Damage to the Host

In order to cause disease, a pathogen must evoke some type of damage to the host. In many cases, the damage facilitates dispersal of the organism, enabling it to infect other hosts. For example, Vibrio cholerae, which causes cholera, induces a severe watery diarrhea; up to 20 liters of microbe-containing fluid can be excreted in one day. In areas of the world with inadequate sewage treatment, this can lead to contaminated water supplies and widespread outbreaks. Bordetella pertussis, which causes whooping cough, facilitates its airborne dispersal by causing severe bursts of coughing. Damage due to infection can be the result of direct effects of the pathogen, such as toxins produced, or indirect effects, such as the immune response.

Exotoxins

A number of Gram-positive and Gram-negative bacterial pathogens produce exotoxins, which are proteins that have very specific damaging effects (table 19.2). These proteins are among the most potent toxins known and are often a major cause of damage to an infected host.

Exotoxins are either secreted by the bacterium or leak into the surrounding fluid following lysis of the bacterial cell. In most cases, the pathogen must colonize a body surface or tissue to produce enough toxin to cause damage. With foodborne intoxication, however, the bacterial cells multiply in a food product where they produce toxin that is then consumed. In the case of botulism, caused by ingestion of botulinum toxin produced by Clostridium botulinum, ingestion of minute amounts of toxin is sufficient to cause paralysis. Like most other exotoxins, botulinum toxin can be destroyed by heating. ■ botulism, p. 672 Exotoxins may act locally, or they may be carried in the bloodstream throughout the body, causing systemic effects. For example, Corynebacterium diphtheriae, the organism that causes diphtheria, grows and releases its exotoxin in the throat. There, the toxin destroys local cells, leading to the formation of a pseudomembrane composed of dead host cells, pus, and blood. This membrane can dislodge and obstruct the airway. The toxin can also be absorbed and carried to the heart and other organs, causing additional damage. ■ diphtheria, p. 568

Because exotoxins are proteins, the immune system can generally produce protective antibodies. Unfortunately, many exotoxins are so powerful that fatal damage can occur before an adequate immune response is mounted. This is why vaccination, using a toxoid, is important in preventing otherwise common diseases such as tetanus and diphtheria (see table 17.1). Passive immunity to a given toxin can be provided by administering an antitoxin; for example, a person who develops symptoms of botulism is administered botulinum antitoxin. Although vaccines against botulinum toxin exist, they are not routinely used because the risks of developing the disease are negligible if prudent food preparation procedures are followed. ■ toxoid, p. 423 ■ antitoxin, p. 420

Many exotoxins can be grouped into functional categories according to the tissues they adversely impact. Neurotoxins damage the nervous system, causing symptoms such as paralysis. Enterotoxins cause symptoms associated with intestinal disturbance, such as diarrhea and vomiting. Cytotoxins damage a variety of different cell types, either by interfering with essential cellular mechanisms or by lysing the cell. Some exotoxins do not fall into any of these groups, instead causing symptoms associated with excessive stimulation of the immune response.

Most exotoxins fall into three general categories that reflect their structure and general mechanism of action—A-B toxins, membrane-damaging toxins and superantigens.

A-B Toxins

A-B toxins consist of two parts—one constitutes the toxic, or active, part (the A subunit) and another (the B subunit) binds

19.8 Damage to the Host 473

Table 19.2 Exotoxins Produced by Various Primary Pathogens

Toxins

Name of Disease; Name of Toxin

Characteristics of the Disease

Reference

A-B TOXINS—Composed of two subunits, A and B.The A subunit is the toxic, or active, part; the B subunit binds to the target cell.

Neurotoxins

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