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Bacterial Atypical Pneumonias: Legionella, Mycoplasma, Chlam
ID Week 1
29
Medical
Graduate
04/13/2010

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Cards

Term
What are unique structural characteristics of Mycoplasma?
Definition

1. Tiny (smallest self-replicating organism, smallest known genome)

2. No cell well. Thus:

-- Pleiomorphic: morphs between round and oblong

-- Insensitive to B-lactams

3. Cell membrane packed with sterols, e.g. cholesterol. (Something needs to give it structure, since there is no cell wall.)

 

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Term
What illness is caused by Mycoplasma? What are the symptoms?
Definition

Mycoplasma causes "walking pneumonia," a mild atypical pneumonia with associated mild bronchitis. 

Note: Walking refers to mild symptoms - patients walk in and walk out.


Symptoms [after long (2-3 week) incubation period]: fever, sore throat, malaise, persistent dry cough

Term
What are some unique lab characteristics of Mycoplasma?
Definition

1. Does not Gram stain (no cell wall).

2. Slow-growing in (sputum) culture; 2-3 weeks. Colonies have characteristic "fried-egg" appearance.

Note: If nothing grows in culture, can't rule out mycoplasma.

3. Require special sterol-containing medium to grow in culture (due to membrane content).

Term
What are virulence factors of Mycoplasma?
Definition

Proteins P1, P30: Adhesins on mycoplasma's specialized apical structure that binds strongly to respiratory epithelium.


Note: Adhesion is a regulated process; at a given time, only 5-25% of mycoplasma are bound. Remaining bacteria are free to be released/transmitted upon coughing. 

Term
How does Mycoplasma evade the immune system?
Definition

Variation of surface antigens yields incomplete immunity.

1. VLPs (variable lipoproteins): Phenotypically unstable surface proteins encoded by multiple, clustered, divergent vlp genes.

Note: Instability due to intragenic insertions/deletions, intergenic recombination due to sequence repeats, phase variation due to insertions/deletions in promoters

2. VSPs (variable surface lipoproteins) are also unstable (due to tandem repeats).

Term
How is Mycoplasma transmitted?
Definition

Respiratory droplets. At any given time, only 5-25% of mycoplasma is bound to respiratory epithelium; remainder is free to spread. 

Note: Contagious to people in close quarters (military barracks, college dorms, families)

Term
How is it possible to get repeat infections of M. pneumoniae?
Definition

Surface protein variability (VLPs and VSPs) means adaptive immunity is incomplete.

Term
What is the pathophysiology of M. pneumoniae?
Definition

Ciliary activity is impaired -> necrosis of respiratory eptihelium.

Note: Mechanism unclear, maybe H2O2


Mild inflammatory response.

Term
How do we treat M. pneumoniae?
Definition

If the infection is limited to upper respiratory tract, do not treat. 

If pneumonia develops, treat with macrolides, tetracyclines, quinolones (not with B-lactams - no cell wall!)

Term
What types of Chlamydia cause atypical pneumonias?
Definition

C. pneumoniae

C. psittaci

Term
Is Chlamydia Gram+ or Gram-?
Definition

Slight trick question: Technically Gram- (two membranes, stains pink), but no peptidoglycan layer.


More importantly, Chlamydia is an obligate intracellular pathogen (energy parasite that steals host ATP).

Term
What is the natural reservoir of C. psittaci?
Definition

Birds.

Term
How do you grow Chlamydia in culture?
Definition

Must be grown in human host cells; no medium to culture Chlamydia has been developed.

Term
Describe the Chlamydia life cycle.
Definition

Cycles between two forms: EB (elementary body) and RB (replicative body).

 

1. Between hosts, Chlamydia exists as EB. EB is a metabolically inert, small body that is stable for extracellular existance.

Note: EB form is not metabolically active, thus not sensitive to inhibitors.


