Alpha versus beta haemolysis on blood agar
Haemolysis, the breaking open of red cells on blood agar (a plate containing red blood cells), is beta if complete, alpha if partial and green, gamma for no change.
Remember
A clear, transparent halo (a zone where the red cells are completely destroyed) on blood agar means beta haemolysis, as with Streptococcus pyogenes, the commonest bacterial cause of sore throat; a green zone is alpha and no change is gamma.
Memory aid
beta bursts the cells completely (clear halo), alpha leaves a green partial zone, and gamma leaves the agar untouched
Use it to recall which haemolysis pattern gives a clear halo, a green partial zone or no change.
Streptococci are Gram-positive cocci (round bacteria that stain purple) that usually form chains. When they grow on blood agar, a culture plate containing red blood cells, one of the main ways to sort them is by what they do to those cells. This is haemolysis, the breaking open of red cells.
Beta haemolysis is complete breakdown of the red cells. The agar goes transparent and light passes through the cleared zone, so a clear halo shows around the bacteria. Alpha haemolysis is only partial: a chemical change to the iron in haemoglobin (the red, oxygen-carrying pigment) leaves a greenish zone instead. Gamma haemolysis means no haemolysis at all, so the agar looks unchanged.
Streptococcus pyogenes is the classic beta-haemolytic species, and group A streptococcus is another name for it. Streptococcus pneumoniae is alpha-haemolytic and the viridans streptococci are mainly alpha-haemolytic, and 'viridans' means green. Streptococcus bovis is an example of gamma.
Streptococci can also be described in other ways, which are not about haemolysis. Lancefield grouping is based on the carbohydrates in the bacterial cell wall, from group A to group V. Sherman's grouping is based on physiological properties and divides streptococci into several groups, such as viridans, enterococcal and lactic. Streptococcus pyogenes is beta-haemolytic and Lancefield group A.
Key facts
- Beta haemolysis
- complete breakdown of red cells, leaving a clear, transparent halo, as with Streptococcus pyogenes.
- Alpha haemolysis
- partial breakdown of red cells with a greenish zone, as with Streptococcus pneumoniae and the viridans streptococci.
- Gamma haemolysis
- no breakdown of red cells, so the agar looks unchanged, as with Streptococcus bovis.
- Blood agar
- a culture plate containing red blood cells, so a bacterium that destroys them leaves a visible mark.
- Lancefield grouping
- based on the carbohydrates in the bacterial cell wall, from group A to group V, and Streptococcus pyogenes is group A.
- True patterns
- alpha, beta and gamma are the true streptococcal haemolysis patterns, and delta and epsilon are not recognised ones.
Exam traps
- Delta haemolysis is not a streptococcal haemolysis pattern, because the true patterns are alpha, beta and gamma.
- A clear halo on blood agar does not point to the viridans group, because viridans streptococci are mainly alpha-haemolytic and give a green zone of partial haemolysis.
- A beta-haemolytic, Lancefield group A streptococcus is not a viridans streptococcus, because viridans streptococci are mainly alpha-haemolytic and are not Lancefield group A.
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