Page 45 - MNUmicrobiology practical 2025

General Microbiology & Immunology (PM 401) second level Semester 4 2024/2025

Simple stain

Objective

To study the morphology of the given bacterial culture (bacterial size, shape and
arrangement to differentiate the various bacteria groups)

Principle

• Basic dyes consist of positively charged charomophores.
• Because of the opposite charges they bind to the negatively charged cell wall

components and cellular Nucleic acid to intensify the colour of the organism.

• Thus, a simple staining technique results in a coloured bacterial cell against a
colourless background. Hence, the bacterial shape and its arrangement can be

studied.
• Example of basic dyes safranin, methylene blue, crystal violet

Simple stain preparation requirements

1. Fixed bacterial smear

2. Basic dye: crystal violet & safranin

3. Bunsen burner.

4. Slide holder.

5. Washing water

6. Microscope with oil immersion lens

Steps

1. Put the slide with the fixed smear uppermost on a staining rack over a sink or

staining tray.

2. Thoroughly cover the smear with stain and leave for, usually, 1 minute.

3. Gently wash off the excess stain with a gentle stream of tape water by holding the

slide parallel to the stream
5. Drying till complete evaporation of water.

General Microbiology & Immunology (PM 401) Practical note Page 34