Colony Picking: How to Pick the Right Colony, Protocol, and Automated Colony Pickers

A student streaks an overnight plate, needs a pure culture for antibiotic testing, and picks a colony from the crowded center of the plate where growth is heaviest. The culture grows, the tests run, and the results come back contradictory, two organisms' worth of reactions in one tube. Nothing was wrong with the media or the reagents. The pick was the error. In the dense part of the plate, colonies had merged, and a single touch of the loop lifted two organisms, not one. Every downstream step inherited that mistake. Colony picking looks trivial, touch a loop to a dot and move it, but the entire value of the step is choosing the right dot: one colony, well separated, truly pure. Get that wrong and everything after it is built on a mixed culture.

What colony picking is

Colony picking is the selection of a single, isolated colony from a culture plate and its transfer to fresh medium to establish a pure culture. Because one colony arises (ideally) from one original cell, picking a well-isolated colony is how a microbiologist obtains a population descended from a single organism, the starting point for identification, susceptibility testing, storage, and most downstream work.

Colony picking is not the same as colony counting. Counting quantifies how many colonies are on a plate (for example, to calculate colony-forming units), and is done with a colony counter. Picking selects one colony to propagate it. The two tasks, and the instruments, are different; for enumeration, see our article on the colony counter. This article is about picking.

Choosing the right colony

The skill in colony picking is not the physical transfer. It is the choice of which colony to pick. Four things decide a good pick.

Isolation. Pick a colony that is well separated from its neighbors, from a part of the plate where colonies stand alone. In dense or confluent growth, colonies touch and merge, and a single pick can lift more than one organism. This is the mistake in the hook. The isolated streaks near the end of a streak plate exist precisely so you have single colonies to pick.

Purity by appearance. Pick a colony that matches the expected morphology of your target organism, size, shape, edge, elevation, color, surface, and looks uniform. A colony that differs from its neighbors, or that has a contaminating speck growing on or beside it, is a warning sign. Picking the odd colony out, unless the odd one is what you are after, risks propagating a contaminant.

A single, discrete colony. Touch only one colony, and touch its center, not the space between two colonies. Take enough growth to inoculate, but you do not need the whole colony; a light touch of a well-isolated colony is enough.

The right plate. Pick from a fresh plate with well-separated colonies. If the plate is old, overgrown, or has no isolated colonies, restreak for isolation first rather than forcing a pick from confluent growth. A good pick starts with a well-streaked plate.

The one-line rule: pick one isolated colony of the expected appearance, from a fresh plate, touching only its center. Everything downstream depends on this being a single pure organism.

Protocol

The manual workflow, with the judgment folded in:

1. Grow the organism on a suitable agar plate and incubate until colonies are well developed and, ideally, well separated.
2. Examine the plate and select a single, well-isolated colony that matches the expected morphology (see above). Work near a flame or in a biosafety cabinet as appropriate for the organism.
3. Using a sterile inoculating loop, needle, or sterile toothpick, lightly touch the center of the chosen colony to pick up a small amount of growth. Use a fresh sterile tool for each colony to avoid cross-contamination.
4. Transfer the picked growth to fresh medium, a new agar plate (streak for isolation), a broth tube, or a biochemical medium, depending on what comes next.
5. Incubate the new culture under the conditions required for that organism.
6. Store the pure culture or proceed to identification, susceptibility testing, or other downstream work.

The critical aseptic point runs through every step: sterilize the loop between picks (flame to red heat and cool, or use a fresh sterile disposable), so you never carry organisms from one pick to the next.

Automated colony pickers

When the number of colonies to pick becomes large, for example in high-throughput screening, synthetic biology, or clone selection, doing it by hand is slow and tiring, and fatigue introduces errors. An automated colony picker is a robotic instrument that does the same job at scale.

How it works. The instrument photographs the plate, and image-recognition software identifies colonies and selects those matching set criteria (size, shape, separation, sometimes color or fluorescence). A robotic arm then picks each selected colony with a sterile pin or tip and transfers it to destination plates, broth, or multiwell plates, repeating unattended across hundreds or thousands of colonies.

What it does well. It removes the repetitive manual labor, applies consistent selection criteria (reducing operator-to-operator variability), works from a single setup (load the plate, set the criteria), and frees staff for other tasks while it runs. It is not fully walk-away, destination plates and consumables still need checking, but it needs far less continuous attention than manual picking.

When it is worth it. Automated pickers are specialized, expensive research instruments. They earn their place when colony numbers are high and repetitive: large screening campaigns, library construction, or clinical labs with heavy isolate volumes. For routine work on a handful of colonies, manual picking is faster and far cheaper. Commercial systems exist from several manufacturers; check current models and specifications directly, since instrument lines change.

How to Remember

Isolated, expected, single, fresh. The four-word checklist for a good pick. The colony must be isolated (standing alone), of the expected appearance, a single colony (touch its center only), from a fresh plate. Miss any one and you risk a mixed or wrong pick.

One colony, one cell, one culture. The whole reason to pick an isolated colony: it descends from a single cell, so your pure culture is one organism. Pick where colonies merge and you break the chain, one touch, two organisms.

Picking selects, counting quantifies. The distinction students blur. A colony picker takes one colony to grow it. A colony counter tallies how many colonies there are. Different job, different instrument.

Sterilize between every pick. The aseptic rule that protects everything. A loop carried from one colony to the next carries organisms with it. Flame or replace between picks, every time.

Key exam facts in one table

Where Students Get Confused

Colony picking versus colony counting. Picking selects one colony to grow a pure culture. Counting quantifies how many colonies are present, often to calculate colony-forming units. They are different tasks with different instruments.

Which colony to pick. The colony must be well isolated. Picking from the dense center of a plate, where colonies have merged, is the most common way to end up with a mixed culture. Pick from where colonies stand alone.

Why one colony equals a pure culture. A single isolated colony grows (ideally) from one original cell, so everything in it is one organism. That is why picking a single colony, not a smear of several, is what gives purity.

"Take the whole colony." You do not need to. A light touch of a sterile loop to the center of a well-isolated colony picks up plenty of cells. Scraping up a large area risks catching neighboring growth.

Automated pickers are not for every lab. They are specialized, costly instruments that pay off only at high colony volumes. For routine picking of a few colonies, manual technique is faster and cheaper. The machine solves a scale problem, not a skill problem.

References

1. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
2. Procop GW, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.