The most commonly isolated gram-positive cocci are best differentiated by their a. both catalase reaction and hemolytic reaction. b. both Gram stain morphology and catalase reaction. c. catalase reaction. d. hemolytic reaction. e. Gram stain morphology.
The correct answer and explanation is:
Correct answer: c. catalase reaction
Explanation:
The most commonly isolated gram-positive cocci are primarily divided into two major genera:
- Staphylococcus
- Streptococcus
Both appear as gram-positive cocci under the microscope, but they can be differentiated most effectively by the catalase reaction, which is a simple biochemical test used to detect the presence of the enzyme catalase.
Catalase Reaction:
- Staphylococci are catalase-positive. They produce the enzyme catalase, which breaks down hydrogen peroxide (H₂O₂) into water and oxygen. When hydrogen peroxide is added to a colony of Staphylococcus, bubbling (oxygen release) is observed.
- Streptococci are catalase-negative. They do not produce catalase, so no bubbling occurs when hydrogen peroxide is added.
This test is quick, easy, and reliable, making it the best single test to differentiate between these two major groups of gram-positive cocci.
Why Not the Other Options?
- a. both catalase reaction and hemolytic reaction: While hemolysis (alpha, beta, gamma) is useful—especially for differentiating types of Streptococcus—it is not required to distinguish Staphylococcus from Streptococcus, which is more fundamental.
- b. both Gram stain morphology and catalase reaction: Although Gram stain confirms gram-positive cocci, it does not differentiate between Staph and Strep, as both look similar microscopically.
- d. hemolytic reaction: Hemolysis patterns are only helpful within Streptococcus species and not for initial differentiation from Staphylococcus.
- e. Gram stain morphology: As stated, both groups appear similar under Gram stain (purple cocci), so this alone is insufficient for differentiation.
Conclusion:
The catalase test is the most useful and commonly used initial method to differentiate Staphylococcus (catalase-positive) from Streptococcus (catalase-negative), making option c the correct answer.