Streptococcal infections

What are the key characteristics of GAS?

• gram positive cocci in chains

• catalase negative

• beta-hemolytic

• Group A (lancefield)

• Further classified by M protein

What is the M protein?

A filamentous structure extending from the cell surface

• may contain Hyaluronic acid capsule

What is used for genotyping Strep?

PCR amplification of the emm gene which codes for the m protein

What does serotyping of strep involve?

• Precipitation of M-protein via specific antisera

• Over 80+ different types identified

What are the major virulence factors for Strep Pyogenes?

• M protein

• Lipoteichoic acid + protein F + protein M (attachment to cell)

• Streptolysin O and L

• Hyaluronidase

• DNAses

• Streptokinase

• Erythrogenic toxin

What is a potential consequence of exposure to GAS?

High levels of antibody can cause sequelae

What kinds of diseases can GAS cause?

• strep throat (pharyngitis)

• scarlet fever

• strep impetigo

• necrotizing fasciitis

• Toxic shock

What toxin is usually involved in scarlet fever?

Erythrogenic toxin

• appearance of skin rash on trunk and face

• strawberry tongue

• can cause desquamation of skin

In streptococcal toxin shock what is the pathology?

• erythematous skin rash

• toxins cause hypotension, pain at infection site, wide spread organ failure and respiratory distress

• Lesion from which organism can be isolated

What are the two GAS related sequelae?

• rheumatic fever

• acute streptococcal glomerulonephritis

What is the primary infection that results in the sequelae rheumatic fever?

Strep pharyngitis, anti-streptolysin O reacts with normal heart valve tissue

What are the primary infections that result in the sequelae acute glomerulonephritis?

• pharyngitis

• scarlet fever

• impetigo

In acute glomerulonephritis what antibodies are attacking the basement membrane of the glomerulus?

• Anti-dnase B

What is collected for GAS testing and what are they used for?

Two swabs, one for antigen testing via laminar flow device and the other for culturing

What is a todd unit?

The reciprocal of the highest dilution of patient serum that inhibits RBC lysis

What type of assay is the ASO reference method for GAS detection?

neutralization reaction

What toxin is used for the ASO reference method?

Anti-streptolysin O

If the patient has antibodies to SLO what will happen to the blood cells in the ASO method?

They will not be lysed when added

What must be run with every patient assay with the ASO method?

• SO control: SO + RBCs, should show complete lysis

• Cell control: buffer + RBCs, should show no lysis

What does weak SO cause in ASO?

False positives as there will be less lysis in the test system which appears as an higher overall titer

What GAS infection is part of the CDC surveillance program?

Necrotizing fasciitis

What is the antibody produced to ASO used for in the laboratory?

Helps confirm recent or past GAS infection, elevated levels indicate prior exposure

What is the antibody produced to Anti-DNAse B used for in the laboratory?

Used to diagnose complications like Rheumatic fever or Glomerulonephritis, positive titers suggest recent infection

What is the antibody produced to Anti-hyaluronidase used for in the laboratory?

Assists in confirming an active infection and evaluating post-infectious sequelae