Learning how to count bacterial colonies is really about consistency. Pick a plate you can defend, count in a pattern, and record enough detail to reproduce the result. Here is a practical workflow for agar plates that ends with a CFU number you can use.
1) Choose a countable plate
Scan all dilutions first, then select the plate(s) that land in a countable range. Many lab protocols use 30 to 300 colonies as a practical target. The FDA guidance for aerobic plate count has updated its suitable range to 15 to 300 colonies per plate. Your method may set different limits, so follow the procedure you are working under.
Do a quick quality check:
- Colonies are distinct (not swarming or smeared by moisture).
- No obvious contamination.
- If you plated duplicates, you can average counts from the same dilution.
2) Set up for visibility
Use bright, even lighting and a background that improves contrast. A grid or simple sector lines help you keep your place. For spiral plated samples, use the spiral grid and colony counter within the ring sectors specified for spiral plating so you do not overcount the high density zone.
An illuminated pad and marking pen setup lets you tick each colony once and reduces missed or double counted colonies.
3) Count in a pattern and mark each colony once
Pick one pattern and stick to it on every plate:
- Work left to right, top to bottom (or the reverse).
- Mark each colony on the outside bottom of the plate as you count. This prevents double counting when you get interrupted.
- If the plate is crowded but still countable, count sector by sector and write subtotals.
4) Apply one boundary rule
You need a rule for colonies that sit on a boundary line. A common approach is to count colonies touching the top and left boundaries and exclude colonies touching the bottom and right boundaries. The exact rule matters less than applying it the same way every time.
5) Handle pinpoints, artifacts, and merged colonies
Pinpoint colonies count if they are true growth and your method includes them. If you cannot reliably tell the difference between a pinpoint colony and a bubble, dust, or agar defect, switch to a clearer plate (often a different dilution) rather than guessing.
6) Convert the count to CFU
Record the raw colony count, plus the dilution and plated volume. The raw data is what makes your result traceable.
CFU per mL = colonies ÷ (dilution × volume plated in mL)
Conclusion
bacterial colony counter is part science, part discipline. Pick a plate you can justify, light it well, and move through it the same way every time. Mark as you go, treat borders consistently, and avoid guessing on messy plates. Then turn counts into CFU with your dilution math and record everything.
FAQ
Q1) How do I count bacterial colonies on spiral plates?
A) Count within the defined ring sectors on the spiral grid, starting from the outer ring and working inward.
Q2) Should I count colonies on the plate edge?
A) Yes, but use one consistent boundary rule so you do not overcount across sections.
Q3) What range is best when counting bacterial colonies?
A) Many protocols use 30 to 300 colonies, while the FDA aerobic plate count guidance lists 15 to 300 as suitable. Follow your method if it specifies a different range.
