Gram-negative bacteria are bacteria which do not turn purple in the Gram staining process used as a basic step in the identification of bacteria. Most bacteria can be divided into either Gram-positive or Gram-negative bacteria, reflecting key differences in the composition of their cell walls. These differences often have a direct influence on what the bacteria does, with some Gram-negative bacteria being pathogenic in nature.
The Gram stain was developed in 1884 by Hans Christian Gram. In this process, bacteria is fixed on a slide and then bathed in crystal violet, the primary staining solution. All of the cells on the slide turn purple, after which a mordant such as iodine is added to fix the color. Then, a decolorizer is added to the slide. If the bacteria is Gram-negative, the decolorizer will wash the crystal violet away, because the permeable cell wall does not allow the crystal violet to stain the bacteria. Then, a secondary stain is added, turning Gram-negative bacteria a pale pink, but having no effect on the already purple Gram-positive bacteria.
These bacteria have thin cell walls with an outer layer composed of proteins and lypopolysaccharide. This outer layer sometimes reacts with the immune system, causing inflammation and infection. In addition to preventing the bacteria from staining, the outer membrane of the cell also helps the bacteria resist an assortment of drugs, making treatment of infections with Gram-negative bacteria rather challenging.
Some examples of Gram-negative bacteria include Legionella, Salmonella, and E. Coli. Numerous other pathogens are also Gram-negative, including some forms of meningitis, a number of bacterial sources of gastrointestinal distress, and spirochetes. Gram-negative bacteria can be stubborn infectious agents, and many sources of lethal infection are Gram-negative, including the bacteria which contribute to secondary infections in hospitals and clinics.
Gram staining can provide insight into the composition of a bacterium's cell wall, so it is a routine step in examining new bacteria in the laboratory. Once bacteria has been subjected to a Gram stain, additional research will be needed to identify the bacteria, the source, and how infections caused by the bacteria might be treated, but the Gram stain provides a good first step. The stain also has the added benefit of highlighting the key structures of bacteria, including the inner structures of the cell, making them easier to see and understand. Gram staining doesn't work on all bacteria, however; Gram-indeterminate and Gram-variable bacteria cannot be identified this way.