Final answer:
The observation of β-hemolysis on a TSA plate with 2% sheep blood is a transition from red to clear around bacterial colonies as red blood cells are completely lysed. The color change to yellow typically indicates an acid reaction, not the result of hemolysis.
Step-by-step explanation:
The question is asking about the observation of hemolysis on a TSA (Tryptic Soy Agar) plate containing 2% sheep blood (SB) after the growth of bacteria. Hemolysis refers to the breakdown of red blood cells and is visible around bacterial colonies growing on blood agar plates. The correct answer to this question would be that the color change observed with β-hemolysis is a transition from red (the color of the blood agar) to clear around the colonies due to the complete lysis of red blood cells.
In β-hemolysis, which is also evidenced in Figure 22.7, hemolytic Streptococcus bacteria completely destroy red blood cells, making the normally opaque red agar turn transparent or clear around the colonies. However, no actual color is mentioned in β-hemolysis, which causes the confusion. The typical appearance of a yellow change is associated with an acid reaction, not with the effect of hemolysis on blood agar, according to the testing guidelines provided.