Streptococcus pneumoniae is a common bacterium residing in the human upper respiratory tract, often without causing illness. However, it can become pathogenic and lead to various infections.
Understanding Hemolysis
Hemolysis is the process where bacteria break down red blood cells, observed on blood agar. This ability helps microbiologists classify and identify bacteria. The breakdown occurs due to substances called hemolysins produced by the bacteria.
Alpha-hemolysis (α-hemolysis) is a partial breakdown of red blood cells, appearing as a greenish discoloration around bacterial colonies. Beta-hemolysis (β-hemolysis) is a complete lysis, resulting in a clear, transparent zone. Gamma-hemolysis (γ-hemolysis) indicates no hemolysis, with no change in the blood agar.
Streptococcus pneumoniae and Alpha Hemolysis
Streptococcus pneumoniae is an alpha-hemolytic bacterium. When grown on blood agar, colonies of S. pneumoniae are typically surrounded by a zone of greenish discoloration. This appearance is sometimes referred to as “green hemolysis” or “partial hemolysis.”
The greenish color results from the bacterium’s production of hydrogen peroxide. This hydrogen peroxide acts on the hemoglobin within the red blood cells, oxidizing it into a greenish derivative called methemoglobin.
Unlike some other hemolytic bacteria that fully lyse red blood cells, S. pneumoniae’s alpha-hemolysis is primarily due to this oxidative process rather than complete cellular breakdown. The red blood cell membranes typically remain intact.
Why Hemolysis Matters for Identification
Observing hemolysis patterns is a foundational step in identifying bacteria in a microbiology laboratory. For Streptococcus pneumoniae, its alpha-hemolytic characteristic on blood agar provides a significant preliminary clue for microbiologists. This helps differentiate it from other bacteria.
While alpha-hemolysis is a strong indicator, it is usually not sufficient for a definitive identification of S. pneumoniae. Other alpha-hemolytic streptococci, such as those belonging to the viridans group, can exhibit a similar greenish discoloration. Therefore, further testing is necessary to confirm S. pneumoniae identity and distinguish it from these similar organisms.
Other Ways to Identify Streptococcus pneumoniae
Beyond observing alpha-hemolysis, microbiologists employ additional tests to confirm the identification of Streptococcus pneumoniae. Two important tests are optochin sensitivity and bile solubility. These tests help differentiate S. pneumoniae from other alpha-hemolytic streptococci that might be present.
Optochin Sensitivity
The optochin sensitivity test involves placing a small disk impregnated with the chemical optochin (ethylhydrocupreine hydrochloride) onto a blood agar plate inoculated with the bacterial sample. S. pneumoniae is typically sensitive to optochin, meaning its growth is inhibited around the disk, creating a clear zone. Most other alpha-hemolytic streptococci are resistant to optochin, so they continue to grow up to the disk.
Bile Solubility
The bile solubility test also aids in identification. This test relies on the unique property of S. pneumoniae cells to undergo rapid lysis in the presence of bile salts, such as sodium deoxycholate. When a drop of bile salt solution is applied to a colony of S. pneumoniae on blood agar, the colony will dissolve or disappear. This occurs because S. pneumoniae possesses an autolytic enzyme that is activated by bile salts, leading to the breakdown of its own cells. Other alpha-hemolytic streptococci generally do not exhibit this bile solubility.