Virulence Plasmids

Virulence plasmids help bacteria infect humans, animals or even plants, by a variety of mechanisms. Some virulence factors are toxins that damage or kill animal cells, others help bacteria to attach to and invade animal cells (Fig. 16.20), whereas yet others protect bacteria against retaliation by the immune system.

Although most strains of Escherichia coli are harmless, occasional rogue strains cause disease. These pathogenic E. coli generally rely on plasmid-borne virulence factors. Wide ranges of toxins are found in different pathogenic E. coli strains, including heat-labile enterotoxin (resembles choleratoxin), heat-stable enterotoxin, hemolysin (lyses red blood cells) and Shiga-like toxin (similar to the toxin of dysentery-causing Shigella). There is a similar variety of adhesins or "colonization factors", adhesin Protein that enables bacteria to attach themselves to the surface of animal cells. Same as colonization factor choleratoxin Type of toxin made by Vibrio cholerae the cholera bacterium colonization factor Protein that enables bacteria to attach themselves to the surface of animal cells. Same as adhesin enterotoxins Types of toxin made by enteric bacteria including some pathogenic strains of E. coli hemolysin Type of toxin that lyses red blood cells virulence factors Proteins that promote virulence in infectious bacteria. Include toxins, adhesins and proteins protecting bacteria from the immune system

FIGURE 16.20 Toxins and Adhesins

Bacteria are able to attack animal cells by attaching to the cellular membrane and releasing toxins. The bacteria contain plasmids that encode adhesins, which are protein filaments able to recognize and attach to cell-surface receptors found on animal cells. One attached, the bacteria secrete toxins, which can penetrate the animal cell membrane and kill the cell.

FIGURE 16.20 Toxins and Adhesins

Bacteria are able to attack animal cells by attaching to the cellular membrane and releasing toxins. The bacteria contain plasmids that encode adhesins, which are protein filaments able to recognize and attach to cell-surface receptors found on animal cells. One attached, the bacteria secrete toxins, which can penetrate the animal cell membrane and kill the cell.

proteins that enable bacteria to stick to the surface of animal cells. Adhesins form filaments that vary in length and thickness, but generally resemble pili. Consequently the symptoms and severity of infection by E. coli vary greatly.

Other enteric bacteria, such as Salmonella typhi (typhoid) and Yersinia pestis (bubonic plague) cause severe infections. They also carry virulence plasmids. In Salmonella the majority of the virulence genes are on the chromosome, but a handful are plasmid-borne. In contrast, in Yersinia several plasmids carry the bulk of the virulence genes. In addition to toxins and adhesins, these "professional" pathogens possess more sophisticated virulence factors that protect against host defenses. Although plasmids have been investigated most intensively in enteric bacteria, it is clear that virulence in many other bacteria often depends on at least some plasmid-borne genes.

The Ti-plasmids can mediate transfer of DNA from bacteria to plant cells.

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