Free medical quiz
Streptococci Quiz
Focused SBA-style practice, followed by the answer and explanation for every question.
- Free now
- 5 questions
- Full bank
- 18 questions
- Answers
- Explained
Free question
Try one with the answer
A question from this quiz. Answer it to see the explanation students get.
Which haemolytic classification describes streptococci that cause partial haemolysis with a greenish discolouration on blood agar?
- A Beta haemolysis causing complete red blood cell lysis with a clear halo You picked ✕
- B Gamma haemolysis with no effect on red blood cells You picked ✕
- C Delta haemolysis, causing partial red blood cell breakdown with a brown discolouration You picked ✕
- D Epsilon haemolysis, causing complete red blood cell breakdown with a yellow halo You picked ✕
- E Alpha haemolysis, causing partial red blood cell breakdown with haemoglobin oxidation You picked ✓
Correct!
Incorrect
The answer is E: Alpha haemolysis, causing partial red blood cell breakdown with haemoglobin oxidation
Explanation
Why E is correct
See Classification of Streptococci. Option E is correct. On blood agar (a culture plate containing red blood cells), alpha haemolysis means the bacteria only partly break down the red cells. The organisms oxidise the iron in haemoglobin (the red oxygen-carrying pigment), and this chemical change leaves a greenish discolouration around the colonies instead of a clear zone. Haemolysis simply means the breaking open of red cells. The alpha-haemolytic group includes Streptococcus pneumoniae and the viridans streptococci; 'viridans' means green. Compare this with beta haemolysis, which is complete breakdown with a clear halo (Streptococcus pyogenes), and gamma haemolysis, which is no breakdown at all (for example Streptococcus bovis). Streptococci can also be described in two other ways that are not about haemolysis: the Lancefield system and the Sherman system. If the stem mentions a colour change on blood agar, think alpha haemolysis straight away.
- Alpha haemolysis: Partial breakdown of red cells, leaving a greenish discolouration instead of a clear zone.
- Cause of the colour: The bacteria oxidise the iron in haemoglobin.
- Examples: Streptococcus pneumoniae and the viridans streptococci, where viridans means green.
Why the other options are incorrect
- A. Beta haemolysis causing complete red blood cell lysis with a clear halo: Beta haemolysis breaks the red cells down completely and leaves a clear halo. A green colour with only partial breakdown is the alpha pattern. Your answer
- B. Gamma haemolysis with no effect on red blood cells: Gamma haemolysis means no breakdown of red cells and no colour change. The green zone in this stem shows that the red cells have been partly changed. Your answer
- C. Delta haemolysis, causing partial red blood cell breakdown with a brown discolouration: Delta haemolysis is not one of the streptococcal haemolysis patterns; the true patterns are alpha, beta and gamma. Your answer
- D. Epsilon haemolysis, causing complete red blood cell breakdown with a yellow halo: Epsilon haemolysis is not a recognised streptococcal haemolysis pattern; only alpha, beta and gamma describe the true effects on blood agar. Your answer
With Premium, questions you miss become Revision Coach flashcards, scheduled for review.
Question preview
What you will practise
More questions from this set. Start the quiz to answer them and see each explanation.
-
1
Under standard blood agar culture conditions, which haemolytic pattern is characteristic of Streptococcus pyogenes?
-
2
Which surface protein helps group A streptococci resist complement-assisted phagocytosis and also promotes attachment to host tissue?
Revise before you test
Review SimpleMed's doctor-written article, then return here to check recall.
Read the topic articleNeed wider coverage?
Explore every Infection topic set, or build a tailored mixed session.
Browse subject quizzes