1. why do you count plates with 20-200 colonies in the spread-plate technique an
ID: 219327 • Letter: 1
Question
1. why do you count plates with 20-200 colonies in the spread-plate technique and plates with 30-300 in the pour plate technique?
2. Are colonies likely to rise from clumps of bacteria rather than single cells? What is meant by the term colony-forming unit (CFU)?
3. why must pipets be changed between dilutions?
QUESTIONS (25 Marks) 1. Why do you count plates with 20-200 colonies in the spread-plate technique and plates with 30- 300 colonies in the pour plate technique? (10 marks) 2. Are colonies likely to arise from clumps of bacteria rather than from single cells? What is meant by the term colony-forming unit (CFU)? (10 marks) 3. Why must pipets be changed between dilutions? (5 marks)Explanation / Answer
1. In pour plate method the innoculum is mixed with the liquid media which is then poured over the plate, upon solidification, the bacteria are distributed throughout the agar and not just on the surface of the agar, which is why a higher range has been specifiec for pour plate. The colonies are visible throughout the depth of the agar. Another reason is that more innoculum volume is used for pour plate method than the spread plate method.
2.CFU refers to the number of viable cells which are able to multiply and grow. in light of this definition, colonies are likely to form by a single cell rather than clumps of different bacteria
3. To measure volumes accurately as each pipette has a particular range and cannot measure all volumes that must be required for dilutions
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