Aerobic Gram-Positive Cocci: Streptococci (Group A,B and viridans)

Question 1

1. Know the reservoirs of Streptococcus pyogenes, Streptococcus agalactiae, and viridans strep.

Answer 1

GAS inhabits skin and the upper respiratory tract as well as variably on nasal and pharyngeal mucosa

GBS colonizes in the genital tract and lower gastrointestinal tract

Question 2

1. Know the methods of transmission of Streptococcus pyogenes, Streptococcus agalactiae, and viridans strep.

Answer 2

GAS person to person or by direct contact with secretions (including respiratory droplets of coughing and sneezing)

GBS: person to person or mother-child

Question 3

2. Describe the major disease caused by Streptococcus pyogenes, Streptococcus agalactiae, and viridans strep.

Answer 3

GAS
pharyngitis/Strep throat (clinical features alone cannot diagnose strep
Scarlet fever- strain lysogenized to produce exotoxin
impetigo (infected wound/bite)
erysipelas (most commonly seen on the legs)
cellulitis
necrotizing fasciitis
streptococcal toxic shock syndrome (TSS)
acute rheumatic fever
post streptococcal glomerulonephritis PSGN

GBS: neonatal bacteremia, pneumonia or meningitis, postpartum infections of mother (puerperal sepsis) and adult infection of skin and soft tissue

Question 4

2. Describe the virulence factors of Streptococcus pyogenes, Streptococcus agalactiae, and viridans strep.

Answer 4

lipoteichoic acid and protein F mediate attachment to epithelial cells by binding fibronectin

M protein (S. pyogenes) promote adherence and have antiphagocytic activity

capsule composed of hyaluronic acid (antiphagocytic)

streptococcal pyrogenic exotoxins (SPEs)- super antigens, mediate rash of scarlet fever

hyemolysins- Streptolysin O and S which lyse leukocytes, platelets, and erythrocytes (O is immunogenic)

additional enzymes: streptokinase, DNase, hyluronidase

GBS: capsular material interferes with phagocytosis and complement activation

Question 5

3. Describe which individuals are at increased risk for developing infections caused by Streptococcus pyogenes, Streptococcus agalactiae, and viridans strep and why.

Answer 5

GAS
school-aged children  (pharyngitis, ARF)
young children (impetigo)
surgical patients, burn patients
post partium women (puerperal sepsis)
GBS
neonates, pregnant and post partum women
adults with immunocompromised (DM, cancer, alcoholism)

Question 6

4. Explain the suppurative complications of streptococcal pharyngitis (caused by direct extension of Group A strep).

Answer 6

pharyngitis/Strep throat (clinical features alone cannot diagnose strep
Scarlet fever- strain lysogenized to produce exotoxin
impetigo (infected wound/bite)
erysipelas (most commonly seen on the legs)
cellulitis
necrotizing fasciitis
streptococcal toxic shock syndrome (TSS)

Question 7

5. Describe the two major nonsuppurative complications (sequelae) of streptococcal infections, including their mechanism of pathogenesis.

Answer 7

acute rheumatic fever: inflammatory reaction of heart valves
post streptococcal glomerulonephritis PSGN: acute inflammation of the renal glomeruli, leading to kidney failure mediated by antigen-antibody complexes

Question 8

5. What is the most successful approach to prevention of nonsuppurative strep complications.

Answer 8

GAS: prompt treatment of strep pharyngitis
GBS: chmoprophylaxis in high risk births, screen expectant mothers

Question 9

6. Explain Gram stain rxns and laboratory tests to id Group A, Group B and viridans strep.

Answer 9

GAS
microscopy: gram positive cocci in chains
culture: facultative anaerobe, producing B-hemolysis
antigen detection, using antibodies to detect the group specific carbohydrate (basis of rapid strep test)
GBS: same as GAS microscopy and culture

Question 10

7. Discuss how these infections can be prevented and treated.

Answer 10

S. pyogenes is very susceptible to penicillin, some macrolide resistance
drainage and surgical debridement is required for serious tissue infection
antibiotic can speed symptom relief, decrease transmission and can prevent RF and peritonsillar abscess

S. agalactiae: DOC penicillin, ceftriaxone is also widely used; vancomycin in penicillin allergy

Question 11

Describe the characteristics of the viridans streptococci.

Answer 11

24 specie of a hemolytic streptococci, considered normal flora of the oral activity, GI tract and GU tract, mostly associated with dental caries; they are highly susceptible to penicillin

Question 12

Describe the characteristics of the peptostreptococci.

Answer 12

anaerobic gram positive cocci that are part of the normal flora of the mouth, vagina and intestines typically involved in mixed infections such as intrabdominal abscesses, aspiration pneumonias and peritonitis