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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Enterobacteriaceae

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Escherichia Coli - Bacteriology

Most strains of E. coli ferment lactose rapidly and produce indole. These and other bio- chemical reactions are sufficient to separate it from the other species.

ESCHERICHIA COLI

BACTERIOLOGY

CLASSIFICATION

Most strains of E. coli ferment lactose rapidly and produce indole. These and other bio- chemical reactions are sufficient to separate it from the other species. There are over 150 distinct O antigens and a large number of K and H antigens, all of which are designated by number. The antigenic formula for serotypes is described by linking the letter (O, K, or H) and number of the antigens present (eg, O111:K76:H7).

PILI

Pili (also called fimbriae) are frequently present on the surface of E. coli strains. Research has shown that some of these structures play a role in virulence as mediators of attach-ment to human epithelial surfaces. Pili show marked tropism for different epithelial cell types, which is determined by the availability of their specific receptor on the host cell surface. Most E. coli express type 1 (common) pili. Type 1 pili bind to the D-mannose residues commonly present on epithelial cell surfaces and thus mediate binding to a wide variety of cell types.

More specialized pili are found in subpopulations of E. coliP pili (also called Pap or Gal – Gal) bind to digalactoside (Gal – Gal) moieties present on certain mammalian cells, including uroepithelial cells and erythrocytes of the P blood group. Other pili bind to intestinal cells and have their own set of specificities. Those binding to human entero-cytes are called colonization factor antigens (CFAs) or bundle-forming pili (BFP), depending on the pathogenic type of E. coli involved and possibly the cell type in the gastrointestinal tract. The specific binding receptors for the enterocyte binding pili are not known.

The genetics of pilin expression is complex. The genes are organized into multi-cistronic clusters that encode structural pilin subunits and regulatory functions. Pili of different types may coexist on the same bacterium, and their expression may vary under different environmental conditions. Type 1 pilin expression can be turned “on” or “off” by inversion of a chromosomal DNA sequence containing the promoter responsible for initiating transcription of the pilin gene. Other genes control the orientation of this switch.

TOXINS

E. coli can produce every kind of toxin found among the Enterobacteriaceae. These in-clude a pore-forming cytotoxin, inhibitors of protein synthesis, and a number of toxins that alter messenger pathways in host cells.

The α-hemolysin is a pore-forming cytotoxin that inserts into the plasma membrane of a wide range of host cells in a manner similar to streptolysin O  and Staphylococcus aureus α-Toxin . The toxin causes leakage of cytoplasmiccontents and eventually cell death.

Shiga toxin is named for the microbiologist who discoveredShigella dysenteriae,andthis toxin was once believed to be limited to that species. It is now recognized to exist in at least two molecular forms released by multiple E. coli and Shigella strains on lysis of the bacteria. In the years following the discovery of this toxin, the term Shiga toxin was reserved for the original toxin, and others were called Shiga-like. In this discussion, the term Shiga toxin will be used for all the molecular variants that have the same mode of action. Shiga toxins are of the AB type. The B unit directs binding to a specific glycolipid receptor (Gb3) present on eukaryotic cells and is internalized in an endocytotic vacuole. Inside the cell, the A subunit crosses the vacuolar membrane in the trans-Golgi network, ex-its to the cytoplasm, and enzymatically modifies 28S-ribosomal RNA of the 60S-ribosomal subunit by removing an adenine base. This prevents the elongation-factor-1 – dependent binding of amino acyl tRNA to the ribosome blocking protein synthesis, leading to cell death.

Labile toxin (LT) is also an AB toxin. Its name relates to the physical property ofheat lability, which was important in its discovery, and contrasts with the heat-stable toxin described below. The B subunit binds to the cell membrane, and the A subunit catalyzes the ADP-ribosylation of a regulatory G protein located in the membrane of the intestinal epithelial cell. This inactivation of part of the G protein causes permanent activation of the membrane-associated adenylate cyclase system and a cascade of events, the net effect of which depends on the biological function of the stimulated cell. If the cell is an enterocyte, the result is the stimulation of chloride secretion out of the cell and the block-age of NaCl absorption. The net effect is the accumulation of water and electrolytes intothe bowel lumen. The structure and biological effect of LT is very similar to choleratoxin.

Stable toxin (ST) toxin is a small (17- to 18-amino acid) peptide that binds to a gly-coprotein receptor, resulting in the activation of a membrane-bound guanylate cyclase. The subsequent increase in cyclic GMP concentration causes an LT-like net secretion of fluid and electrolytes into the bowel lumen.

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