Bacterial Diseases

How do bacteria cause disease?

Disease-causing agents are called pathogens. Although pathogens can come from any taxonomic group, bacteria and viruses are among the most common. All currently known prokaryotic pathogens are bacteria. This is why the discussion here is restricted to pathogenic bacteria, and it excludes archaea. However, in the future scientists may well discover that some archaea are associated with disease.

The French chemist Louis Pasteur was the first person to show convincingly that bacteria cause disease. Pasteur helped to establish what has become known as the germ theory of disease when he showed that bacteria were responsible for a number of human and animal diseases.

Disease Mechanisms Bacteria produce disease in one of two general ways. Bacteria cause disease by destroying living cells or by releasing chemicals that upset homeostasis. Some bacteria destroy living cells and tissues of the infected organism directly, while some cause tissue damage when they provoke a response from the immune system. Other bacteria release toxins (poisons) that interfere with the normal activity of the host. Figure 20–15 lists some common human diseases caused by bacteria.

▸ Damaging Host Tissue One example of a pathogen that damages host tissue is the bacterium that causes tuberculosis. This pathogen is inhaled into the lungs, where its growth triggers an immune response that can destroy large areas of tissue. The bacterium also may travel through blood vessels to other sites in the body, causing similar damage.

▸ Releasing Toxins Bacteria that produce toxins include the species that causes diphtheria, and the species responsible for a deadly form of food poisoning known as botulism. Diphtheria has largely been eliminated in developed countries by vaccination, but outbreaks of botulism still claim many lives.