How the plates are made
Each series starts from a hidden concentration. The countable plate gets 30 to 149 colonies; every other plate comes from the same culture scaled by its dilution, and must be over 300 or under 15, so only one dilution counts under any of the three ranges. The colonies drawn are the count in the key, never overlapping, with quadrant lines to count a quarter at a time.
A worked plate count
In set BAM3AP (AP / A-level), six tubes each take 1 mL into 9 mL, and 0.1 mL of tubes 3 to 6 is plated. Plate A is a lawn, plate B has 595 colonies and plate D has 2, so plate C (tube 5, 60 colonies) is the one to count. Tube 5 is 10⁻⁵, so the culture held 60 ÷ (10⁻⁵ × 0.1 mL) = 6.0 × 10⁷ CFU/mL, and its 25 mL held 1.5 × 10⁹ CFU.
What each level includes
High school: mostly 1 mL into 9 mL steps, with an odd 1 mL into 99 mL (and 1 mL into 4 mL in the dilution factor questions), one plate per tube, 1 mL or 0.1 mL plated. AP / A-level adds 0.1 mL into 9.9 mL steps, 1:2 and 1:5 steps and the amount of culture on a plate. College adds duplicate plates to average, odd transfer volumes (0.5 mL into 4.5 mL, 10 mL into 90 mL) and food samples blended 1:10 and reported per gram.
Also here: zone of inhibition practice, measuring clear zones around antibiotic discs and finding their area with πr².
Questions
How do you calculate CFU/mL from a serial dilution?
Count the colonies on the countable plate, then divide by the dilution of its tube times the volume you plated. 60 colonies from a 10⁻⁵ tube with 0.1 mL plated gives 60 ÷ (10⁻⁵ × 0.1) = 6.0 × 10⁷ CFU/mL. Multiply by the volume of the original for the total CFU.
Which plate do you count?
The one inside the countable range. Textbooks use 30 to 300 colonies, APHA uses 25 to 250, and FDA BAM changed to 15 to 300 in March 2025. Plates above the range are too many to count (TNTC); plates below it are too few to count (TFTC) and not reliable. Every set here has exactly one dilution that fits all three.
How do you find the dilution factor of each tube?
Each step is (transfer + diluent) ÷ transfer: 1 mL into 9 mL is 1:10, 0.1 mL into 9.9 mL is 1:100. Multiply the steps to get a tube's total dilution, so three 1:10 steps make 1:1,000 (10⁻³).
Why do you divide by the volume plated?
Because CFU/mL is per millilitre. If only 0.1 mL went on the plate, a whole millilitre would have held ten times as many colonies. Forgetting it makes the answer ten times too small.
How do you round a plate count?
FDA BAM reports two significant figures, rounded only at the end: 1.37 × 10⁷ becomes 1.4 × 10⁷. When the third digit is exactly 5, BAM rounds up after an odd digit and down after an even one. The sets here never land on a case where the two ways differ, so ordinary rounding gives the same answer.
How do you use C1V1 = C2V2?
Put in the three numbers you know and solve for the fourth. To make 50 mL of 10% glucose from a 40% stock, V1 = 10 × 50 ÷ 40 = 12.5 mL of stock, topped up with 37.5 mL of water.
Sources, checked October 24, 2026: FDA, Bacteriological Analytical Manual chapter 3, Aerobic Plate Count (January 2026 edition); OpenStax Microbiology (2016), 9.1 How Microbes Grow; OpenStax Chemistry 2e (2019), 3.3 Molarity; OCR A Level Biology A specification, version 4.1 (April 2026); OCR, Challenging maths skills in A Level Biology (May 14, 2021); AQA AS and A-level Biology 7401/7402 specification, version 1.5 (November 26, 2021, PDF).