Outer membrane proteins

— OmpA (outer membrane protein A) and the murein lipoprotein form a bond between outer membrane and murein.

— Porins, proteins that form pores in the outer membrane, allow passage of hydrophilic, low-molecular-weight substances into the periplasmic space.

— Outer membrane-associated proteins constitute specific structures that enable bacteria to attach to host cell receptors.

— A number of Omps are transport proteins. Examples include the LamB proteins for maltose transport and FepA for transport of the siderophore ferric (Fe3+) enterochelin in E. coli (see also p. 13).

■ Lipopolysaccharide (LPS). This molecular complex, also known as endotoxin, is comprised of the lipoid A, the core polysaccharide, and the O-specific polysaccharide chain (Fig. 3.12).

— The Lipopolysaccharide Complex

Lipid

Lipoid A

- Diglucosamine

- Fatty acids

Fatty acids Diglucosamine Phosphate-

Core polysaccharide

Lipid

Lipoid A

- Diglucosamine

- Fatty acids

Different Different sugars sugars

(heptoses) (heptoses)

3-deoxy-octulosonic acid)

Fatty acids Diglucosamine Phosphate-

Inner core region

Outer core region

O-specific polysaccharide chain

Repeating units consisting of 3-8 sugars

(XXXXXX)

Different Different sugars sugars

(heptoses) (heptoses)

3-deoxy-octulosonic acid)

Fig. 3.12 The three-part lipopolysaccharide complex (LPS) of Gram-negative bacteria is anchored in the outer membrane by means of its lipid moiety. LPS is also known as endotoxin.

— Lipoid A is responsible for the toxic effect. As a free substance, or bound up in the LPS complex, it stimulates—by binding together with the LPS binding protein (LBP) to the CD14 receptor of macrophages—the formation and secretion of cytokines that determine clinical endotoxin symptomatology. Interleukin 1 (1L-1) and tumor necrosis factor (TNF) induce an increased synthesis of prostaglandin E2 in the hypothalamus, thus setting the "thermostat" in the temperature control center higher, resulting in fever. Other direct and indirect endotoxin effects include granulopoiesis stimulation, aggregation and degeneration ofthrombocytes, intravasal coagulation due to factor VII activation, a drop in blood pressure, and cachexia. LPS can also activate the alternative complement pathway. Release of large amounts of endotoxin can lead to septic (endotoxic) shock. Endotoxin is not inactivated by vapor sterilization. Therefore, the parent materials used in production of parenteral pharmaceuticals must be free of endotoxins (pyrogens).

— The O-specific polysaccharide chain is the so-called O antigen, the fine chemical structure of which results in a large number of antigenic variants useful in bacterial typing (e.g., detailed differentiation of salmonella types) (see p. 284f.).

L-forms (L = Lister Institute). The L-forms are bacteria with murein defects, e.g., resulting from the effects of betalactam antibiotics. L-forms are highly unstable when subjected to osmotic influences. They are totally resistant to betalactams, which block the biosynthesis of murein. The clinical significance of the L-forms is not clear. They may revert to the normal bacterial form when betalactam therapy is discontinued, resulting in a relapse.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment