Blood Agar is a bacterial growth medium that can distinguish normal from pathogenic bacteria based on the effect of bacterial hemolytic enzymes on red blood cells.
* What Is a Growth Medium? *
A growth medium (plural: media) is a mixture of nutrients, moisture and other chemicals that bacteria require for growth. Media are used to grow bacterial colonies (millions of bacteria having arisen through the binary fission of a single progenitor).
* Using Media to Identify Bacteria *
Some media can be used to do more than just grow bacteria; specialized agars can aid in bacterial identification. Differential and selective media are special types of agar that can exclude certain types of bacteria and even test for certain bacterial metabolic capabilities. MacConkey's (MAC), Blood agar (BAP) and Mannitol Salt (MSA) are three examples of these specialized types of media.
* Blood Agar Is Not a Selective Medium *
If a bacterial growth medium is selective, that means that it grows only certain types of microbes while inhibiting the growth of other types of microbes. Blood agar is an enriched medium that provides an extra rich nutrient environment for microbes. Therefore, BAP is not a selective growth medium, since it supports the growth of a wide range of organisms.
* Blood Agar Is a Differential Medium *
A growth medium is considered differential if, when specific microbes are present, the medium or bacterial colonies themselves exhibit a color change that provides information about their identity.
Blood agar (BAP) is a differential growth medium which microbiologists use to distinguish clinically significant bacteria from throat and sputum cultures. BAP contains 5% sheep blood. Certain bacteria produce enzymes (hemolysins) that act on the red blood cells to lyse or break them down.
* Hemolysis Patterns of Blood Agar *
Beta hemolysis means that the bacteria's hemolytic enzymes completely beak down the blood cells. The -hemolysis pattern results in the media displaying clear halos around bacterial colonies.
Alpha hemolysis (-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 colony, indicating incomplete hemolysis.
Gamma hemolysis is essentially no hemolysis at all. The bacteria have no effect on the red blood cells, and there is no change to the color of the medium.
* When Is Blood Agar (BAP) Used? *
Blood agar is usually inoculated from a patient's throat swab, because the medical laboratory is trying to detect the presence of Group A beta hemolytic Streptococci (a Gram-positive round shaped 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.
* Sources *
Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
* This article originally appeared in Suite101 online magazine.