Practical

Question 1

A student used a solution series to investigate the number of cells present in a liquid culture of bacteria

Describe how he made a 1 in 10 dilution and then used this to make a 1 in 1000 dilution of the original liquid culture of bacteria

Answer 1

1. Add 1 part (bacteria) culture to 9 parts (sterile)
   liquid (to make 10^-1
   dilution);
2. Mix (well);
3. Repeat using 9 parts fresh (sterile) liquid and 1
   part of 10^-1
   and 10^-2
   dilutions to make 10^-3
   dilution;
   OR
   Add 1 part 10^-1
   (suspension) to 99 parts (sterile)
   liquid (to make 10^-3
   dilution);

Question 2

The student looked at cells in the 1 in 10 dilution during his preliminary work. He decided not to use this dilution to determine the number of cells in the undiluted liquid culture.

Suggest an explanation for the student’s decision

Answer 2

1. Count unlikely to be accurate / repeatable /
   reproducible / reliable;
2. Because too many cells;
   OR
   Because cells overlapping / not spread out;