A sensitivity test is a series of bacterial cultures performed in a medical laboratory on a specimen obtained from an infected individual. Specimens can be swabs of a wound or wound discharge, blood, urine or sputum (spit).
The researcher will prepare several dishes of solid media, coat the dishes with the bacteria isolated from the specimen and then place several paper discs impregnated with different antibiotics on top of the media. As the bacteria multiply in the culture, they will form a visible lawn on the top of the solid media. However, this lawn will not develop near discs containing an antibiotic the bacteria is sensitive to. In this manner, a researcher can determine the most effective antibiotic to use to treat the infection. Antibiotic selection is the main goal of a sensitivity test.
The primary reason to add blood to the solid media is to support the growth of the bacteria. However, a secondary goal of a sensitivity test is to identify the strain of bacteria present. In this case, the blood in the media may act as an indicator.
Certain types of bacteria produce enzymes which cause haemolysis (the breakdown of blood). There are different types of haemolysis. Bacteria with enzymes which produce beta haemolysis will have a clear area around the colony. Those which produce alpha haemolysis will be surrounded by a greenish area. The green color is because this type of haemolysis results in the production of a by-product called biliverdin. If no haemolytic enzymes are produced, the color of the media will be unchanged.
Knowing which type of haemolytic enzymes, if any, a bacterial colony produces can help the researcher to narrow down the strain of the bacteria.