Gram-positive  Staphylococcus viewed  @ 1000xTM

How to Prepare & Heat Fix a Bacterial Smear - P2
Staining Bacteria
Once a bacterial sample is heat fixed to a microscope slide, there are many staining techniques that can be used to view the bacteria. A simple stain is the use of one dye to increase contrast between the specimen and background when viewed through a microscope. 

Differential staining 
procedures are more complex, using a series of dyes to distinguish different types of bacteria based on some chemical or structural attribute of the cell.

Differential staining is often used for general identification of bacteria, but does not allow for identification of the exact species. 

​Page last updated 3/2016

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Staphylococcus viewed 
@ 1000xTM.
Sources & Resources
  • Schauer Cynthia (2007) Lab Manual to Microbiology for the Health Sciences, Kalamazoo Valley Community College.
  • Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.

Tutorial on Compound Light Microscope Parts & Operation
When doing a differential stain it is best to use known positive (+) and negative (-) bacterial controls in addition to the unknown bacteria that is being identified. Controls allow for comparison of the unknown to a known bacteria that tests positive for that stain and another that is a known negative.

The following are examples of bacterial controls that could be used in differential stains:

Gram Stain
  • + controls: StaphylococcusBacillusStreptococcus
  • - controls: Escherichia, Salmonella, Enterobacter 

    Acid-fast Stain
  • + controls: Mycobacteria, Nocardia
  • - controls: any type of bacteria that does not have a waxy cell wall

    Endospore Stain​
  • + controls: Bacillus, Clostridium (Clostridium are pathogens, so not a great choice for a control)
  • - controls:  Any type of bacteria that does not produce endospores

After staining, the bacteria can be viewed using a compound light microscope at a total magnification of 1000xTM. 

About Us
Examples of differential stains include the Gram stainAcid-fast stain and Endospore stain

Three slides of stained bacterial smears. Controls are outside circles. Each unknown is in the center. Slides left to right: Gram stain, acid-fast stain and endospore stain.
Three slides of stained bacterial smears. Controls are outside circles. Each unknown is in the center. Slides left to right: 
Gram stainacid-fast stain and endospore stain.
Microbiology Lab Videos
from the free STEM 
education site 
Science Prof Online
Virtual Microbiology

You have free access to a large collection of materials used in two college-level introductory microbiology courses (8-week & 16-week). The Virtual Microbiology Classroom provides a wide range of free educational resources including PowerPoint Lectures, Study Guides, Review Questions and Practice Test Questions.
Prokaryotic Cell, Mariana Ruiz