Gram-positive bacterium , any of various types of bacteria that are characterized by having a thick peptidoglycan cell wall and by the absence of an outer membrane composed of lipopolysaccharide. Learn about the difference between Gram positive and Gram negative bacteria , including why you need to be able to tell them apart. Gram-positive bacteria are a group of bacteria that retain the crystal violet stain in the Gram stain test , appearing purple under a microscope. They have a thick peptidoglycan layer in their cell wall, which contributes to their ability to retain the stain. Learn how to identify gram - positive bacteria by their color and shape under a microscope. Find out the common types of gram - positive bacteria and the infections they can cause, such as pneumonia, strep throat, and anthrax.
