B-hemolytic, golden colonies ("Au"reus)

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"Grape-like" clusters of Gram+ cocci

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S. aureus: Nares, skin, mucous membranes.

Note: Colonization can be transient or persistent. About 30% of healthcare workers are colonized by S. aureus.

S. epidermidis: Skin.

Note: More common but less virulent than S. aureus

S. Saprophyticus: Urinary tract.

Catalase test distinguishes Streptococci from Staphylococci. All staphylococci are catalase+ (bubbles in H2O2), all streptococci are catalase-.

Coagulase test differentiates Staphylococcus species. S. aureus is coagulase+, S. epidermidis and S. saprophyticus are coagulase- (+ test: serum will clot)

Note: Clinically, we refer to S. aureus versus coagulase-negative staphylococci; nobody cares which one.

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How are Staphylococcus and GAS (S. pyogenes) distinguished with:

1. Gram stain?

2. Culture colonies?

3. Biochemistry?

1. Both are Gram+, but Staphylococci grow in clusters while GAS grow in chains.

2. Both are beta-hemolytic, but Staphylococci grow large, yellow colonies whereas GAS grow small, white colonies.

3. Staphylococci are catalase+, all streptococci are catalase-.

1. Antiphagocytic factors

-- Protein A inhibits opsonization by IgG. Binds to Fc (constant region) of immunoglobulins, preventing Fc receptor from binding,  

-- Clumping factor A (ClfA) and Efb inhibit opsonization by C3b by blocking its generation.

Note: ClfA also binds fibrinogen, which acts as a physical barrier to C3b opsonization, analogous to M protein in GAS.

2. Abscess formation: Coagulase cleaves host fibrinogen --> fibrin --> clot, which acts as a protective wall around the organism but also limits its access/spread.

Note: Peptidoglycan and teichoic acid activate complement pathway/ inflammation leading to purulence, though unlike pneumococcus, Staphylococcus doesn't lyse so there is much less inflammation.

3. Catalase defends against PMN reactive oxygen species.

Toxins

1. Pyrogenic toxins

-- Staphylococcus Enterotoxins (SE) A, B, C, G, L, T: Food poisoning

Note: SE-A is phage-encoded

-- Superantigens: Toxic Shock Syndrome Toxin (TSST) stimulates non-specific binding to T-cell receptors/MHC complexes, leading to excessive T-cell/cytokine response.

Note: TSS also associated with GAS

2. Pore-forming toxins (blood cell lysis): hemolysins (lyse RBCs, e.g. alpha-toxin); leukocidin (lyses WBCs).

3. Exfoliatin: Toxin targets an epidermal cadherin (desmoglein), causing skin blistering (Staphylococcal scalded skin syndrome, SSSS).

4. Secreted enzymes (common to all Staphylococci): Hyaluronidase, lipase, DNAse. Particularly important to break down protective fibrin clot and promote spread.

1. Adhesion to host cell ECM via fibronectin binding protein (fnbA)

2. Inflammation via peptidoglycan and teichoic acid, which activate complement pathway (though not to same extent as in S. pneumoniae, where cell wall components are released due to bacterial lysis)

3. Abscess formation: Coagulase seeds abscess by forming clot around S. aureus infection.

S. aureus has a two-stage infective cycle. Differential expression of virulence factors will either promote colonization (replication) or spreading (transmission). 

Note: Similar to Legionella, Chlamydia

Adhesion and immune evasion factors:

-- Fibronectin binding protein

-- Coagulase

-- Antiphagocytic factors: protein A, C3b inhibitors (clumping factor A)

Toxins and secreted enzymes: Hylauronidase, hemolysins, leukocidin, staphylococcal enterotoxin, etc. 

Threshold amounts of quorum-sensing molecule (e.g. AIP) triggers a two-component regulatory system.

1. AIP binds to sensing progtein AgrC, which autophosphorylates.

2. Phosphate transfered to response regulator (activator protein) AgrA, which modulates gene expression.

Note: Quorum sensing coordinates behavior of a bacterial population, enhancing virulence by only "attacking once the army is big enough."

Horizontal gene transfer from other bacteria (or phages). Staphylococcal resistance mechanisms are not derived from DNA mutations, but rather whole new virulence factors.

-- B-lactamase gene encoded on a transposon.

-- Methicillin resistance coded pathogeniticy island.

-- [Insertion sequence in Enterococcus (not S. aureus) conferred genomic plasticity.]

Note: Bacteriophages also transfer genes to S. aureus, though they generally encode toxins (e.g.  SE-A).

1. Transposons: "Jumping genes" recognizable by their flanking inverted repeats. They encode their own excision/insertion gene (transposase), can jump between chromosomal DNA and plasmid, and often confer antibiotic resistance if conjugated to another bacterium. E.g. B-lactamase gene in S. aureus.

2. Insertion Sequences: Transposons with no recognizable phenotype (still have flanking inverted sequence and transposase). Can inactivate genes at insertion site, cause rearrangements, and act as insertion site for similar elements in the future.

E.g. Hospital-derived E. faecium acquired IS16, which conferred genomic plasticity leading to future acquisition of antibiotic resistance genes.

2. Pathogenicity Islands: large blocks of DNA containing multiple, functionally-related genes. They are recognizable by their high GC content relative to the rest of the bacterial genome.

E.g. Staphylococcal cassette chromosome mecA (SCCmecA) 1-4 are pathogenicity islands, each of which include mecA, a novel PBP that confers methicillin resistance to MRSA.

1. Penicillin: B-lactamase makes most Staphylococcus resistant.

2. Methicillin: mecA gene codes for novel penicillin binding protein (PBP2a) which confers resistance to all B-lactams.