Microscopes & Cultures - Bacterial Colonies Areas (GCSE Biology)

Bacterial Colonies – Areas

Calculating the Colony Size

The colony size is the number of bacteria in a specific population.

To work out the colony size you need to know the mean division time, which is the average time taken for each bacteria to divide to form two daughter bacteria.

This can be used to find out how many times the bacteria have divided in a time period. If both the time period and the mean division time are in the same units (e.g. minutes) then you can find the number of divisions. The following formula can be used:

= Total number of cells in population

= Initial number of cells

n = number of divisions

Example

A bacteria has a mean division time of 20 minutes. How many cells will be present in a population after 4 hours?

• Calculate how many divisions there will be in 4 hours.

4 hours = 4 x 60 = 240 minutes

240 / 20 = 12 divisions

• Substitute the values into the equation.

Nt = N0 x 2n

Nt = 1 x 212

Nt = 4,096 bacterial cells in the population

Calculating the Area of a Bacterial Colony

You can calculate the area of a bacterial colony using the following formula:

Area = πr²

Area = Area of the colony

π = the value of Pi

r = radius (this is half of the diameter)

Example

Calculate the area of the largest bacterial colony.

• Measure the radius of the colony. An easier way to do this is to measure the diameter and half this.
  Diameter = 20mm
  Radius = 20/ 2 = 10mm
• Calculate the area using the above formula.
  Area = πr²
  Area = π x 10²
  Area = 314.2mm²

Calculating the Areas of Zones of Inhibition

You can also use the same method to calculate the areas of the zones of
inhibition.

Example

Compare the effectiveness of antibiotics B and C using their zones of inhibition.

• Measure the radius of B’s zone of inhibition. An easier way to do this is to measure the diameter and half this.
  Diameter = 15mm
  Radius = 15/ 2 = 7.5mm
• Calculate the area using the above formula.
  Area = πr²
  Area = π x 7.5²
  Area = 176.7mm²
• Do the same for C.
  Diameter = 10mm
  Radius = 10/ 2 = 5mm
  Area = πr²
  Area = π x 5²
  Area = 78.5mm²
• Compare your results.
  Antibiotic B is a stronger antibiotic as it produced a 2.25 x larger zone of inhibition than antibiotic C.