Skills applied: CP13 microbial growth, CP14 antibiotics

CP13: growth in liquid culture

Method recap (see The bacterial growth curve, Unit 4): inoculate sterile broth with yeast or bacteria, incubate, and sample at intervals.

Measuring population size:

• turbidity with a colorimeter (absorbance rises with cell density; fast, but counts dead cells too);
• haemocytometer counts of cells in a known volume (can use a stain such as methylene blue to tell living from dead yeast);
• dilution plating: serially dilute, spread 0.1 cm³ on agar, count colonies (each from one viable cell) on a plate with roughly 30 to 300 colonies.

Cells per cm³ = colonies ÷ volume plated x dilution factor.

CP14: effect of antibiotics

Method recap (see Core practical 14, Unit 4): spread bacteria evenly on agar (a lawn), add antibiotic discs (or wells) with a control disc, incubate at 25 °C, measure the clear zone.

• Measure two diameters at right angles and average; area = πr².
• Same concentration and volume of antibiotic, same bacterial density, same incubation time and temperature.
• Compare two antibiotics with a t-test on repeats; several concentrations with Spearman's rank.
• Bacteriostatic antibiotics stop growth; bactericidal antibiotics kill bacteria (the January 2024 Unit 6 paper asked about these terms).

Aseptic technique

Flame loops and bottle necks; work near a Bunsen; minimal lid opening; sterile pipettes and spreaders (dip in ethanol, flame, let cool); tape lids; incubate at 25 °C; autoclave everything afterwards.

Worked example

0.1 cm³ of a 10⁻⁵ dilution produces 34 colonies. Calculate the number of bacteria per cm³ in the original culture.

1. Colonies per cm³ of the diluted sample = 34 ÷ 0.1 = 340
2. Multiply by the dilution factor 10⁵
3. = 3.4 x 10⁷

Answer: 3.4 x 10⁷ bacteria per cm³

Check yourself

1. 0.1 cm³ of a 10⁻⁴ dilution gives 56 colonies. The original culture has a x 10⁶ bacteria per cm³. What is a?
   Show answer

   5.6. 56 / 0.1 = 560; x 10⁴ = 5.6 x 10⁶.

2. A haemocytometer square holds 0.004 mm³ and contains on average 12 yeast cells. Calculate cells per mm³.
   Show answer

   3000 cells per mm³. 12 / 0.004 = 3000 cells per mm³.

3. Zones (mm) for antibiotic X: 21, 23, 22, 24, 20; antibiotic Y: 17, 19, 18, 16, 18. Calculate t to 2 decimal places.
   Show answer

   5.05. Means 22.0 and 17.6; s = 1.581 and 1.140; t = 4.4 / √(0.500 + 0.260) = 5.05.

4. For the zone data, t = 5.05 with 8 degrees of freedom (critical value 2.31). What is the conclusion?
   • Accept the null hypothesis: no significant difference
   • Reject the null hypothesis: antibiotic X gives significantly larger zones than Y
   • There is a significant correlation
   • The antibiotics are identical
   Show answer

   Reject the null hypothesis: antibiotic X gives significantly larger zones than Y. 5.05 > 2.31.

5. Why are plates with about 30 to 300 colonies used for counting?
   • Only these plates grow bacteria
   • These plates are sterile
   • Colonies must be over 300 to see them
   • Fewer gives large sampling error; more means colonies overlap and cannot be counted
   Show answer

   Fewer gives large sampling error; more means colonies overlap and cannot be counted. A countable range balances accuracy and practicality.

6. What is a disadvantage of measuring culture growth by turbidity?
   • It takes many days
   • It cannot distinguish living from dead cells
   • It needs no calibration
   • It can only measure fungi
   Show answer

   It cannot distinguish living from dead cells. Dead cells still scatter light.

7. What is a bacteriostatic antibiotic?
   • One that kills bacteria
   • One that only works on viruses
   • One that makes bacteria grow faster
   • One that stops bacteria from growing and dividing without killing them
   Show answer

   One that stops bacteria from growing and dividing without killing them. Bactericidal antibiotics kill; bacteriostatic ones stop growth.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson Examiners' Report WBI16/01, January 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.