Differences between Gram positive and Gram negative bacteria :

• Gram positive bacteria have cell walls composed mostly of a substance unique to bacteria known as peptidoglycan, or murein. These bacteria stain purple after Gram staining.
• Gram negative bacteria have cell walls with only a thin layer of peptidoglycan and an outer membrane with a lipopolysaccharide component not found in Gram positive bacteria. Gram negative bacteria stain red or pink after Gram staining.

Gram Positive Cocci

Gram positive cocci refer to Gram positive bacteria that are spherically shaped. Two genera of Gram positive cocci noted for their role as human pathogens are Staphylococcus and Streptococcus. Staphylococcus are spherical in shape and their cells appear in clusters after they divide. Streptococcus cells appear as long chains of cells after division. Examples of Gram positive cocci that colonize the skin include Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pyogenes.

While all three are part of the normal human microbiota, they can cause disease under certain conditions. Staphylococcus epidermidis form thick biofilms and can cause infections associated with implanted medical devices.Some Staphylococcus aureus strains, such as methicillin-resistant Staphylococcus aureus (MRSA), have become resistant to antibiotics and can lead to the development of serious illness. Streptococcus pyogenes can cause strep throat, scarlet fever, and flesh-eating disease.

Gram Negative Cocci

Gram negative cocci refer to Gram negative bacteria that are spherically shaped. Bacteria of the genus Neisseria are examples of Gram negative cocci that cause disease in humans. Neisseria meningitidis is diplococcus, meaning that its spherical cells remain in pairs after cell division. Neisseria meningitidis causes bacterial meningitis and can also cause septicemia and shock.

Another diplococcus bacterium, N. gonorrhoeae, is the pathogen responsible for the sexually transmitted disease gonorrhea.6 Moraxella catarrhalis is a Gram negative diplococcus that causes ear infections in children, upper respiratory system infections, endocarditis, and meningitis.

Gram negative coccobacillus bacteria have bacterial shapes that are in between spherical and rod shaped. Bacteria of the genus Haemophilus and Acinetobacter are coccobacilli that cause serious infections. Haemophilus influenzae can cause meningitis, sinus infections, and pneumonia. Acinetobacter species cause pneumonia and wound infections.