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Overview of Gram-Negative Bacteria

By

Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Feb 2020| Content last modified Feb 2020
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Gram-negative bacteria are classified by the color they turn after a chemical process called Gram staining is used on them. Gram-negative bacteria stain red when this process is used. Other bacteria stain blue. They are called gram-positive bacteria. Gram-negative and gram-positive 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.

Gram-negative bacteria are enclosed in a protective capsule. This capsule helps prevent white blood cells (which fight infection) from ingesting the bacteria. Under the capsule, gram-negative bacteria have an outer membrane that protects them against certain antibiotics, such as penicillin. When disrupted, this membrane releases toxic substances called endotoxins. Endotoxins contribute to the severity of symptoms during infections with gram-negative bacteria.

Gram-negative bacterial infections include the following:

Gram-negative bacteria can cause many serious infections, such as pneumonia, peritonitis (inflammation of the membrane that lines the abdominal cavity), urinary tract infections, bloodstream infections, wound or surgical site infections, and meningitis.

Gram-negative bacteria are increasingly becoming resistant to antibiotics. Bacteria may be resistant because of any of the following:

  • They are naturally resistant to certain antibiotics.

  • They acquire genes from bacteria that have become resistant.

  • Their genes mutate.

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