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Line 4: '''Gram-negative bacteria''' are [[bacteria]] that do not retain the [[Crystal violet|crystal violet stain]] used in the [[Gram stain]]ing method of bacterial differentiation.<ref name=Baron>{{cite book| pmid = 21413343| vauthors = Baron S, Salton MR, Kim KS| chapter = Structure| title = Medical Microbiology| editor = Baron S| edition = 4th| publisher = University of Texas Medical Branch at Galveston| year = 1996| chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK8477/| isbn = 978-0-9631172-1-2| access-date = 2021-08-18| archive-date = 2021-07-06| archive-url = https://web.archive.org/web/20210706013613/https://www.ncbi.nlm.nih.gov/books/NBK8477/| url-status = live}}</ref> Their defining characteristic is their [[Cell envelope|cell envelopes]], which consists of a thin [[peptidoglycan]] [[cell wall]] sandwiched between an inner ([[Cytoplasm|cytoplasmic]]) [[Cell membrane|membrane]] and an [[Bacterial outer membrane|outer membrane]].{{cn|date=November 2023}} These bacteria are found in all environments that support life on [[Earth]]. Within this category, notable species include the [[model organism]] ''[[Escherichia coli]]'', along with various [[pathogenic bacteria]], such as ''[[Pseudomonas aeruginosa]]'', ''[[Chlamydia trachomatis]]'', and ''[[Yersinia pestis]]''. They pose significant challenges in the medical field due to their outer membrane, which acts as a protective barrier against numerous [[Antibiotic|antibiotics]] (including [[penicillin]]), [[Detergent|detergents]] that would normally damage the inner cell membrane, and the [[antimicrobial]] enzyme [[lysozyme]] produced by animals as part of their [[innate immune system]]. Furthermore, the outer [[Lipid_bilayer|leaflet]] of this membrane contains a complex [[lipopolysaccharide]] (LPS) whose [[lipid A]] component can trigger a toxic reaction when the bacteria are [[Lysis|lysed]] by immune cells. This reaction may lead to [[septic shock]], resulting in [[hypotension|low blood pressure]], [[respiratory failure]], [[hypoxia (medical)|reduced oxygen delivery]], and [[lactic acidosis]].<ref name="ww.ncbi.nlm.nih.gov">{{cite book |pmid=21413321 |last1=Pelletier |first1=Lawrence L. |editor=Baron S |title=Medical Microbiology |date=1996 |publisher=University of Texas Medical Branch at Galveston |isbn=978-0-9631172-1-2 |edition=4th |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK8290/ |chapter=Microbiology of the Circulatory System |access-date=2021-05-12 |archive-date=2022-04-13 |archive-url=https://web.archive.org/web/20220413142416/https://www.ncbi.nlm.nih.gov/books/NBK8290/ |url-status=live }}</ref> Several [[Antibiotics#Classes|classes of antibiotic]]s have been developed to target gram-negative bacteria, including [[aminopenicillin]]s, [[ureidopenicillin]]s, [[cephalosporin]]s, [[beta-lactam]]-[[Β-Lactamase inhibitor|betalactamase inhibitor]] combinations (such as [[Piperacillin/tazobactam|piperacillin-tazobactam]]), [[Antifolate|folate antagonist]]s, [[quinolone]]s, and [[carbapenem]]s. Many of these antibiotics also cover [[Gram-positive bacteria|gram-positive]] bacteria. The antibiotics that specifically target gram-negative organisms include [[aminoglycoside]]s, [[monobactam]]s (such as [[aztreonam]]), and [[ciprofloxacin]]. | |||