cells, which release IFN-γ to recruit and activate macrophages
- Activated macrophages differentiate into epithelioid cells, which aggregate and fuse into multinucleated giant cells (Langhans giant cells)
- Aggregation of epithelioid cells, multinucleated giant cells and CD4+ T cells around a necrotic cellular center leads to the formation of caseating granulomas
What is a caseating granulomas?
Aggregation of epithelioid cells, multinucleated giant cells and CD4+ T cells around a necrotic cellular center
After recovery from the primary infection, resistance to the organism is mediated by ___________
cellular immunity
Cellular immunity occur by?
by CD4-positive T cells and macrophages
*The CD4 positive T cells are Th-1 helper T cells
What is the role of Circulating antibodies after primary infection?
They are formed but they play no role in resistance and are not used for diagnostic purposes.
Who are at higher risk for disseminated, life threatening tuberculosis?
AIDS , acquired immunodeficiency syndrome
How is the prior infection detected?
by Positive tuberculin skin test
What is the tuberculin skin test due to?
due to a delayed hypersensitivity reaction
What is the antigen in tuberculin skin test?
purified protein derivative , PPD, injected intradermally
How is the tuberculin skin test performed?
Evaluated by measuring the diameter of the induration surrounding the skin test site 48 hours later
What is observed in a positive tuberculin skin test?
Induration (thickening), not simply erythema (reddening) must be observed
What are the determined factors that judge the diameter required in the Tuberculin test?
—-> Induration of 5 mm or more is positive in :
a person who has deficient cell-mediated immunity (e.g., AIDS patients) or has been in close contact with a person with active tuberculosis
—-> Induration of 10 mm or more is positive in :
a person with highrisk factors, such as a homeless person, intravenous drug users, or nursing home residents
—-> Induration of 15 mm or more is positive in :
all other individuals i.e: with no known risk factors
What does a positive tuberculin Test indicates ?
result indicates previous infection by the organism but not necessarily active disease
When does the tuberculin test becomes positive?
4 to 6 weeks after infection
Which vaccine also cause a positive test?
bacillus Calmette-Guérin (BCG)
In BCG the reactions are how much in mm?
usually only 5 to 10 mm and tend to decrease with time
Is it possible for vaccinated people to be infected by M.tuberculosis?
Yes, People with PPD reactions of 15 mm or more are assumed to be infected with M. tuberculosis even if they have received the BCG vaccine.
The skin test itself does not induce a positive response in a person who has not been exposed to the organism.
What is Nramp?
A gene that determines natural resistance to tuberculosis
- People who have mutations in the Nramp gene have a much higher rate of clinical tuberculosis than those with a normal allele
Where is the NRAMP protein located and what is its role?
in the membrane of the phagosome in macrophages
and plays an important role in killing the organism within the phagosome
What are the general clinical findings of M.tuberculosis?
- Fever (low grade), fatigue, night sweats, and weight loss are common.
- Many organs can be involved
- Pulmonary tuberculosis causes cough and hemoptysis
What are the main features of Primary pulmonary tuberculosis?
- Ghon focus: a granuloma typically located in the middle/lower lung lobes.
- Ghon complex: formed by the Ghon focus, regional (hilar) lymph node, and the linking lymphatic vessels
- Includes latent tuberculosis infection (asymptomatic) and active
primary tuberculosis
When does Primary pulmonary tuberculosis occur ?
after initial exposure to M. tuberculosis
What is the Secondary tuberculosis?
A condition in which a latent infection with M. tuberculosis becomes symptomatic (e.g., due to the patient becoming immunocompromised) and contagious
Where does the 2ndry tuberculosis infect?
Usually affects the upper lobes of the lungs because of higher oxygen tension
What are the nonspecific findings in pulmonary tuberculosis?
1.Pallor
2.Clubbing
3.Generalized wasting
What are the chest examination findings in pulmonary tuberculosis?
By Percussion
1. Dullness over areas of consolidation
- Hyperresonance over areas of cavitation
By Auscultation
1. Amphoric (hallow) breath sounds over areas of cavitation
- Rhonchi
- Crackles
- Diminished breath sounds over areas of consolidation (or pleural effusion
What is the disease that cause lymphatic spread of M. tuberculosis from the lung during primary TB infection?
Tuberculous hilar lymphadenopathy
A patient with primary infection is characterized by tender nodules along the extensor surfaces of the tibia and ulna , what is this ?
Erythema nodosum
What is Scrofula?
mycobacterial cervical lymphadenitis that presents as swollen, non-tender lymph nodes, usually unilaterally.
Scrofula is less commonly caused by Mycobacterium scrofulaceum.
What is the most common extrapulmonary manifestation of tuberculosis?
Lymphadenitis (Hematogenous dissemination)
What is the characterization of Gastrointestinal tuberculosis?
characterized by abdominal pain and diarrhea accompanied by more generalized symptoms of fever and weight loss
What is the cause of gastrointestinal tuberculosis?
