Agar plates are the petri dish of legend — a solid jelly of nutrients where bacteria grow into visible colonies. Here's how to make your own and streak them to isolate a single colony.
A single bacterial cell, given food and warmth, divides until millions of descendants form a visible dot — a colony. Because every cell in that colony came from one ancestor, picking a colony gives you a pure population. The trick to getting single, well-separated colonies is the streak plate.
Dissolve LB agar powder in water at the ratio on the pack (typically ~35 g/L). Swirl until no clumps remain.
Autoclave / pressure-cook at 121 °C for 15 minutes, or microwave in short bursts until fully boiling and clear (watch it — it boils over easily). This kills everything in the medium.
Let the agar cool until the bottle is hand-warm (~50 °C). Too hot and it warps the dishes and creates condensation. Too cool and it sets before you pour.
Working near a flame or in a still-air box, lift each lid just enough, pour a thin layer (~15–20 mL, enough to cover the base), and replace the lid. Bubbles? Briefly pass the flame over the surface to pop them.
Leave plates undisturbed 15–30 min to solidify. Store inverted (agar-side up) in a bag in the fridge; let them reach room temperature before use.
Write on the base (not the lid) with the date and sample. Work with the plate lid-down, lifting it clamshell-style only while streaking.
Touch your loop to the sample, then gently zig-zag it across one quarter of the plate. Don't gouge the agar.
Flame the loop (or take a fresh one) and let it cool. Drag it through the edge of quadrant 1 two or three times, then zig-zag into a fresh quarter. This dilutes the cells as you go.
Sterilize again, pull from the previous quadrant into the next. By quadrant 4, so few cells remain that they land far apart — and grow into isolated single colonies.
Flip the plate (agar up, so condensation doesn't drip onto colonies) and incubate at ~30–37 °C. Colonies usually appear in 16–24 hours.
| You see… | Likely cause | Fix |
|---|---|---|
| No separated colonies, just a smear | Loop not sterilized between quadrants | Flame/replace the loop each quadrant so cells actually dilute out. |
| Cracked or dented agar | Loop pressed too hard | Streak lightly. The loop should glide, not dig. |
| Water droplets all over the lid | Poured too hot / incubated upright | Cool agar before pouring; always incubate inverted. |
| Extra colony types you didn't add | Contamination | Revisit sterile technique. Re-sterilize media and tools. |
Grab pre-poured LB agar plates and sterile petri dishes from the shop, or the Microbiology Starter Kit for the full set.