RS08 - Strep Throat

What is the causative agent of strep throat?

Streptococcus pyogenes (Group A Streptococcus, GAS).

What is the Gram stain and morphology of Streptococcus pyogenes?

Gram-positive cocci that grow in chains.

Is Streptococcus pyogenes catalase positive or negative?

Catalase negative.

What genus includes catalase-negative cocci that grow in chains?

Genus Streptococcus.

What catalase reaction differentiates streptococci from staphylococci?

Streptococci are catalase negative; staphylococci are catalase positive.

What is the typical growth pattern of streptococci?

Chains.

What is the typical growth pattern of staphylococci?

Clusters

Which staphylococcal species is coagulase positive?

Staphylococcus aureus.

What is alpha-hemolysis?

Incomplete hemolysis with “greening” of agar.

Which streptococci are alpha-hemolytic?

Viridans streptococci (normal oral microbiota).

What is beta-hemolysis?

Complete hemolysis with blood agar appearing yellow/clear.

Which organism shows beta-hemolysis?

Streptococcus pyogenes (Group A Streptococcus).

What is gamma-hemolysis?

No hemolysis; greenish-white appearance.

Which organism is gamma-hemolytic?

Enterococcus faecalis (formerly Group D strep).

Which Lancefield group does Streptococcus pyogenes belong to?

Group A.

What organism is in Lancefield Group B?

Streptococcus agalactiae.

Which organisms are not part of the Lancefield grouping system?

Streptococcus pneumoniae and viridans streptococci.

What is the clinical name for strep throat?

Streptococcal pharyngitis.

What are common diseases caused by S. pyogenes?

Streptococcal pharyngitis, scarlet fever, impetigo, erysipelas, cellulitis, necrotizing fasciitis.

What are post-streptococcal sequelae?

Conditions resulting from prior GAS infection, including acute rheumatic fever (ARF) and acute glomerulonephritis (AGN).

What is the composition of the S. pyogenes capsule?

Hyaluronic acid.

What is the function of the S. pyogenes capsule?

Nonimmunogenic and antiphagocytic.

What is M protein?

An antiphagocytic surface protein that prevents interaction with complement.

How many M protein types exist?

More than 80 types.

What does protein F bind to?

Fibronectin.

What is the role of protein F?

Major adhesin for bacterial attachment and entry into epithelial cells.

What are streptococcal pyrogenic exotoxins (SPEs)?

Pyrogenic exotoxins that act as superantigens.

Which streptococcal virulence factor is a superantigen?

SPEs (SpeA, SpeB, erythrogenic toxin).

What is a superantigen?

A toxin that causes massive immune stimulation (>1000-fold) by stimulating 1 in 5 T cells.

What disease manifestation are SPEs responsible for?

Scarlatiniform rash and invasive disease.

What age group is most affected by strep throat?

Children 5–15 years old.

How is strep throat transmitted?

Direct contact via respiratory droplets or oral/nasal secretions.

What is the incubation period of streptococcal pharyngitis?

5 days.

What percentage of school-aged children may be asymptomatic carriers?

15–20%.

What are hallmark symptoms of strep throat?

Sudden onset sore throat, fever ≥101°F, painful swallowing, headache.

Which symptoms are typically absent in strep throat?

Cough, rhinorrhea, hoarseness, oral ulcers, conjunctivitis.

What oral findings are associated with strep throat?

Pharyngeal and tonsillar erythema, enlarged tonsils with exudate, palatal petechiae, anterior cervical lymphadenopathy.

What is the preferred diagnostic method for strep throat?

Throat culture.

How does S. pyogenes respond to bacitracin?

Bacitracin sensitive.

What rapid test can detect GAS?

Rapid antigen-detection test (RADT).

What should follow a negative RADT?

Confirmation with throat culture.

What is the treatment of choice for streptococcal pharyngitis?

Penicillin V or amoxicillin.

What are the benefits of early antibiotic treatment?

Shortens fever, toxicity, infectivity, and prevents complications.

How long is the recommended antibiotic course?

10 days.

When can patients return to school or work?

Afebrile and 24 hours after starting antibiotics.

What causes scarlet fever?

GAS strains that express erythrogenic toxin (SPE).

Is scarlet fever a sequela or a symptom of disease?

Symptom of disease.

When does the rash of scarlet fever appear?

On the second day after initial symptoms of pharyngitis.

What are key oral findings in scarlet fever?

White strawberry tongue progressing to red strawberry tongue; palatal hemorrhagic spots.

What is acute rheumatic fever (ARF)?

A nonsuppurative inflammatory disease following untreated GAS pharyngitis.

When does ARF typically occur after infection?

2–3 weeks after infection.

What is the most serious complication of ARF?

Carditis with heart valve damage.

What mechanism causes ARF?

Molecular mimicry involving circulating antigens and auto-antigens.

What is acute glomerulonephritis (AGN)?

An acute inflammatory disorder of the renal glomerulus.

What types of infections can lead to bacterial endocarditis?

Repeated GAS infections or bacteremia following surgery (viridans streptococci or enterococci).