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Plasma membrane found in gram negative bacteria

The bacterial outer membrane is found in gram negative bacteria. Gram negative bacteria form two lipid bilayers in their cell envelopes an inner membrane (IM) that encapsulates the cytoplasm, and an outer membrane (OM) that encapsulates the periplasm. The composition of the outer membrane is distinct from that of the inner cytoplasmic cell membrane among other things, the outer leaflet of the outer membrane of many gram negative bacteria includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin and in some bacteria such as E. coli it is linked to the cell's peptidoglycan by Braun's lipoprotein. Porins can be found in this layer.

Outer membrane proteins are membrane proteins with key roles associated with bacterial cell structure and morphology; cell membrane homeostasis; the uptake of nutrients; protection of the cell from toxins including antibiotics; and virulence factors including adhesins, exotoxins, and biofilm formation. There are a number of outer membrane proteins that are specifically virulence related. Outer membrane proteins consist of two major classes of protein transmembrane proteins and lipoproteins. The transmembrane proteins form channels or pores in the membrane called porins, and actively pumping efflux channels. The outer membranes of a bacterium can contain a huge number of proteins. In E. Coli for example there are around 500,000 in the membrane. O antigen repeat units are then polymerised in the periplasm by the Wzy polymerase and ligated to the lipid A core moiety by the WaaL ligase. The LPS transport machinery is composed of LptA, LptB, LptC, LptD, LptE. This supported by the fact that depletion of any one of these proteins blocks the LPS assembly pathway and results in very similar outer membrane biogenesis defects. Moreover, the location of at least one of these five proteins in every cellular compartment suggests a model for how the LPS assembly pathway is organised and ordered in space.

If lipid A, part of the lipopolysaccharide, enters the circulatory system it causes a toxic reaction by activating toll like receptor TLR 4. Lipid A is very pathogenic and not immunogenic. However, the polysaccharide component is very immunogenic, but not pathogenic, causing an aggressive response by the immune system. The sufferer will have a high temperature and respiration rate and a low blood pressure. This may lead to endotoxic shock, which may be fatal. The bacterial outer membrane is physiologically shed as the bounding membrane of outer membrane vesicles in cultures, as well as in animal tissues at the host–pathogen interface, implicated in translocation of gram negative microbial biochemical signals to host or target cells.