- caused by either M. tuberculosis when it is swallowed after being coughed up from a lung lesion
- or by M. bovis when it is ingested in unpasteurized milk products.
Which type of TB resemble millet seeds?
Miliary tuberculosis (multiple disseminated lesions)
Which TB is associated with skeletal system?
- Tuberculous meningitis
- Tuberculous osteomyelitis, especially vertebral osteomyelitis (Pott’s disease).
What other organs involve TB?
• Oropharyngeal tuberculosis typically presents as a painless ulcer accompanied by local adenopathy.
• Renal tuberculosis, dysuria, hematuria, and flank pain occur.
What is “Sterile pyuria”?
It is a characteristic finding.
The urine contains white blood cells, but cultures for the common urinary tract bacterial pathogens show no growth. However, mycobacterial cultures are often positive
Most infections with M. tuberculosis are__________
asymptomatic
What is the most important determinant of whether overt disease occurs or not in TB?
adequacy of the host’s cell mediated immune (CMI) response
Which people have a higher rate of reactivation in latent infections?
AIDS and Diabetics patients
Which staining is utilized for TB?
Acid-fast staining
What is the sample taken to be stained?
sputum or other specimens
What makes mycobacteria acid-fast?
It’s high lipid content in the cell wall
Which version or type of acid fast stain is used?
Kinyoun version of the acid-fast stain
or the older Ziehl-Neelsen
Which stain is used for rapid screening purposes ?
Auramine stain
What is the role of Auramine stain?
It Binds to mycolic acid present in the cell wall
What is the result of auramine stain?
A reddish-yellow rods fluorescence visualized by fluorescence microscopy
Which media is used for M.tuberculosis culturing?
- Löwenstein-Jensen agar
- Liquid BACTEC medium
How is M.tuberculosis differentiated from other mycobacteria in LJ agar?
by biochemical niacin and catalase production
How is M.tuberculosis identified in Liquid BACTEC medium?
the growth can be detected by the production of radioactive carbon dioxide in about 2 weeks
Mycobacteria metabolize the 14 C labeled substrate and release radioactive labeled 14 Co2
What does PCR detect for TB?
detect either the ribosomal RNA or the DNA of the organism
What is the benefit of PCR tests?
- highly specific, but their sensitivity varies
-more useful in deciding initiation therapy prior to obtaining the culture results
In sputum specimens that are acid-fast stain_______, the sensitivity is high, but in “smear-negative” sputum, the sensitivity is significantly______
Positive ; lower
Mention the Drug resistance tests
- Susceptibility tests
- molecular tests (PCR)
Describe the Susceptibility tests
It should be performed
However, the organism grows very slowly, and susceptibility tests usually take several weeks, which is too long to guide the initial choice of drugs.
To address this problem, molecular tests (PCR) are available
What does the molecular test detect?
Molecular test (PCR) , detect mutations in the chromosomal genes that encode either catalase gene that mediates resistance to isoniazid or RNA polymerase gene that mediates resistance to rifampin.
Which enzyme is isolated from fireflies that produces flashes of light in the presence of adenosine triphosphate (ATP)?
Luciferase
Explain what happens if the patient is resistant and sensitive to luciferase.
Resistant(not damaged by the drug) :
- the luciferase will produce the normal amount of light
Sensitive (to the drug) :
- less ATP will be made and less light produced
State the 2 approaches to the diagnosis of latent infections.
- PPD skin test (tuberculin skin test )
- Interferon-γ release assay (IGRA)
What are the 2 versions of IGRA?
Quantiferon-TB and T-spot.TB
What is the problem with PPD test?
There are problems both in the interpretation of the PPD test and with the person returning for the skin test to be read.
PPD is preferred to be done to who?
In children < 5 years of age
What does IGRA do?
blood cells from the patient are exposed to antigens from M. tuberculosis, and the amount of interferon-γ released from the sensitized T cells is measured
IGRA is not influenced by whether a person has been___________________ ________________
previously immunized with the BCG vaccine
What are the used antigens in Quantiferon-TB?
Single mixture of synthetic peptides representing ESAT-6, CFP-10 & TB7.7
What are the used antigens in T-spot TB?
Separate mixtures of synthetic peptides representing ESAT-6 & CFP-10
What is the principle of T-spot TB?
ELISA Immunospot that detects Interferon-γ in purified peripheral blood mononuclear cells
What is the principle of Quantiferon-TB?
ELISA that detects Interferon-γ in whole blood
What is the specific measurements of Quantiferon-TB and T-spot TB?
Quantiferon-TB:
- IFN-γ concentration
T-spot TB:
-Number of IFN-γ producing cells (spots)
What is the test results of IGRA?
- Positive test indicates active TB or latent TB
- Negative
- Indeterminate, and commonly seen in: Immunosuppressed states and children < 5 years of age
Mention the advantages of IGRAs
- Requires a single patient visit to conduct the test.
- Results can be available within 24 hours.
- Prior BCG vaccination does not cause a false-positive IGRA test result. The test detect interferon-γ released from the sensitized T cells in response to the antigens which are absent in BCG
