why is the bacteria heat fixed to slide using the bunsen burner

this is when doing a gram stain

It so that the bacteria isn't wash away during the staining process. It also kills the bacteria to prevent them from being a danger to the stainer. The heat fixing technique is used in most staining techniques ie TB staining, it however isn't used in the staining of cryptosporidium as it would kill the parasite. You also want to make sure that your smear of the organsim on the slide isn't to thick as it will be washed away even after heat fixing, being to thick can also affect the staining process and prevent the stains from being absorbed properly.

The gram stain differentiates bacteria into 2 easily recognisable group Gram +(purple) and Gram -(pink). The differences between these 2 groups of organism is the thickness of their peptidoglycan layer which is located in the cell wall.

Gram + organisms have thicker pepetidoglycan layers so they retain the first stain which is crystal violet, iodine is added which acts as a mordant, these 2 stains form the crystal violet-iodine complex that is not easily removed by the addition of alcohol/acetone. The counter stain carbol fuscin/neural red is added and can't be absorbed due to the already formed complex. It is simply washed away. Therefore the organisms appear purple under the microscope.

Gram - organism have thinner peptidoglycan layer so the first stain Crystal violet is washed away. So there is no formation of a crystal violet-iodine complex so the last stain Carbol fuscin/neutral red is the one that is absorbed. Therfore the organism appear pink under the microscope.