Microbial Basics

Microbes—including bacteria and fungi—are single‑cell organisms that reproduce rapidly, making them ideal for laboratory study. When a new microbe is isolated from nature, it is first cultured in a liquid medium known as broth, which contains sterilized water, salts, sugars, and other nutrients that support swift bacterial growth in a flask.

Agar Plates: The Solid Growth Medium

Agar, a seaweed‑derived gelatinous substance, is blended with nutrients to form a semi‑solid gel. This medium provides a smooth, inert surface that allows a single bacterium to multiply into a visible colony.

Essential Streaking Tools

Three items are required: an agar plate, a small alcohol lamp (to sterilize), and a sterile wire loop. The loop carries a tiny amount of broth as it is transferred onto the plate.

Step‑by‑Step Streaking Protocol

1. Heat the loop in the alcohol flame until it turns red‑hot; this sterilizes the tip.
2. Dip the loop into the broth, allowing a minute droplet to cling to the tip.
3. Streak the loop back and forth across one quarter of the plate, drawing a continuous line.
4. Re‑sterilize the loop and repeat the streaking perpendicular to the first line, covering another quarter.
5. Continue streaking across new sections, each time sterilizing the loop to progressively dilute the bacterial suspension.
6. The final, lightly streaked area should contain isolated colonies, each derived from a single cell.

Place the plate in an incubator (or at room temperature) and allow 12–24 hours for colonies to develop.

Why This Matters

Accurate isolation is critical for downstream applications—antibiotic screening, mutagenicity assays, and biochemical pathway studies all rely on pure cultures. Mastery of agar streaking ensures reproducible, trustworthy results.