Streptococcus pneumoniae (strep pneumo) is a common bacterium found in the human respiratory tract. It can cause various infections, from mild to severe. Understanding its characteristics is important for identifying and treating the diseases it causes.
Understanding Hemolysis
Hemolysis refers to the breakdown of red blood cells by bacteria, observed when culturing them in a laboratory. Microbiologists grow bacteria on blood agar, which contains red blood cells, to determine hemolytic activity. The appearance of the agar around bacterial colonies provides visual cues about how the bacteria interact with these blood cells.
There are three primary types of hemolytic patterns. Alpha hemolysis is partial lysis of red blood cells, resulting in a greenish discoloration around bacterial colonies on blood agar. This green color arises from hemoglobin oxidation.
Beta hemolysis involves complete lysis of red blood cells, creating a clear, transparent zone around colonies. Gamma hemolysis describes the absence of hemolysis, meaning there is no change or discoloration in the blood agar. These bacteria do not produce enzymes that break down red blood cells. These distinct visual patterns are valuable initial indicators for bacterial identification.
Streptococcus Pneumoniae and Its Hemolytic Pattern
Streptococcus pneumoniae is an alpha-hemolytic bacterium, a key feature observed when grown on blood agar. Its colonies typically produce a characteristic greenish discoloration in the surrounding agar. This greenish hue is a direct result of the bacteria’s partial lysis of red blood cells.
The partial breakdown of red blood cells by S. pneumoniae involves hydrogen peroxide production, which oxidizes hemoglobin. This process converts red hemoglobin into a greenish compound called methemoglobin. Observing this green zone around colonies is often one of the first indicators of S. pneumoniae presence in a clinical sample.
This alpha-hemolytic pattern is a consistent characteristic of Streptococcus pneumoniae. It helps laboratory professionals narrow down possibilities when identifying bacteria from patient samples. The appearance of these colonies provides valuable preliminary information.
Identifying Streptococcus Pneumoniae in the Lab
Observing alpha hemolysis is an initial step in laboratory identification of Streptococcus pneumoniae. This characteristic helps guide microbiologists toward further specific tests. However, alpha hemolysis alone is not sufficient for definitive identification because other bacteria, such as Streptococcus viridans, also exhibit this pattern on blood agar.
To differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci, additional confirmatory tests are necessary. One is the optochin susceptibility test, where a disk containing optochin is placed on the bacterial culture. S. pneumoniae is susceptible to optochin, meaning its growth is inhibited around the disk, creating a clear zone.
Another confirmatory test is the bile solubility test. This test exploits the property of S. pneumoniae cells to lyse in bile salts. When a bacterial suspension is mixed with bile, S. pneumoniae cells dissolve, resulting in a clear solution, while other alpha-hemolytic streptococci remain turbid. These tests, used with initial alpha hemolysis observation, allow for identification of Streptococcus pneumoniae.