What are Gram-Positive and Gram-Negative Bacteria?
Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain, appearing purple under a microscope. Examples include *Staphylococcus aureus* and *Streptococcus pneumoniae*. Gram-negative bacteria, such as *Escherichia coli* and *Salmonella*, have a thinner peptidoglycan layer and an outer membrane, appearing pink after counterstaining with safranin.
