Download Lecture6Dec01Mycobacteria

The stages of an infection
Incubation period: no symptoms, but organism is growing and spreading
Prodrome period: non-specific symptoms, fever, weakness; disease is
communicable.
Specific Illness: rash, cough, etc.
Convalescence: Increasing levels of antibodies, specific immune response
Recovery: sometimes with a persistent carrier state.
Mycobacteriae
Mycobacterium tuberculosis
- agent of tuberculosis (TB; Hansen’s Disease)
Mycobacterium leprae
- agent of leprosy
Nontuberculosis mycobacteria (NTM):
- opportunistic, rare (in otherwise healthy individuals)
“Not so ‘typical’ bacteria”
Bacteria that have special shapes, sizes or staining properties
Acid-fast bacteria
Tuberculosis is an old disease.
DNA tests on mummy show TB killed ancient Egyptians (600 BC).
Tuberculosis has been a major disease for a
long time.
During the Industrial Revolution, nearly 100% of
Europeans were infected and 25% died.
White Death or White plague
Consumption
Vampirism
Tuberculosis remains a major disease.
 Leading cause of bacterially related death
 Kills more than HIV, malaria & other tropical diseases
 One-third of the global population is infected
 8 million develop active TB every year
 2 million die/year
HIV infection sharply increases risk for TB (25% of deaths
from TB are in patients also infected with HIV).
About 90% of those infected have asymptomatic, latent TB infection.
10% have a lifetime chance active TB disease.
If untreated, the death rate for these active TB cases is more than 50%.
Predisposing factors
Immunosuppressed
Immunocompromised (HIV/AIDS)
Reduced general health
close contact (homeless shelter, nursing homes,
dormitories, schools, prisons)
Mycobacterium tuberculosis
Acid-fast bacillus (non-motile slender rod)
Obligate aerobe; replicates in oxygenated tissues
Doubling time of 15-20h
Increased resistance to chemical agents (acidic or alkaline),
antibiotics, reactive oxygen species, osmotic lysis via
complement deposition and dehydration
Grows readily inside macrophages, hiding from the host's immune
system
Waxy coat – nearly water proof
The Mycobacterial cell wall
Proteins and fatty acids
Lipoarabinomannan
Mycolic acids
Arabino galactan
om
PG
PG
PG
im
im
im
Gram +
Gram -
Acid fast
Ziehl-Neelsen stain
•Stain with fuchsin (red dye) in
detergent
•Wash with acid (3% HCl) to remove
dye from all but “acid-fast” bacteria.
•Counterstain with dimethylene blue
A. Non acid-fast bacteria
B. Acid-fast bacteria
Everything else (other bacteria and white blood cells)
– stained with counter stain only
Mycobacterium tuberculosis
Obligate aerobe; replicates in oxygenated tissues
Mycolic acid modifies murein cell wall (waxy fatty
acid, low permeability, interferes with Gram-stain;
resistant to drying, chemicals, disinfectants,
phagocyte killing)
Transmission
Transmission can only occur from people with active — not latent — TB
Transmission
Air-borne droplets are inhaled.
Bacteria are ingested by alveolar macrophages.
However, the macrophages are not activated and are unable
to destroy the intracellular organisms.
Respiratory System
Virulence mechanism
The probability of transmission depends on:
number of infectious droplets expelled by a carrier
effectiveness of ventilation
duration of exposure
Phagosomes containing Mtb evade fusion
with lysosomes
Alveolar macrophage
Intracellular pathogens: protected from serum,
antibody, complement and antibiotics
Virulence factors
No classic bacterial virulence factors such as toxins, capsules and
fimbriae, but…
Mycobacteria can grow intracellularly.
Slow generation time.
High lipid concentration in cell wall.
The course of TB infection
1- 3 weeks post infection. Bacteria multiply within unactivated macrophages
until the macrophages burst. Other macrophages move in and also phagocytose
Mycobacteria. Repeated cycles of phagocytosis and cell lysis.
3- 8 weeks: Lymphocytes begin to infiltrate. T-cell activation. Liberation of
lymphokines. Activation of macrophages. Increased ability to kill Mycobacteria and
Mycobacterium-infected cells.
Containment of infection: The immune response results in the formation of
granulomas – walling off the bacteria. The center of the granuloma - "caseous
necrosis", semi-solid or "cheesy" consistency.
Latent infection: Mycobacteria cannot multiply
within these granulomas (low pH and anoxic
environment).
Mycobacteria can persist within these granulomas
for extended periods.
Latency; most people remain in this state (positive
PPD test).
