The main difference between Gram-positive and Gram-negative bacteria is their cell wall, which affects how they react to Gram staining, a way to classify bacteria.

Gram-positive bacteria have a thick wall made of peptidoglycan, a strong material that traps the crystal violet dye during Gram staining. This makes them look purple under a microscope. Their thick wall gives them strength and protection. They don’t have an outer membrane, but some have teichoic acids that help keep their shape and may play a role in causing disease. Common examples are Staphylococcus and Streptococcus.

Gram-negative bacteria have a thinner peptidoglycan wall and an extra outer membrane made of lipopolysaccharides (LPS). In Gram staining, the crystal violet washes away, and they take up a red dye called safranin, making them look pink or red under the microscope. Their outer membrane provides extra protection and makes them less sensitive to some antibiotics. However, this outer layer also has endotoxins that can trigger strong immune reactions in humans. Examples include Escherichia coli and Salmonella.

In short, Gram-positive bacteria have a thick cell wall and look purple, while Gram-negative bacteria have a thinner wall with an extra membrane and look red or pink. This outer membrane also makes Gram-negative bacteria more resistant to certain antibiotics.