Final answer:
Bacteria that look reddish-pink after Gram staining are classified as Gram-negative due to their thin peptidoglycan cell walls, which do not retain the crystal violet stain and instead take up the safranin counterstain.
Step-by-step explanation:
After Gram staining, any bacterium that looks reddish-pink under the microscope is Gram-negative. This is because Gram-negative bacteria have a thinner peptidoglycan cell wall that does not retain the initial crystal violet stain used during the Gram staining process. During staining, after the application of the crystal violet, iodine is added which forms a complex with the dye making it less soluble. The decolorizer, usually alcohol or acetone, is then applied, which dehydrates the thick peptidoglycan layer in Gram-positive bacteria, trapping the crystal violet stain. However, in Gram-negative bacteria, the thinner peptidoglycan layer cannot retain the stain, and it is washed away. Subsequently, the counterstain safranin is added, which imparts the reddish-pink color to the Gram-negative bacteria, such as Escherichia coli.