The slides need to be heat fixed during the direct stain but not during the negative stain because heat fixing is used to kill the bacteria on the slides so the bacteria does not spread.
Heat fixation is not used in negative staining because the main aim is the experiment to view bacteria that has not been distorted by harsh staining or heat fixing.Heat can fix the shrink cells. Mostly bacterial capsule are composed of polysaccharide , some genera produce polypeptide capsules that material is very moist and heating will caused to shrink it.
It is ensure that the elimination of contaminating organisms from the smear preparation. The cells to the slide and cognates the bacterial proteins, effectively killing the bacteria.
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