Overview
This lecture explains the streak plate technique for isolating pure bacterial cultures, covering aseptic methods, streaking steps, incubation, and interpreting colony results.
Purpose of the Streak Plate Technique
- The streak plate technique is used to obtain pure cultures of bacteria.
- It separates individual bacterial cells on a solid nutrient agar surface.
Preparation and Labeling
- Label the bottom of the Petri plate with initials, date, medium type, and source of inoculum.
- Labeling the bottom ensures correct identification even if lids are switched.
Aseptic Inoculation Steps
- Use a loopful of bacterial culture (e.g., Staphylococcus aureus) to inoculate the plate.
- Hold the loop like a pencil, flame it until red-hot, and let it cool before use.
- Flame the mouth of the culture tube before and after obtaining the inoculum.
Four-Quadrant Streak Method
- Streak the first quadrant with as many close streaks as possible, keeping the loop in contact with agar.
- Flame and cool the loop between each quadrant to prevent cross-contamination.
- Streak from previous to next quadrant with only a tiny fraction of bacteria each time.
- Avoid touching the first quadrant when streaking into the center of the plate.
Incubation and Results
- Incubate the plate inverted at 35–37°C for at least 24 hours to prevent condensation on the agar.
- Proper incubation allows discrete colonies to form, each from a single cell or group.
- Different bacterial species produce colonies with specific shapes, colors, and sizes.
- Pure cultures yield identical colonies; mixed cultures show colonies with different appearances.
Key Terms & Definitions
- Pure culture — a population of identical bacteria derived from a single cell.
- Aseptic technique — procedures that prevent contamination by unwanted microorganisms.
- Colony — a visible mass of bacteria originating from one cell or a group.
- Inoculum — the sample of bacteria transferred to the agar.
- Four-quadrant streak — sequential streaking method to isolate bacteria.
Action Items / Next Steps
- Record all data for colony appearance and numbers.
- Follow your instructor's instructions for disposal or further incubation of plates and tubes.