Streptococci

Question 1

What are the characteristics of Streptococci?

Answer 1

Gram positive
Pairs or chains
Facultative anaerobes (some are obligate anaerobes)
Non-sporing
Non-motile
Catalase negative
Requires enriched media
Optimum growth at 37 degrees Celsius 
5% CO2
Haemolytic on blood agar

Question 2

What are the types of haemolytic shown by streptococcal bacteria?

Answer 2

Alpha haemolysis
Beta haemolysis
Gamma haemolysis

Question 3

What does alpha haemolysis look like?

Answer 3

Greenish zone forms around the streptococcus colonies due to partial haemolysis of erythrocytes

Question 4

What does beta haemolysis look like?

Answer 4

The area around the colonies is clear due to complete haemolysis of erythrocytes

Question 5

What does gamma haemolysis look like?

Answer 5

No haemolysis, no zone is formed around the colonies

Question 6

How are streptococci classified?

Answer 6

1. The type of haemolysis seen on blood agar
2. Biochemical reactions
3. Serological detection of cell wall antigen

Question 7

What are the major pathogenic serogroups of streptococcus?

Answer 7

Group A streptococcus

Group B streptococcus

Question 8

How is streptococcal pathogenesis mediated?

Answer 8

Adherence: adherence
                    Signalling 
                     Nutritional adaptation 
                     Host modulation
&

Colonisation

Question 9

Colonisation

Answer 9

Streptococci produce multiple adhesions and can colonise different sites in the human body.
Pioneer bacteria bind to host cell surface molecules.
Pioneers>settlers>society>community

Question 10

To what serogroup of does S.pyogenes belong to?

Answer 10

Group A streptococcus (GAS)

Question 11

Information on S.pyogenes

Answer 11

• leading pathogenic bacteria in GAS
• infects children and adults
• 600 million cases of GAS pharyngitis (“strep throat”) worldwide
• 100 million worldwide cases of GAS pyoderma
• 500,000 deaths annually

Question 12

Main sites of infection?

Answer 12

Respiratory tract
Bloodstream
Skin

Question 13

How is the S.pyogenes transmitted?

Answer 13

Skin contact ( via skin sores or wounds)
Via respiratory droplets or mucus

Question 14

What are the virulence factors of S.pyogenes?

Answer 14

Capsular polysaccharide (C-substance, hyaluronic acid)
Cell wall peptidoglycan
Lipoteichoic acid (LTA)

Surface proteins:

• M protein, fimbrial proteins
• fibronectin-binding protein (Protein F)
• cell-bound streptokinase

Question 15

What molecules mediate invasion of epithelial cells by group A streptococci?

Answer 15

Invasins are required for high frequency invasions.

• M protein: mediate the attachment to host cells.
• Sfbl, a fibronectin binding protein

Question 16

Sfbl

Answer 16

This protein is involved in the adherence of the bacteria to the host cell and the internalisation of the bacteria by the host cell.
It is also a bridging molecule that interacts with host ECM proteins. This allows the bacteria to evade the host’s defences.

Question 17

M protein

Answer 17

• major virulence factor
• expressed by all GAS isolates
• composed of series of repeated regions (ABC)
• made of 11 amino acids
• involved in evasion of phagocytosis
• encoded by the emm genes
• resistance to infection is related to M protein antibody production
• antibodies for 1 type of M protein does no protect against any other types of M proteins.

Question 18

What did Lancefield do in 1930?

Answer 18

Lancefield described streptococcal serogroups A to S