Gram-negative bacteria are found everywhere, in virtually all environments on Earth that support life. The gram-negative bacteria include the model organism Escherichia coli, as well as many pathogenic bacteria, such as Pseudomonas aeruginosa, Neisseria gonorrhoeae, Chlamydia trachomatis, and Yersinia pestis. They are an important medical challenge, as their outer membrane protects them from many antibiotics (including penicillin); detergents that would normally damage the peptidoglycans of the (inner) cell membrane; and lysozyme, an antimicrobial enzyme produced by animals that forms part of the innate immune system. Additionally, the outer leaflet of this membrane comprises a complex lipopolysaccharide (LPS) whose lipid A component can cause a toxic reaction when these bacteria are lysed by immune cells. This toxic reaction can include fever, an increased respiratory rate, and low blood pressure — a life-threatening condition known as septic shock.
Several classes of antibiotics have been designed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam-betalactamase combinations (e.g. pipercillin-tazobactam), Folate antagonists, quinolones, and carbapenems. Many of these antibiotics also cover gram positive organisms. The drugs that specifically target gram negative organisms include aminoglycosides, monobactams (aztreonam) and Ciprofloxacin.
Gram-negative bacteria display these characteristics:
Along with cell shape, gram-staining is a rapid diagnostic tool and once was used to group species at the subdivision of Bacteria. Historically, the kingdom Monera was divided into four divisions based on gram-staining: Firmacutes (+), Gracillicutes (−), Mollicutes (0) and Mendocutes (var.). Since 1987, the monophyly of the gram-negative bacteria has been disproven with molecular studies. However some authors, such as Cavalier-Smith still treat them as a monophyletic taxon (though not a clade; his definition of monophyly requires a single common ancestor but does not require holophyly, the property that all descendents be encompassed by the taxon) and refer to the group as a subkingdom "Negibacteria".
Bacteria are traditionally divided into the two groups: gram-positive and gram-negative, based on their gram-staining response. Gram-positive bacteria are also referred to as monoderms having one membrane, and gram-negative bacteria are also referred to as diderms, having two membranes. These groups are often thought of as lineages, with gram-negative bacteria more closely related to one another than to gram-positive bacteria. While this is often true, the classification system breaks down in some cases. A given bacteria's staining result, bacterial membrane organization, and lineage groupings do not always match up. Thus, gram-staining cannot be reliably used to assess familial relationships of bacteria. However, staining often gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane.