A streak plate is a method in microbiology used to separate microorganisms on an agar surface so that discrete colonies can be obtained from a mixed sample.
Explanation
The technique involves dragging an inoculating loop or swab loaded with a microbial sample across a solid growth medium in a pattern that successively dilutes the inoculum. Typically, the loop is flamed or replaced between streaks and the plate is rotated to create separate regions. As the concentration of cells decreases, individual cells are deposited far enough apart to grow into isolated colonies. Isolated colonies are critical for obtaining pure cultures, which allow precise identification and downstream analyses such as biochemical testing, antibiotic susceptibility and genetic studies. Variations of streaking include the quadrant streak, T‑streak and zig‑zag streak. Proper aseptic technique, including sterilization of the loop and avoiding re‑entry into previous streaks, reduces contamination. The method is simple and inexpensive, making it a staple in teaching and research laboratories.
Applications and examples
Streak plates are used to isolate Escherichia coli from water by streaking the sample across a nutrient agar plate, flaming the loop between quadrants. A clinician may streak a throat swab onto blood agar to isolate Streptococcus pyogenes and observe hemolysis patterns. Pure colonies obtained from streak plating can be subcultured to slants for storage or used in biochemical assays. Antibiotic‑producing organisms can be isolated by streaking soil samples onto selective media. Another common application is streaking a mixed culture onto MacConkey agar to differentiate lactose fermenters from non‑fermenters.
The streak plate technique remains a fundamental skill in microbiology for obtaining isolated colonies from mixed samples. Mastery of streak plating and adherence to aseptic practices allow accurate identification and characterization of microorganisms.
Related Terms: Isolation streak, Pure culture, Inoculating loop, Agar, Colony morphology