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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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

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).

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. 

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).

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)

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

Ciliary activity is impaired -> necrosis of respiratory eptihelium.

Note: Mechanism unclear, maybe H₂O₂

Mild inflammatory response.

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!)

C. pneumoniae

C. psittaci

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).

Birds.

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

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.

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

Chlamydia is considered Gram-. Why? 

How does it differ from typical Gram- bacteria

Like Gram-:

1. Stains pink

2. Has inner and outer membrane

Unlike Gram-:

1. Obligate intracellular organism

2. No peptidoglycan layer

Carbol fuchsin (Gimenez)

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.

Gram- rod

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

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.

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

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

Elderly or immunocompromised patients

Smokers/drinkers - damaged airways

Type IV secretion (homologous to plasmid conjugation).

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

Nutrient depletion: ppGpp 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.