In order to grow bacteria in laboratory settings, the microbes must be provided with everything that they need to survive and thrive. Microbes are typically cultured onto TSY (tryptic soy) agar, a gelatin-like medium that has been supplemented with nutrients, moisture and sometimes buffers to keep the pH within a tolerable range. TSY agar is a general, all-purpose medium that will grow many different types of microbes.
Sometimes bacteria are cultured onto more specific types of growth media, agar that has additives that either exclude some types of bacteria (selective media) or produce a color change in the presence of certain of microbes (differential media).
Blood Agar (BAP)
BAP Is Not Selective
Blood agar is an enriched medium that provides an extra rich nutrient environment for microbes. It is not selective, since it supports the growth of a wide range of organisms.
BAP Is Differential
Blood agar is, however, a differential growth medium. Microbiologists use BAP to distinguish clinically significant bacteria from throat and sputum cultures.
BAP contains 5% sheep blood and is the deep red color of blood. The presence of blood in the agar is useful in identifying certain microbes, because some bacteria produce enzymes (hemolysins) that act on the red blood cells to lyse or break the cells down, creating hemolysis patterns on the agar.
Hemolysis Patterns of Blood Agar
Beta hemolysis means that the bacteria's hemolytic enzymes completely beak down the blood cells. When ß-hemolytic bacteria grow on BAP, their presence results in halos of clear agar surrounding the bacterial colonies.
Alpha hemolysis patterns result when bacteria are present that can only partially break down the BAP's blood cells. This results in an incomplete hemolysis pattern that looks like a bruise on the medium surrounding an alpha-hemolytic colonies.
The third type of hemolysis pattern, Gamma, actually means no hemolysis occurred at all; that bacteria growing on the BAP had no effect on the red blood cells, resulting in no color change to the medium.
When Is Blood Agar Used?
BAP is typically inoculated from a patient’s throat swab, when the medical lab is trying to detect the presence of Group A beta hemolytic Streptococci (a Gram-positive round bacteria that causes Beta hemolysis on blood agar). The major human pathogen in this group is Streptococcus pyogenes, the causative agent of strep throat. Normal throat flora will exhibit alpha or gamma hemolysis.
Photo Credit
Blood Agar plates with bacterial growth. National Cancer Institute.
