What is the main purpose of the Gram stain?

The primary goal of the Gram stain is to differentiate bacterial species based on the structural differences in their cell walls. This staining technique allows for the classification of bacteria into two main groups: Gram-positive and Gram-negative.

Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain used in the process, appearing blue or purple under a microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which does not retain the crystal violet but takes up the counterstain (safranin), appearing pink or red. This differentiation is crucial in microbiology as it not only assists in the identification of bacterial species but also informs treatment options, as Gram-positive and Gram-negative bacteria often respond differently to antibiotics.

The other choices do not accurately represent the main function of the Gram stain. Identifying viruses is not relevant because the Gram stain specifically targets bacteria. Similarly, while the Gram stain can provide some information about cell structure, it is not primarily used for counting cells or assessing bacterial metabolism.