6. Streptococci

Question 1

What do streptococci look like on a gram stain?

Answer 1

Gram positive - purple

Chains of cocci - string of pearls

Question 2

How are streptococci classified by haemolysis?

Answer 2

alpha haemolysis - partial breakdown
beta haemolysis - complete breakdown
non-haemolytic (gamma)

Question 3

What does alpha hemolytic strep look like on blood agar?

Answer 3

Green appearance

Question 4

What does beta haemolytic strep look like on blood agar?

Answer 4

Clear halo visible

Question 5

What is an example of an alpha haemolytic strep?

Answer 5

Strep. pneumoniae - wet appearance due to capsule

Question 6

What is an example of beta haemolytic strep?

Answer 6

Strep pyogenes

Question 7

What are the virulence factors of streptococci pygogenes?

Answer 7

Hyaluronic acid capsule
M Protein
Streptolysins
DNAases
Hyaluronidase
Streptokinase
Exotoxins

Question 8

What are M proteins, how do they act as virulence factors?

Answer 8

Proteins on bacterial surface that inhibit complement activation and phagocytosis.
Also help with adherence.

Question 9

What does hyaluronidase do?

Answer 9

Disrupts ground substance of connective tissue, aiding spread of infection.

Question 10

What does streptokinase do?

Answer 10

Converts plasminogen to plasmin, lysing clots.

Question 11

How does the bacterial capsule act as a virulence factor?

Answer 11

Made of hyaluronic acid, non-immunogenic as same as found in CT. Prevent phagocytosis,

Question 12

What do streptolysins O and S do?

Answer 12

Cell lysis of erythrocytes, neutrophils and platelets, release of lysosomal enzymes.

Question 13

What is the significance of the exotoxin released by strep pyogenes?

Answer 13

Streptococcal pyrogenic toxin
“super antigen family”
Cleaves IgG bound to group A strep

Question 14

What infection does strep pyogenes commonly cause in people aged 5-15?

Answer 14

Streptococcus pharyngitis

Question 15

How it Strep pharyngitis spread?

Answer 15

Droplet spread

Question 16

What are complications that can arise from strep pharyngitis?

Answer 16

Scarlet fever
Acute rheumatic fever
Acute post-streptococcal glomerulonephritis

Question 17

What suppurative (pus-forming) complications can occur following strep pharyngitis?

Answer 17

Peritonsillar abscess
Retropharyngeal abscess
Meningitis
Brain abscess

Question 18

How does scarlet fever arise from strep pyogenes infection?

Answer 18

Strep. pyogenes exotoxin strain

Question 19

What are the symptoms of scarlet fever?

Answer 19

High fever, sepsis, arthritis, jaundice

Question 20

What is acute rheumatic fever?

Answer 20

Inflammatory disease affecting the heart, joints and CNS

Question 21

What is acute post-streptococcal glomerulonephritis and how is it caused by strep pyogenes?

Answer 21

Acute inflammation of the renal glomerulus as due to antigen-antibody complexes in the glomerulus

Question 22

What skin infections can strep pyogenes cause?

Answer 22

Impetigo
Erysipelas
Cellulitis
Necrotising fasciitis

Question 23

Which age group is impetigo most common?

Answer 23

2-5 year olds

Question 24

What complication is most common following strep progenies impetigo?

Answer 24

Most common cause of glomerunephritis

No rheumatic fever

Question 25

What is necrotising fasciitis?

Answer 25

Infection of deeper subcutaneous tissues and fascia leading to rapid, extensive necrosis.

Question 26

How can streptococci cause toxic shock syndrome?

Answer 26

Can happen when group A strep enters deeper tissues and blood stream.
Release of super antigen exotoxins that stimulate T cells through MHCII, inducing mass cytokine release.