Gram-negative bacteria

Microscopic image of gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)

Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation.[1] Their defining characteristic is their cell envelope, which consists of a thin peptidoglycan cell wall sandwiched between an inner (cytoplasmic) membrane and an outer membrane.[2] 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 antibiotics (including penicillin), 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 leaflet of this membrane contains a complex lipopolysaccharide (LPS) whose lipid A component can trigger a toxic reaction when the bacteria are lysed by immune cells. This reaction may lead to septic shock, resulting in low blood pressure, respiratory failure, reduced oxygen delivery, and lactic acidosis.[3]

Several classes of antibiotics have been developed to target gram-negative bacteria, including aminopenicillins, ureidopenicillins, cephalosporins, beta-lactam-betalactamase inhibitor combinations (such as piperacillin-tazobactam), folate antagonists, quinolones, and carbapenems. Many of these antibiotics also cover gram-positive bacteria. The antibiotics that specifically target gram-negative organisms include aminoglycosides, monobactams (such as aztreonam), and ciprofloxacin.

  1. ^ Baron S, Salton MR, Kim KS (1996). "Structure". In Baron S (ed.). Medical Microbiology (4th ed.). University of Texas Medical Branch at Galveston. ISBN 978-0-9631172-1-2. PMID 21413343. Archived from the original on 2021-07-06. Retrieved 2021-08-18.
  2. ^ Silhavy, T. J.; Kahne, D.; Walker, S. (2010-05-01). "The Bacterial Cell Envelope". Cold Spring Harbor Perspectives in Biology. 2 (5): a000414. doi:10.1101/cshperspect.a000414. ISSN 1943-0264. PMC 2857177. PMID 20452953.
  3. ^ Pelletier, Lawrence L. (1996). "Microbiology of the Circulatory System". In Baron S (ed.). Medical Microbiology (4th ed.). University of Texas Medical Branch at Galveston. ISBN 978-0-9631172-1-2. PMID 21413321. Archived from the original on 2022-04-13. Retrieved 2021-05-12.