What type of bacteria takes up the counterstain during gram staining?

In the context of gram staining, the counterstain is typically safranin, which is used to differentiate between gram-positive and gram-negative bacteria. Gram-negative bacteria are characterized by their thinner peptidoglycan layer and outer membrane, which allows them to take up the counterstain after the initial crystal violet stain is washed out during the staining process.

When a sample is treated with iodine, the crystal violet forms a complex that is retained in gram-positive bacteria due to their thicker peptidoglycan layer. However, when treated with alcohol or acetone, gram-negative bacteria have their outer membrane disrupted, causing them to lose the crystal violet stain. As a result, these bacteria can then absorb the safranin counterstain, appearing red or pink under the microscope, while gram-positive bacteria remain purple due to retention of the crystal violet stain.

This differentiating characteristic is crucial for various diagnostic and treatment purposes in microbiology, as it helps in identifying bacterial species and guiding appropriate antimicrobial therapy.