Infection - Streptococci

Question 1

How does streptococci appear on a gram stain?

Answer 1

Gram positive (purple) cocci in chains

Question 2

What are the three possible reactions after adding streptococci to a blood plate?

Answer 2

• non-haemolytic (gamma) do nothing
• viridans streptococci turn it green (alpha haemolysis)
• streptococcus pyogenes turn it yellow (beta haemolysis)

Question 3

True or false - encapsulated bacteria are much more pathogenic than non-encapsulated?

Answer 3

True - the capsule protects them

Question 4

Why is streptococcus pyogenes called ‘pyogenes’?

Answer 4

Because it forms pus

Question 5

How are different strains of streptococcus identified?

Answer 5

They are reacted with different antibodies. If it reacts with antibody A then it will be group A etc.

Question 6

What type of strep is group A usually?

Answer 6

Streptococcus pyogenes

Question 7

What kind of strep is group B usually?

Answer 7

Streptococcus agalactiae - important cause of neonatal sepsis due to being normal vaginal flora

Question 8

What are the four Sherman groups of streptococci?

Answer 8

Pyogenic, viridans, enterococcal and lactic

Question 9

What is the best way of identifying bacteria?

Answer 9

Looking at the 16S ribosomal RNA sequence

Question 10

How does the hyaluronic acid capsule benefit strep pyogenes?

Answer 10

• inhibits phagocytosis by neutrophils and macrophages

- poor immunogenicity due to similar to human connective tissue

Question 11

How do M proteins help strep pyogenes?

Answer 11

• resistant to phagocytosis as they inhibit activation of alternative complement pathway on bacterial cell surface
• over 150 different stereotypes so difficult to become immune to them

Question 12

What do adhesins do to help strep pyogenes?

Answer 12

They allow adherence, which is the first step in colonisation/infection

Question 13

How do streptolysins O and S help strep pyogenes?

Answer 13

They cause lysis of erythrocytes, neutrophils and platelets

Question 14

What is the function of DNAses A, B C and D?

Answer 14

They degrade DNA, so it can be expelled and used as a net

Question 15

What is the function of hyaluronidase?

Answer 15

Degradation of hyaluronic acid in connective tissue

Question 16

What is the role of streptokinase?

Answer 16

Dissolution of clots through conversation of plasminogen to plasmin

Question 17

What is the function of streptococcal pyrogenic exotoxins?

Answer 17

They cleave IgG bound to group A strep. They are a member of superantigenic Spe family

Question 18

Which type of strep causes streptococcal pharyngitis?

Answer 18

Strep pyogenes

Question 19

What are the clinical features of strep pharyngitis?

Answer 19

• abrupt onset sore throat
• malaise, fever, headache
• lymphoid hyperplasia
• tonsillopharyngeal exudates

Question 20

Someone with streptococcal pharyngitis develops high fever, sepsis, arthritis and jaundice. What have they now contracted?

Answer 20

Scarlet fever

Question 21

Give some complications of streptococcal pharyngitis

Answer 21

• peritonsular cellulitis/abscess
• retropharyngeal abscess
• mastoiditis, sinusitis, otitis media
• meningitis, brain abscess
• acute rheumatic fever
• acute post-streptococcal glomerulonephritis

Question 22

Give some skin infections caused by streptococcus pyogenes

Answer 22

• impetigo
• erysipelas
• cellulitis
• necrotising fasciitis

Question 23

What is impetigo?

Answer 23

Very superficial childhood infection (usually 2-5 years), where strep pyogenes has an initial skin colonisation followed by it entering to intradermal area. Can lead to glomeruonephritis

Question 24

What is erysipelas?

Answer 24

Dermis infection with lymphatic involvement of face and lower limbs by strep pyogenes. Usually preceded by pharyngitis

Question 25

What is cellulitis?

Answer 25

Infection of skin and subcutaneous tissue. Impaired lymphatic drainage and illicit drug injection are important risk factors

Question 26

What is necrotising fasciitis?

Answer 26

Infection of deeper subcutaneous tissues and fascia leads to rapid, extensive necrosis. Usually secondary to skin break and causes severe pain. High fever, fulminant course, high mortality

Question 27

What is streptococcal toxic shock syndrome?

Answer 27

Deep tissue infection with strep pyogenes and bacteraemia leading to vascular collapse and organ failure. Streptococcal pyrogenic exotoxins stimulate T-cells through binding to MHC class II antigen-presenting cells and T cell receptors, inducing monocytes cytokines and lymphokines. M-protein fibrinogen complexes form