L19: Streptococci

Question 1

How do microorganisms cause disease?

Answer 1

Virulence factors

• Exotoxins
• Endotoxins

Cause host cellular damage

• Direct
• Consequent to host immune response

Question 2

What is streptococci?

Answer 2

Gram positive cocci
Bacteria forms chains
Stains purple

Question 3

How can streptococci be classified?

Answer 3

Haemolysis–> how good they are at breaking down RBC
Lancefield antigen–> based on cell wall antigens
Sherman group–> pyogenic, viridans, lactic or enterococcal

Question 4

How does the haemolysis system of classification work?

Answer 4

alpha haemolysis–> partial haemolysis of RBC–> goes green like a bruise

beta haemolysis–> full haemolysis (complete breakdown), translucent area forms around

non haemolytic haemolysis (gamma)–> no haemolysis

Question 5

Why would you use a blood agar plate?

Answer 5

Blood contain nutrients
Iron
Required for some bacteria to cultivate

Question 6

How does the Lancefield classification system work?

Answer 6

Further groups beta haemolytic streptococci
Groups A-G
Based on presence or absence of antigenic carbohydrate on cell wall
Uses antibodies to recognise different bacteria, some will react and others wont

Question 7

How does the Sherman group classification system work?

Answer 7

Separated into four groups
- Pyogenic
- Viridans 
- Lactic 
- Enterococcal 
Based on different factors*
Only the pyogenic and viridans group still exists 

*Based on ability to grow at 10 and 45 degrees, to survive at 60 degrees for 30 minutes, grow at pH 9.6 in 0.1% methylene blue and at different concentrations of sodium chloride

Question 8

What is the classification of Streptococcus pyogenes?

Answer 8

Lancefield group A beta-haemolytic streptococcus

Question 9

What are the virulence factors for Streptococcus pyogenes?

Answer 9

Lots of different types of bacteria within the Streptococcus pyogenes species so lots of different virulence factors

1) Hyaluronic acid capsule
2) M protein
3) Adhesins, including lipoteichoic acid, M protein, fibronectin binding proteins
4) Streptolysins O and S
5) Dnase A, B, C and D
6) Hyaluronidase
7) Streptokinase
8) Streptococcal pyrogenic exotoxins

Question 10

How does the hyaluronic acid capsule act as a virulence factor?

Answer 10

Inhibits phagocytosis by neutrophils and macrophages

Similar to host connective tissue so protects it from defence systems

Question 11

How does M protein work?

Answer 11

Different variants
Resistant to phagocytosis–> inhibits activation of alternative complement pathway on bacterial cell surface
>150 antigenically different serotypes

Question 12

How does the adhesins work?

Answer 12

Includes lipoteichoic acid, M protein, fibronectin binding proteins
Adherence is the first step toward colonisation

Question 13

How does streptolysins O and S work?

Answer 13

Cause lysis of erythrocytes, neutrophils and platelets

Question 14

How does the DNAses A, B, C and D act as virulence factors?

Answer 14

Degradation of DNA

In response to NETs (neutrophil extracellular traps)–> DNAse break them down releasing the bacteria

Question 15

How does hyaluronidase work as a virulence factor?

Answer 15

Degradation of hyaluroinc acid in connective tissue allowing the bacteria to spread more easily

Question 16

What does the virulence factor streptokinase do?

Answer 16

Dissolution of blood clots through conversion of plasminogen to plasmin
Prevents clots stopping the spread of bacteria

Question 17

What does the virulence factor streptococcal pyrogenic exotoxins do?

Answer 17

Cleave IgG bound to Group A strep

Member of superantigenic Spe family –> cause excess T cell proliferation

Question 18

What does Streptococcal pyrogenes cause?

Answer 18

Streptococcal pharyngitis

Infection at back of the throat including tonsils

Question 19

What are the clinical features of Streptococcal pharyngitis? How is it transmitted?

Answer 19

Sore throat 
Enlarged tonsils 
Lymphoid hyperplasia
Malaise, fever, headache
Tonsillopharyngeal exudates 
Droplet spread

Question 20

How is streptococcal pharyngitis treated?

Answer 20

Will resolve without treatment
Develop M proteins specific antibody–> protection
Treatment given to prevent late immune complications of Streptococcal pharyngitis

Question 21

What are some of the potential complications that can arise from streptococcal pharangitis?

Answer 21

Scarlet fever–> Red rash caused by pyrogenic exotoxin, local or haematogenous spread, associated with high fever, sepsis, arthritis and jaundice
Suppurative complications–> complications of pus
- Retroperitoneal abscess–> bacteria collects in retroperitoneal space behind tonsil and spreads down to mediasinum–> infection and clotting
- Peritonsillar cellulitis/abscess
- Mastoiditis, sinusistis, otitis media
- Meningitis, brain abscess

Question 22

What are the immune complications of streptococcal pharyngitis?

Answer 22

Immune complications means late complication occurs after the infection has been clears due to immune system
Antigens cross react with own antigens–> causes autoimmune condition
- Acute Rheumatic fever
- Acute post-streptococcal glomerulonephritis

Question 23

What is acute rheumatic fever?

Answer 23

Cause inflammation of the heart (pericarditis or myocarditis), joints and CNS
Rheumatogenic M types –> certain M type caused by a specific type of Streptococcal pyrogene–> rheumatic fever
Possible mechanism
-Autoimmune provoked by previous pharyngitis
-Serum sickness
-Binding of M protein to collagen
- ASO, ASS induced injury
If streptococcal pharyngitis is treated early–> don’t get this

Question 24

What is acute post-streptococcal glomerulonephritis?

Answer 24

Inflammation of renal glomerulus

Caused again by specific M types (different to ones causing acute rheumatic fever)

Question 25

What skin infections can be caused by streptococcus pyogenes?

Answer 25

Impetigo
Erysipelas
Cellulitis
Necrotising fasciitis

Question 26

What is Impetigo?

Answer 26

Crusty lesions form on the skin
Childhood infections 2-5 years
Initial skin colonisation, followed by intradermal innoculation
No ARF but impetigo is most common cause of glomeruonephritis

Question 27

What is erysipelas?

Answer 27

Invasive skin infection of the deep layers of the skin
Involvement of lymphatic system
Face and lower limbs
Lower limb infections usually secondary to invasion of skin via trauma, skin disease or local fungal infection

Question 28

What is cellulitis?

Answer 28

Skin and subcutaneous tissue infection
Impaired lymphatic drainage and illict injecting drug use important risk factors
Flucoxacillin treated

Question 29

What is necrotising fasciitis?

Answer 29

Infection of deeper subcutaneous tissue and fascia
Causes rapid extensive necrosis
Usually result of break in skin
Severe pain, even before gross clinical changes
High fever, fulminant (severe sudden onset) course, high mortality (20-70%)
Loose limbs and lots of tissue
Tissue turns to mush

Question 30

What is streptococcal toxic shock syndrome?

Answer 30

• Deep tissue infection with strept pyrogenes
• With bacteraemia (bacteria in blood), vascular collapse, organ failure–> sudden death
• Streptococcal pyrogenic exotoxins stimulate T cells inducing monocyte cytokines and lymphokines
• M protein fibrinogen complex formation–> binds to integrins on surface of polymorphonuclear leukocytes activating the cells
• Cells adhere to endothelium and degranulate–> release large amount of hydrolytic enzymes and respiratory burst (ROS) causing damage to endothelium
• Causes hypotension, DIC and organ damage