Some growth media, such as Blood Agar (BAP), can be used to do more than just grow bacteria; specialized agars can be used to help identify microbes.
* Blood Agar Is a Differential Medium *
A growth medium is considered differential if, when specific microbes are present, the medium exhibits a color change due to the metabolic capabilities of the bacteria growing on it. This color change provides information about the bacteria’s identity.
Blood agar (BAP) is a differential growth medium used to distinguish clinically significant bacteria from throat and sputum cultures. In clinical diagnosis, Blood agar is usually inoculated from a patient’s throat swab.
BAP contains 5% sheep blood. Certain bacteria produce enzymes (hemolysins) that act on the red blood cells in BAP to lyse or break them down.
* Hemolysis Patterns of Blood Agar *
There are three possible hemolysis patterns that can be observed when organisms are growing on Blood Agar (BAP):
* Beta hemolysis means that the bacteria's hemolytic enzymes completely beak down the blood cells. The β-hemolysis pattern results in the media displaying clear halos around bacterial colonies. Streptococcus pyogenes, strep throat bacteria, is the B-hemolytic organism that clinicians are looking for when they plate a throat sample onto BAP.
* Alpha hemolysis (α-hemolysis) means that the bacterial enzymes only partially break down the blood cells. This results in the media showing a yellowish/greenish/brownish discoloration (like a bruise) around the colonies, indicating incomplete hemolysis.
* Gamma hemolysis is essentially no hemolysis at all; that the bacteria have no effect on the red blood cells and there is no change to the color of the medium.
* What If a Blood Agar Plate Shows Beta Hemolysis? *
Beta hemolysis on BAP does not always indicate strep throat. There are other microbes that will produce B-hemolysis, including some Gram negative enteric bacteria (poop bacteria).
First look at the colony morphology—the shape and surface characteristics of the dots on the agar. These dots are colonies; millions of bacteria that have resulted from the one initial bacterium multiplying. Streptococcus pyogenes forms very tiny (punctiform) colonies, in contrast to Gram-negative bacteria, which typically form larger, slimy-looking colonies.
In addition to examining the colony morphology, there is a more precise way to determine if the B-hemolytic bacteria are Strep. Plating another throat sample onto Bood Agar, then adding a bacitracin antibiotic disk will help reveal if the unknown bacteria are S. pyogenes. Bacitracin inhibits the growth of Streptococcus. Therefore, if, after incubation, the plate shows Beta-hemolytic colonies that will not grow near the bacitracin disk, this is indicative to Strep throat bacteria.
* Sources *
Schauer, Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.