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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 Overview of Gram-Positive Bacteria 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... read more . 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:

  • They are naturally resistant to certain antibiotics.

  • They acquire genes from bacteria that have become resistant.

  • Their genes mutate.

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