Lecture 2: Gram Positive Bacteria II Flashcards
What are Streptococci bacteria?
The genus Streptococcus consists of gram-positive coccus shaped bacteria.
Streptococci divide in one plane, and thus form chains of bacteria.
They are around 0.5 - 1.2 micrometres in diameter.
Catalase negative, therefore cannot break down hydrogen peroxide, a harmful by-product of aerobic respiration.
They are facultatively anaerobic.
How can types of cocci be differentiated?
Streptococci can be differentiated by performing a catalase test. As staphylococci are catalase positive and streptococci aren’t.
Staphylococci can be further tested through coagulase tests. Staphylococcus aureus and intermedius are coagulase positive, all others are negative.
Staphylococci do not require enriched blood agar. Streptococci do (fastidious).
How can different groups of streptococci be differentiated?
Serological tests:
- e.g. Antistreptolysin-O antibodies are only produced in response to strep A infections.
Haemolysis:
- e.g. Group A and B strep are beta-haemolytic (group C mostly beta-haemolytic).
Cell arrangement:
- e.g. Streptococcus pneumoniae are diplococcal, strep A/B are streptococcal.
Biochemical tests:
- e.g. Strep A bacitracin sensitive, strep B is resistant.
What forms of haemolysis are there?
Alpha haemolysis.
Beta haemolysis.
Gamma haemolysis.
Describe alpha haemolysis.
Red blood cells are partially lysed, leaving a green area with a light halo around it.
Describe beta haemolysis.
Red blood cells are fully lysed, clear zone of haemolysis.
Describe gamma haemolysis.
No haemolysis occurs.
What strains of streptococcus are there for each type of haemolysis?
Alpha haemolysis:
- Streptococcus pneumoniae
- Viridans streptococci
Beta haemolysis:
- Mainly Lancefield groups A-C (Strep.pyogenes, Strep.agalactiae, Strep.equi, Strep. Strep dysgalactiae)
Gamma haemolysis:
- Enterococci (similar to streptococci but changed from group D strep to own category). e.g. Enterococcus faecalis.
What grouping method is there for Streptococci?
Lancefield grouping, developed in 1938 by Rebecca Lancefield.
It includes groups A to H, and K to V.
It is based on the antigens present.
The most significant groups for humans are A, B, C, D, E, F, and G.
What animal’s blood is used for blood agar, why not use human?
Horse blood.
What are group A streptococci?
GAS, or Streptococcus pyogenes.
These are small 1-2 mm white colonies.
There is a large zone of beta haemolysis on blood agar at 24hrs.
It is sensitive to bacitracin (an antibiotic that can’t be used for treatment).
Pathogenic species are often capsulated (hyaluronic acid capsule - via hyaluronan synthase).
What virulence factors does GAS have?
M protein - Strongly anti-phagocytic. Also binds serum factor H (alternative pathway) to its membrane, FH disrupts C3 convertase, preventing opsonisation and lysis.
Hyaluronic acid capsule is used as ‘camouflage’ against WBCs.
Produces 2 other membrane bound proteins called Streptolysins (Streptolysin O and S, the former being oxygen labile, while S isn’t).
What is the difference between streptolysin O and streptolysin S, how is it significant in relation to infections?
Streptolysin O is oxygen-labile unlike S, meaning it is inactive in the presence of oxygen. Infection sites are low oxygen environments due to neutrophils and macrophages requiring lots of O2 to generate ROS to combat bacteria. Due to this, this may explain why GAS infections are more local and don’t as easily become systematic.
What is protein M’s influence on the complement system?
It can bind factor F to bacterial cells, increasing the decay of C3 convertase in alternative pathway (C3bBb).
It can also bind to factor 4 binding protein (C4BP), that increases the rate of decay of classical and lectin pathway C3 convertase (C4bC2b).
(Factor F circulates in blood, attaches to host cells due to their distinct surface molecules, and helps decay C3 convertase, protein M hijacks this system).
What important enzymes does GAS have?
2 Streptokinases, these break down blood clots and facilitate infection in infected and damaged tissue.
4 distinct deoxyribonucleases, these depolymerise DNA, meaning less dense pus in abscesses, and it can spread faster (Neutrophil extracellular traps). (Spd1 introduced by phages).
C5a peptidase, which breaks down C5a complement protein, an anaphylatoxin, less signalling to WBCs.
Hyaluronidase, breaks down hyaluronic acid in ECM, facilitating infection.
What toxins can GAS produce?
3 distinct erythrogenic toxins (pyrogenic toxins) that stimulate macrophages and T helper cells to release cytokines that result in fever, rash, and TSS. (introduced by phages)
Haemolytic toxins (streptolysin O and S).
