Why is it necessary to heat fix bacterial smears prior to staining?

Heat fixing bacterial smears kills the bacterial cells so that they are fixed in place and ready for staining. Heat fixing also adheres the bacterial cells to the slide and allows the sample to take up the stain more easily.

Fixation is a critical step in preparing slides and samples in microbiology, pathology and histology. Fixing bacteria or another type of sample to a slide ensures that the sample will not decay over time and also terminates any type of chemical or biological reactions that could occur on the slide. It preserves cells and tissue samples and allows them to be studied through staining methods at a later date.