Lecture 30 - Legionella Flashcards
Where does the causative agent of Legionaire’s disease come from?
Environment
• soil
• water
No human or animal reservoir
Describe Legionnaire’s disease
Acute form of pneumonia
- requires serious medical intervention
What is Pontiac fever?
Milder form of Legionnaire’s disease
• flu like
• self limiting
What determines whether the infection causes Legionnaire’s or Pontiac fever?
Unclear
• patient factors?
• strain?
How many species of Legionella are there?
more than 50
Which is the most common species of Legionella?
Legionella pneumophila
Which serogroup is the most common cause of Legionnaire’s disease?
Serogroup 1
90 % of cases
Why is it an emerging pathogen
Only been identified from the late 1970’s
Only recently has it caused outbreaks, due to man-made water systems
Describe the Gram classification of Legionella
Gram negative
Describe the trends of Legionnaire’s disease in AUstralia
Not very common
• only a few cases reported
• since most people clear the infection
2000
• very large outbreak at Melbourne Aquarium
Describe the result of most cases of infection with Legionella
Most people will feel ill, but then clear the infection
Tip of the iceberg:
• immunocompromised will get full blown disease
Describe what happened at the Melbourne Aquarium
2000
Just before the Aquarium opened
Legionella in the air conditioning
SPread to many people working around the site
Since then, tight regulations on cooling towers etc.
What are the different places where Legionella can be picked up?
Community acquired
• eg Melbourne Aquarium
Nosocomial
How is Legionella transmitted?
Aerosols Man made water sources • cooling towers • spas • fountains • car wash • showers
Cannot be transferred from person to person
What are the risk factors for Legionnaire’s disease?
• Immune deficiency • Drinking • Male gender • Old age • Co-morbidity: - smoking - poor lung function
Why is Legionella called an accidental pathogen?
Infection in a human is a dead end for the pathogen
It can not be spread further from there
Why does Legionella grow in man made water sources?
Higher than normal temperature
Favourable conditions for the bacteria
What are some important Legionella outbreaks?
Philadelphia 1976
• American Legion convention
Netherlands, 1999
• Whirlpool spa
How are water sources tested?
Water sample taken
Growth on medium
Viable count test
How did they identify Legionella initially?
Philadelphia outbreak
Initially, didn’t know what is was
Took a biopsy
Growth in pig cells
How is Legionella pneumophila cultured?
Charcoal agar (iron and cysteine) • because it is very fastidious
Dessicated white colonies
How is Legionella treated?
Antibiotics - Erythromycin
Macrolides
Auinolones
How is Legionella detected in the lab?
Describe the features of this test
Urine antigen test • very high specificity • rapid • non-invasive • only serogroup 1
What is seen in the urine in people with Legionella infection?
LPS in the urine
What does high specificity of a test mean?
Very few false positives
Describe the pathogenesis of Legionella
- Taken into lung
- Taken up by alveolar macrophages
- Avoids phagolysosome fusion
- Recruits host cell vesicles
- Recruits rough ER into the vaculole
- Grows and replicates in alveolar macrophages
- Macrophage dies
- Released to infect more macrophages
Describe the appearance of the alveolar macrophage after infection with Legionella
Very large vacuole forms
Full of Legionella bacteria
How does Legionella survive in the environment?
Not free living
Must infect amoebae
• survives and resplicates in amoebae
• kills amoebae and spreads further
What are amoebae?
Single celled phagocytes
Take up bacteria as a food source
Training ground for pathogens
Describe the role of amoebae in transmission of Legionella
- Amoebae in cooling towers
* Amoebae protect the bacteria from disinfectants and biocides
What is the difference between Legionella and other intracellular bacteria pathogens?
Legionella not at all evolved to survive in human cells
Describe normal phagocytosis
- Phagosome formed around bacteria
- Fusion of lysosomes
- Early endosome → lysosome
- Degradation of bacteria
• hydrolytic enzymes
• ROS
• RNS
How does Legionella avoid killing once phagocytosed?
- prevents lysosomal fusion
- recruitment of ER exit vesicles
- vesicles coat the outside of the vacuole
- vacuole look like ER → obtains ribosomes, allowing replication
What are LCVs?
Legionella containing vacuoles
- don’t acidify
- don’t fuse with lysosomes
- come to resemble rER
What makes the LCV resemble ER?
Retains ER markers on the membrane
- Type IV secretion system in membranes of bacterium
- Interacts with host cell membrane
- Transfers proteins into the host cell
What is the name of the T IV SS of Legionella?
Dot / Icm secretion system
Defective organelle trafficking
Intracellular multiplication
Where is Dot/Icm SS found?
Poles of bacterium
How many different proteins are injected by the type IV SS?
300 ‘effector’ proteins
What do the effector proteins do?
- similar to eukaryotic proteins
- mimic eukaryotic function
→ take over eukaryotic cell processes such as: • mitochondrial activity • gene transcription and translation • apoptosis • GTPase activity • vesicle trafficking
Do many bacteria have these ‘eukaryotic proteins’
No, almost exclusively Legionella have them
Due to close evolution of Legionella with amoebae
Describe how Legionella targets host GTPases
Legionella has taken up a Sec7 domain from a eukaryotic cell
Sec7 domain in the RalF
RalF is a Guanine exchange factor
- RalF recruits Arf1 (a GTPase) to the LCV
- RalF activates Arf1
- Arf1 controls vesicle fusion
What is the function of GTPases?
Regulate many processes in cells
• vesicle trafficking, for example
- GEF swaps GDP → GTP
- GTPase now active
What is Arf 1?
A GTPase in the host cell
• controls vesicle fusion
What is a GEF?
Guanine exchange factor
Activates GTPase
What is Sec7?
A domain in RalF (a GEF in Legionella)
Describe the immune response to Legionella
- inflammatory cytokine response
- neutrophils
- NK
- T and B cells response
Which cytokines are required for an effective immune response?
TNF, IL-12, IFN-gamma
What is required for clearance of Legionella?
T and B cell responses
