Fig. 1. The image shows colonies of some bacterial species that exhibit different hemolys patterns. The colonies have been lit from above during photography. However, the easiest way to observe haemolysis is with the illumination from below and by looking at the plate in the "right" angle. The following bacteria have been used to illustrate hemolysis:
A. Streptococcus uberis, causing no hemolysis. This is sometimes is called γ-hemolysis, which is a bit unfortunate.
- Click on the image to enlarge it.
Figs. 2. This image shows the same agar plates as in Fig. 1, but the colonies have been lit from below during photography, because it is the easiest way to observe the haemolysis. - Click on the image to enlarge it.
Hemolysis (Brittish spelling: haemolysis) means that the red blood cells (erythrocytes) burst apart (hemolyse) and release the cell contents (hemoglobin). Some bacteria produce so-called hemolysins, which give them hemolytic capacity. Most hemolysins are proteins (enzymes or porins), but there are also other types of hemolysins like rhamnolipids and biological detergents (biosurfactants).
Hemolysins are membrane distupting exotoxins which can be divided into two groups: toxins with enzymatic activity, and channel-forming toxiner (= porins).
Enzymatically active hemolysins are often lipases such as α-toxin of Clostridium perfringens, which is a phospholipase. When lipase cleaves lipids in plasma membranes of the host animal cells, the membrane will become fragmented and the cell contents leak out.
Porins are composed of subunits, but are secreted by the bacterium in monomeric form. In the cell membranes of the host animal, the monomers aggregate to channel-forming polymers (heptamers), which makes it impossible for the ion gradient across the plasma membrane of the host cell to be maintained and the osmotic pressure in the cell will increase until it lyses.
A function of hemolysins is that the bacteria can utilize hemolysis to release and utilize nutrients from the host animal cells. Iron e.g., is essential to many pathogenic bacteria, but is only present in very low concentrations outside the cells. If the bacteria have access to free hemoglobin, it can utilize the iron, which is bound to the heme groups of hemoglobin. Hemolysins do not act only on erythrocytes, but can also lyse other types of cells.
Identification of bacteria based on haemolysis
By cultivation on blood agar, bacteria can be differentiated based on their capacity to secrete hemolysins. The hemolysis will cause a clearing zone of the blood agar around the colonies. Bacteria can cause different types of hemolysis:
Note that the α-hemolysin of staphylococci causes complete hemolysis, whereas their β-hemolysin causes incomplete hemolysis.
The capacity to produce hemolysins may vary between different strains of a particular bacterial species. Blood agar plates with bacteria having different hemolysis patterns are shown in Fig. 1 and 2. Note that in Fig. 1A and 2A hemolysis cannot be observerd. In Fig. 1B, one can discern a thin hemolysis zone and in Fig. 2B, the clear β-hemolysis is evident around all colonies. In Fig. 1C it is possible to discern hemolysis around some colonies, and in Fig. 2C, one can clearly see the green α-hemolysis around some colonies (white arrows). In Fig. 1D one can see the outer hemolysis zone (white arrow) and in Fig. 2D, it is possible to see both the clear inner β-hemolysis zone and outer turbid α-hemolysis zone (white arrows).