Gram Positive: Streptococci

Question 1

Major characteristics of Streptococci.

Answer 1

• Gram positive cocci that line up in strips of bundles (like button candy), as opposed to forming clusters (seen in Staphylococci)
• catalase negative (Staph is catalase positive)
• microaerophilic/aerotolerant anaerobes or facultative anaerobes

Question 2

How are Streptococci classified?

Answer 2

• based on hemolytic activity: beta (completely lyse RBCs; clear halo around colony), alpha (partially lyse; green discoloration around colony), gamma (unable to lyse)
• also based on Lancefield antigens (these are specific carbohydrates in the peptidoglycan cell wall): groups A through S exist

Question 3

What are the five major Streptococci pathogens?

Answer 3

• Lancefield group A, B, D, S. pneumoniae, and the Viridans group (the last two lack Lancefield antigens)
• group A beta-hemolytic strep (AKA Streptococcus pyogenes)
• group B beta-hemolytic strep (AKA Streptococcus agalactiae)
• group D strep (enterococci and non-enterococci); can be alpha or gamma-hemolytic)
• Strep pneumoniae are alpha-hemolytic
• Strep viridans are mainly alpha-hemolytic

Question 4

Group A beta-Hemolytic Strep

Answer 4

• AKA Streptococcus pyogenes
• local invastion/exotoxin release causes: strep throat (pharyngitis), skin infections, scarlet fever, streptococcal toxic shock syndrome
• delayed antibody mediated disease causes: rheumatic fever and post-streptococcal glomerulonephritis
• major virulence factors: M protein (antigenic; prevents phagocytosis), streptolysin O (antigenic; oxygen labile; destroys RBCs and WBCs), streptolysin S (non-antigenic; oxygen stable), erythrogenic/pyrogenic exotoxin (cytokine release, rash, etc.)
• treat with penicillin (group A remains sensitive) and clindamycin (prevents synthesis of the exotoxin)

Question 5

What is scarlet fever?

Answer 5

• scarlet fever is a potential complication of infection with group A beta-hemolytic strep (Strep. pyogenes) that secretes a pyrogenic exotoxin
• the toxin results in fever and a scarlet-red rash that starts at the neck and trunk and spreads across the body (but tends to spare the face)

Question 6

What is toxic shock syndrome?

Answer 6

• a potential complication of infection with group A beta-hemolytic strep (Strep. pyogenes) that secretes a pyrogenic exotoxin
• the toxin stimulates TNF and IL-1, and results in fever, rash, nausea, and vomiting
• associated with septic shock
• also occurs due to Staph aureus’ TSST-1, which is far more deadly than Strep pyogenes’

Question 7

What is rheumatic fever? How do patients classically present? What is a major complication and how do we prevent it?

Answer 7

• an antibody mediated complication of infection with group A strep (Strep pyogenes) due to molecular mimicry
• patients present with fever, myocarditis, migratory arthritis, Sydenham’s chorea, subcutaneous nodules, and erythematic rash
• usually occurs in 5-15 year olds following an untreated strep throat (note that it does NOT follow a skin infection)
• major complication: rheumatic valvular heart disease (mitral valve > aortic valve) via repeated bouts of rheumatic fever (give these patients continuous doses of prophylactic penicillin!)

Question 8

What is post-streptococcal glomerulonephritis? How do patients classically present?

Answer 8

• an antibody mediated complication of infection with group A strep (Strep pyogenes) due to immune complex deposition in the glomerulus that induces complement activation
• patients present with fluid retention and tea/coke colored urine (hematuria)
• usually occurs about 1 week after pharyngitis OR skin infection

Question 9

Group B Strep

Answer 9

• (beta-hemolytic)
• AKA Streptococcus agalactiae
• major pathogen for neonates as the bacteria is found in the birth canal of about 25% of women; meningitis, pneumonia, and sepsis
• (neonatal meningitis caused mainly by: Listeria monocytogenes, Escherichia coli, and Strep agalactiae)

Question 10

Group D Strep

Answer 10

• (alpha or gamma-hemolytic); normal gut flora that can cause bacteremia, UTIs, biliary tract infections, subacute bacterial endocarditis (SBE)
• Enterococci: E. faecalis and E. faecium; these are the 2nd to 3rd most common nosocomial infectious agent (hard to eradicate; VRE: vancomycin resistant enterococci)
• non-Enterococci: Strep bovis and Strep equinus; Strep bovis bacteremia is associated with colon cancer

Question 11

Streptococci Viridans

Answer 11

• most are alpha-hemolytic (“viridans” for “verde” for “green”)
• normal gut flora, especially in dental plaque and saliva
• results in cavities, subacute bacterial endocarditis (SBE), and abscesses of the brain and abdominal organs (patients with Strep viridans bacteremia should get a CAT scan to check for abscesses)

Question 12

What is SBE?

Answer 12

• subacute bacterial endocarditis
• mainly due to the Viridans group of strep; dental manipulations result in massive bacteremia, allowing implantation onto the heart (usually onto a previously damaged valve via rheumatic heart disease, etc.)
• SBE is a slow development of fever, fatigue, anemia, and murmurs secondary to valve destruction (compare to the much more rapid onset of healthy valve destruction in acute infectious endocarditis via Staph. aureus in IV drug users)
• note that SBE can also be caused by group D streptococci

Question 13

Streptococcus Pneumoniae

Answer 13

• (alpha-hemolytic)
• the MOST common cause of pneumonia and meningitis in adults, and the MOST common cause of otitis media in children
• at least 84 polysaccharide capsular serotypes, use a Quellung reaction to differentiate
• (major causes of otitis media in children: Strep pneumoniae, Haemophilus influenzae, Moraxella catarrhalis)

Question 14

Both Strep. pneumoniae and Strep. viridans are alpha-hemolytic and lack Lancefield antigens - how can we tell them apart?

Answer 14

• differentiate based on Optochin sensitivity

- Optochin will inhibit Strep pneumoniae, but not Strep viridans