Overview
This lecture covers the process of preparing a smear of microorganisms on a slide for microscopic examination, including sterilization, sample transfer, air drying, and heat fixing.
Purpose of Smear Preparation
- Smears are thin preparations of microorganisms on a slide for microscopic observation.
- Smears allow staining, which helps identify organisms and perform diagnostic (differential) stains.
- Stained smears can indicate whether an organism is dangerous or its biological family.
Tools and Sterilization
- The wire loop is the primary tool for transferring microorganisms in microbiology labs.
- The loop is sterilized by flaming it with a Bunsen burner or an incinerator until red hot.
- Allow the wire loop to cool before transferring organisms to avoid killing the cells.
Sample Transfer Techniques
- For liquid cultures, transfer two loopfuls of organism to the center of the slide and spread thinly.
- For solid cultures, add two loopfuls of water to the slide, then add a clump of bacteria and break it up.
Air Drying and Heat Fixing
- Allow the smear to air dry completely before heat fixing to prevent cell destruction and ensure cells adhere to the slide.
- Heat fixing can be done by passing the slide through a Bunsen burner flame six times for about one second each time.
- Proper heat fixing kills the cells but preserves their structure for staining.
Staining and Observation
- Once heat fixed, the smear is ready for staining with techniques like Gram staining to observe organisms under a microscope.
Key Terms & Definitions
- Smear — A thin layer of microorganisms spread on a microscope slide.
- Wire loop — A metal tool used to transfer microorganisms, sterilized by heating.
- Aseptic technique — Procedures used to prevent contamination by unwanted microorganisms.
- Heat fixing — The process of killing and attaching organisms to a slide by heating.
- Gram stain — A common differential staining technique used to classify bacteria.
Action Items / Next Steps
- Practice smear preparation using both liquid and solid cultures.
- Review steps for air drying and correct heat fixing procedure.
- Prepare for upcoming lab session on staining techniques.