Lab 4: Simple Staining | BIOS242 Week 2 | Microbiology Lab Guide

Learning Objectives:

By the end of this lab, students will be able to:

• Understand the principles of staining in microbiology.
• Perform a simple stain procedure to enhance specimen visibility.
• Prepare wet mount slides for microscopic observation.

Overview:

In light microscopy, stains are used to increase contrast between specimens and their background, improving visualization—especially for transparent microorganisms like bacteria. Stains contain chromophores that impart color, and they bind to cellular structures based on charge interactions.

• Basic stains (positively charged) bind to negatively charged cell structures (e.g., nucleic acids, cell walls).
• Acidic stains (negatively charged) bind to positively charged cellular components.

Common basic stains include Crystal Violet, Safranin, and Methylene Blue.

Simple staining uses a single stain to highlight the size, shape, and arrangement of bacteria, whereas differential staining uses multiple stains to distinguish specific bacterial features.

Lab Activities:

• Perform simple staining on bacterial and yeast cultures.
• Observe stained specimens under a microscope.
• Prepare wet-mount slides of yeast and cheek cells for live-cell observation.

Materials Required:

• Bacterial cultures: S. epidermidis, B. subtilis, S. cerevisiae (yeast)
• Inoculating loop, glass slides, staining reagents (Crystal Violet or instructor-selected stain)
• Incinerator, staining trays, microscope with immersion oil
• Lens paper, bibulous paper, tongs, and PPE

Lab Safety Notes:

• Always wear gloves and PPE before starting the experiment.
• Use aseptic techniques to prevent contamination.
• Apply only 1-2 drops of stain per slide to avoid excess staining.