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Staphylococci and Other Gram-positive Bacteria Quiz
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Which combination best describes enterococci on routine laboratory tests?
- A Alpha-haemolytic diplococci that are often optochin-sensitive and bile-soluble You picked ✕
- B Anaerobic Gram-positive rods that often form endospores You picked ✕
- C Catalase-positive Gram-positive cocci often in irregular clusters You picked ✕
- D Catalase-negative Gram-positive cocci often in pairs or short chains You picked ✓
- E Coagulase-positive Gram-positive cocci often in irregular clusters You picked ✕
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Incorrect
The answer is D: Catalase-negative Gram-positive cocci often in pairs or short chains
Explanation
Why D is correct
See Coagulase-negative Staphylococci and Enterococci. Option D is correct: enterococci are Gram-positive cocci (round bacteria that keep the purple Gram stain) that usually lie in pairs or short chains and are catalase-negative. Catalase is an enzyme that breaks hydrogen peroxide down into water and oxygen, so a drop of peroxide fizzes on a catalase-positive organism such as Staphylococcus but stays still on an enterococcus. Enterococci live in the bowel as part of the normal gut flora (the bacteria that normally live there) and cause infection when they reach other sites, mainly the urinary tract, the bloodstream (bacteraemia) and the heart valves (endocarditis). They also tolerate bile and high salt, which helps laboratories separate them from ordinary streptococci. Pairs and chains alone never prove the genus, because streptococci look similar, so catalase is the first quick screen. A memory aid: staphylococci form clusters and fizz; enterococci and streptococci form pairs and chains and stay still.
- Shape: Gram-positive cocci that usually lie in pairs or short chains.
- Catalase: negative, so hydrogen peroxide does not fizz as it does with Staphylococcus.
- Screening: pairs and chains alone do not prove the genus, so catalase is the first quick screen.
Enterococci, in contrast, grow in the presence of bile salts, so those two clues point away from them.
Why the other options are incorrect
- A. Alpha-haemolytic diplococci that are often optochin-sensitive and bile-soluble: Alpha-haemolytic diplococci that are often optochin-sensitive and bile-soluble describe Streptococcus pneumoniae. Enterococci are not optochin-sensitive diplococci, and unlike pneumococci they grow in the presence of bile salts. Your answer
- B. Anaerobic Gram-positive rods that often form endospores: Anaerobic rods that often form endospores suggest clostridia, not enterococci. Enterococci are cocci and do not produce endospores, the dormant survival structures made by these rods. Your answer
- C. Catalase-positive Gram-positive cocci often in irregular clusters: This describes staphylococci rather than enterococci: catalase-positive Gram-positive cocci often form irregular clusters, whereas enterococci are catalase-negative and usually lie in pairs or short chains. Your answer
- E. Coagulase-positive Gram-positive cocci often in irregular clusters: This describes Staphylococcus aureus rather than enterococci: coagulase-positive Gram-positive cocci often form irregular clusters, while enterococci are catalase-negative and lie in pairs or short chains. Your answer
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