2. Once endocytosed, EB differentiates to RB form, which replicates by binary fission (using host ATP). This all occurs within endosome.

Note: RB form somehow stops formation of phagolysozome.


3. Secondary differentiation back to EB form, which is then exocytosed or released by cell lysis.

Term
How do we treat Chlamydia? How many membranes must the drug cross?
Definition

Tetracyline or erythromycin. (No peptidoglycan wall, so B-lactams won't work).

 

Drug must cross 4 membranes

1. Host PM

2. Vacuolar PM

3. Bacterial outermembrane

4. Bacterial inner membrane

Term

Chlamydia is considered Gram-. Why? 

How does it differ from typical Gram- bacteria

Definition

Like Gram-:

1. Stains pink

2. Has inner and outer membrane

 

Unlike Gram-:

1. Obligate intracellular organism

2. No peptidoglycan layer

Term
What stain identified Legionella in lung biopsies?
Definition

Carbol fuchsin (Gimenez)

Term
Legionella has special requirements for culture. What are they?
Definition

Buffered charcoal yeast extract (BCYE) media

Extra cysteine

Extra iron

 

Note: Special stain requirement: silver stain or carbol fuchsin (Gimenez). Will not stain with Gram stain!


Note: Slow-growing (3-5 days), so not visible in standard sputum culture.

Term
What are some structural characteristics of Legionella?
Definition

Gram- rod

Note: Does not stain with Gram stain. Need silver or carbol fuchsin

Term
Describe the life cycle of Legionella.
Definition

Legionella cycles between replicative and transmissive forms, depending on nutrient supply. 

--Replicative (log) phase: when environment is nutrient-rich (suppresses formation of flagella using csrA gene)

-- Transmissive (stationary) phase: forms flagella for motility (expresses flaA gene), initiates autolysis

 

Note: Legionella can also survive in biofilms, or in a spore-like mature intracellular (MIC) form that is resistant to antibiotics.

Term
Legionella employs a replicative/transmissive life cycle. Which other bacteria that cause atypical pneumonia do this?
Definition
Chlamydiae (EB and RB forms)
Term
How is Legionella transmitted?
Definition

Inhalation of aerosols of contaminated water. (Human lung is a dead end for Legionella.)

Note: Hotel A/C systems particularly risk, risk for nosocomial infection

Term
Legionella grows in amoebae. How does it infect humans?
Definition

Amoebae and macrophages are similar enough that Legionella can be transferred from inhaled amoebae to infect human alveolar macrophages.

Term
Most people exposed to Legionella will not get sick. What are risk factors the predispose people to infection by Legionella?
Definition

Elderly or immunocompromised patients

Smokers/drinkers - damaged airways

Term

 

What is the difference between standard sec-pathway secretion of proteins and type IV secretion pathways?

 

Definition

Sec-pathway secretion is simple, standard secretion of proteins into ER (using SRPs, peptidases)

Type IV secretion pathway, which is homologous to conjugation for DNA transfer (uses pillus).
Note: Legionella employs type IV secretion to transfer proteins from vacuole to host cell

Term
What secretion pathway does Legionella employ to transfer its proteins to the host macrophage/amoeba?
Definition

Type IV secretion (homologous to plasmid conjugation).

Term
How does Legionella evade phagolysozomes?
Definition

Uses type IV secretion to release proteins -> association with rER. Bacteria can replicate within protective rER vacuole.

Term
What are environmental cues that trigger differentiation of Legionella to one form (replicative/transmissive) or the other?
Definition

Nutrient depletionppGpp is an alarmone involved in the stringent response, a reaction to amino acid starvation. ppGpp modulates RNA polymerase activity, upregulating transcription of transmissive form proteins.

Note: Threonine transporter PhtA is also necessary for Legionella replication. PhtA expression will trigger switch to replicative form.

Note: RelA and SpoT sense amino and fatty acid depletion, respectively, and together trigger synthesis of ppGpp.

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