Gram-positive bacteria are classified by the color they turn after a chemical called Gram stain is applied to them. Gram-positive bacteria stain blue when this stain is applied to them. Other bacteria stain red. They are called gram-negative. Gram-positive and gram-negative bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.
All bacteria may be classified as one of three basic shapes: spheres (cocci), rods (bacilli), and spirals or helixes (spirochetes). Gram-positive bacteria may be cocci or bacilli. (See figure How Bacteria Shape Up.)
Some Gram-positive bacteria cause disease. Others normally occupy a particular site in the body, such as the skin. These bacteria, called resident flora, do not usually cause disease.
Gram-positive bacilli cause certain infections, including the following:
Gram-positive cocci cause certain infections, including the following:
Gram-positive bacteria are increasingly becoming resistant to antibiotics. For example, methicillin-resistant Staphylococcus aureus (MRSA) bacteria are resistant to most antibiotics that are related to penicillin. Methicillin is a type of penicillin. MRSA strains are commonly involved in infections acquired in health care facilities and can cause infections acquired outside health care facilities (community-acquired infections).
(See also Overview of Bacteria.)