Granuloma formation
Granuloma: alveolar macrophages release TNFa and other proinflammatory cytokines, which recruit more lymphocytes.
The granuloma contains the infection but does not clear it.
Granuloma formation is a hallmark of
tuberculosis
Chest X-ray (radiograph)
Granuloma rupture releases infectious
bacteria
Months, years later - reactivation.
The caseous centers of the granulomas liquefy. The liquid is
conducive to Mycobacteria growth, and the organism begins to
rapidly multiply extracellularly. Rupture of the surrounding bronchus
- “cavitation”.
Rapid spread of Mycobacteria to other parts of the lung.
Granuloma rupture releases infectious
bacteria
Granuloma rupture releases infectious
bacteria
Weakened immunity leads to granuloma rupture and release of infectious
bacteria.
Disseminated (miliary) TB: spread throughout body,
small tubercles form (infants and debilitated)
Active TB is highly infectious and fatal if untreated
(slow progression of tubercles and erosion of air
passages and blood vessels).
A Person with Latent TB Infection
A Person with TB Disease
• No symptoms
• Symptoms that may include:
- a bad cough that lasts 3 weeks or longer
- pain in the chest
- coughing up blood or sputum
- weakness or fatigue
- weight loss
- no appetite
- chills
- fever
- sweating at night
• Does not feel sick
• Usually feels sick
• Cannot spread TB bacteria to others
• May spread TB bacteria to others
• Usually has a test result indicating TB
infection
• Usually has a test result indicating TB
infection
• Needs treatment for latent TB infection to
prevent active TB disease
• Needs treatment to treat active TB
disease
The slow growth complicates TB diagnosis,
therapy and research.
4-8 weeks for clinical isolation
Months required for antibiotic susceptibility tests
Antibiotics must be taken for months.
Research is slowed by time required for bacterial growth.
Diagnosis
Tuberculin skin test
Stain sputum samples with Ziehl-Neelsen stain
Chest X-ray or scans
Tuberculin skin test
Injection of tuberculosis antigens intradermally (PPD = purified protein
derivative)
A hypersensitivity response is seen 2 days later in people infected with M.
tuberculosis (= delayed-type hypersensitivity response, a.k.a. DTH response).
A positive response is a firm, red, raised bump (filled with macrophages and
neutrophils).
Test does not distinguish between latent and active infection.
Therapy
4 different antimicrobial agents
6-9 months
Complications of TB therapy
Problems emerge with the appearance of extreme
multiple drug-resistant (MDR) bacteria.
MDR Mtb are isolates with accumulated mutations in
multiple target genes.
MDR acquired by step-wise accumulation of
mutations in response to selective pressure of
antibiotic treatment. (NB: antibiotics are not
mutagens!)
Vaccines
No successful vaccine has been developed.
Mycobacterium bovis causes tuberculosis in cattle.
Bacillus Calmette-Guerin (BCG) is an attenuated strain of
M.bovis which is used as a live vaccine.
BCG provides incomplete protection, but reduces severity of
disease.
BCG vaccination compromises the tuberculin skin test (PPD).
Vaccines
TB infection is controlled by cell-mediated immunity
(not antibodies)
Treatment and vaccine development for intracellular
pathogens is difficult.
Vaccine antigens must be delivered to MHC I
pathway.
Mycobacterium leprae
Obligate intracellular pathogen.
Grows best at lower temperatures.
Transmission inefficient, person-to-person via secretions, aerosols, close
contact.
Infection of skin and nerves (chronic granulomatous disease of peripheral
nerves and nasal mucosa).
Mycobacterium leprae
Disfiguring skin sores and nerve damage.
Progressive debilitation: loss of sensation in the hands and feet, facial
deformities and loss of sight.
People affected by the disease and untreated can become severely
disabled and unable to care for themselves.
Historically more feared than tuberculosis. More social stigmatization.
Mycobacterium leprae
M. leprae is difficult to study:
-Slow growing
-Cannot be cultivated
Therapy
Complicated treatment
3 antibiotics for 1-2 years
Once a person is treated, within a few weeks he is no longer contagious but
physical deformities persist.
12 million cured
How do you know if a patient is sick with an
infectious disease?
Fever (but also present in autoimmune diseases and cancer)
Inflammation (red, hot, tender, swollen)
Inflammo = “I set alight; I ignite”
Dolor = “pain”
+
Calor = “heat”
+
Rubor = “redness” +
Tumor = “swelling”
Functio laesa = “loss of function”
Exudate (pus, discharge)
Leukocytosis (increased white blood cell count in the blood)
Disfigurement associated with leprosy