Session 6: Streptococci: Streptococcus pyogenes

Question 1

How will streptococci appear on blood culture?

Answer 1

As gram positive (purple)

In chains and each segment of the chains is globular

Question 2

How can streptococci be classified?

Answer 2

By haemolysis
By based on cell wall antigens
By whether they are pyogenic, viridans, enterococcal or lactic streptococci

Question 3

What are the different subgroups of classification of streptococci by haemolysis? Give the degree of haemolysis in each case.

Answer 3

Alpha haemolysis (partial haemolysis)
Beta haemolysis (complete breakdown)
Gamma haemolysis (no haemolysis)

Question 4

Give an example of streptococci in each haemolytic group.

Answer 4

Alpha haemolysis: viridans streptococci like streptococci pneumoniae

Beta haemolysis: Streptococcus pyogenes

Gamma haemolysis: Enterococus faecalis

Question 5

How can you tell whether the streptococcus is alpha, beta or gamma haemolytic?

Answer 5

By the use of a blood agar plate.

Question 6

How will the appearance of the blood agar plate differ between the three haemolytic classifications?

Answer 6

Alpha: will appear somewhat green/brownish
Beta: Will appear much more white and the blood will disappear
Gamma: will barely have a shift of appearance, there might be pale silvery dots.

Question 7

What are the virulence factors of streptococcus pyogenes?

Answer 7

Hyaluronic acid capsule
M proteins
Adhesins
Streptolysins
DNAses
Hyaluronidase
Streptokinase
Streptococcal pyrogenic exotoxins

Question 8

Briefly explain the mechanism of action of: Hyaluronic acid capsule

Answer 8

Inhibits phagocytosis by neutrophils and macrophages and immune system doesn’t response as readily due to the similarity to human connective tissue hyaluronate.

Question 9

Briefly explain the mechanism of action of: M protein

Answer 9

Resistance to phagocytosis by inhibition of activation of alternative complement pathways.
Multiple of nucleotide variants of M-gene meaning antibodies might not recognise them.

Question 10

Briefly explain the mechanism of action of: Adhesins

Answer 10

Adherence of the bacteria for colonisation and infection

Question 11

Briefly explain the mechanism of action of: Streptolysins

Answer 11

Lysis of erythrocytes, neutrophils and platelets

Question 12

Briefly explain the mechanism of action of: DNAses

Answer 12

Degradation of DNA

Question 13

Briefly explain the mechanism of action of: Hyaluronidase

Answer 13

Degradation of hyaluronic acid in connective tissue

Question 14

Briefly explain the mechanism of action of: Streptokinase

Answer 14

Dissolution of thrombus by converting inactive plasminogen to active plasmin

Question 15

Briefly explain the mechanism of action of: Streptococcal pyrogenic exotoxins

Answer 15

Active secretion of toxins

Question 16

What is a common infection that is caused by Streptococcus pyogenes?

Answer 16

Streptococcal pharyngitis (sore throat)

Question 17

What patients are most susceptible to Strep pharyngitis?

Answer 17

5-15 years of age
Over-crowding places
People who have been treated with antibiotics regularly for Strep pharyngitis (no antibodies for it)

Question 18

Clinical features of Strep pharyngitis.

Answer 18

Abrupt onset of sore throat
Malaise, fever and headache
Lymphoid hyperplasia
Tonsillopharyngeal exudates

Question 19

How do you make a diagnosis of Strep pharyngitis?

Answer 19

Check for clinical features. Do a throat swab and then blood agar plate if needed.

Question 20

How do you treat Strep pharyngitis?

Answer 20

Usually you leave it to its own device. Antibiotics will work but will probably only make it better 1 or 2 days in advance.
The problem with treating with antibiotics is that they might not develop M-protein specific antibodies which means that they might get infection again and again.

Question 21

What are the complications of Strep pharyngitis?

Answer 21

Scarlet fever
Suppurative complications
Acute rheumatic fever
Acute post-streptococcal glomerulonephritis
Skin infections
Streptococcal toxic shock syndrome

Question 22

How is Strep pharyngitis transmitted?

Answer 22

Usually via coughs (droplets) airborne

Question 23

Briefly explain Scarlet fever. Mechanism of infection, spread of infection and clinical features.

Answer 23

Due to infection of Strep pharyngitis with streptococcal pyrogenic exotoxin strain of Strep. pyogenes.

It can be of local or haematogenous spread that results in high fever, sepsis, arthritis and jaundice.

Question 24

What are suppurative complications of Strep pharyngitis.

Answer 24

Such as peritonsillar cellulitis/abscess
Retropharyngeal abscess
Mastoiditis, sinusitis and otitis media
Meningitis and brain abscess in case of infection entering via carotid artery and haematogenous spread.

Question 25

Briefly explain acute rheumatic fever.

Answer 25

Inflammation of heart, joints and CNS.
From pharyngitis
Rheumatogenic M types are the cause.
Mechanisms such as:
Auto-immune
Sreum sickness
Binding of M protein to collagen
ASO, ASS induced injury

Question 26

Briefly explain post-streptococcal glomerulonephritis.

Answer 26

Acute inflammation of renal glomerulus
M type specific
Antigen-antibody complexes in glomerulus

Question 27

What are some common streptococcus pyogenes skin infections that can result from pharyngitis?

Answer 27

Impetigo
Erysipelas
Cellulitis
Necrotising fasciitis

Question 28

Briefly explain impetigo.

Answer 28

Childhood infection 2-5 years
Skin colonisation followed by intradermal inoculation
No acute renal failure but impetigo is most common cause of glomerulonephritis

Question 29

Briefly explain Erysipelas

Answer 29

Dermis infection with lymphatic involvement
Face and lower limbs
Facial lesions
Lower limb infections

Question 30

Briefly explain cellulitis

Answer 30

Skin and subcutaneous tissue infection
Impaired lymphatic drainage and illicit injecting drug use is an important risk factor.
Can also be caused by S. aureus

Question 31

Briefly explain necrotising fasciitis

Answer 31

Infection of deeper subcutaneous tissue and fascia
Rapid and extensive necrosis
Usually secondary to skin break
Severe pain
High fever and high mortality

Question 32

How do you necrotising fasciitis?

Answer 32

Usually by antibiotics and debridement of dead tissue.

Question 33

Explain mechanism of streptococcal toxic shock syndrome.

Answer 33

Group A strep enters into deeper tissue and bloodstream.
Strep. pyrogenic exotoxins stimulate T-cells though binding to mHC class II antigen presenting cells inducing monocyte cytokines and lymphokines.
M-protein fibrinogen complex formation occurs.

Adherence of leucocytes to endothelium where they perform respiratory burst. Endothelium gets damaged causing vascular leakage and hypercoagulability.

Hypotension, DIC and organ damage will ensue.