<P> Gram - negative bacteria are a group of bacteria that do not retain the crystal violet stain used in the gram - staining method of bacterial differentiation . They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane . </P> <P> 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 . </P>

The cell wall of gram-negative bacterial cells is external to the