Bacterial pathogenicity

How do bacteria make us ill? That is an important question to answer, for understanding bacterial pathogenicity can lead to better ways of treatment and prevention of infectious diseases. Bacteria need to be able to remain in, or in close contact to, our body, and to multiply there, before it can do harm. Food poisoning is the only exception to this rule as we will see below. Our body has developed several strategies to make life as difficult as possible for pathogenic bacteria (see our exhibit the immune system to learn about these strategies) and bacteria have in turn worked out how to deal with our defenses.

• How some bacteria cause disease and others don't (Source: Natl. Inform. Program on Antibiotics).

Pathogenic bacteria have certain characteristics that they need, and use, to cause disease. These so-called virulence factors have specific functions in the successive steps that result in an infection. An infection can be seen as a miniature battle between bacteria and host, the first trying to remain present, and to feed and multiply, while the host is trying to prevent this. The resulting infection is a process with three possible outcomes: the host wins and the bacteria are removed (possibly with the help of medication) so that the host can recover; the bacteria win the ultimate battle and kill their host (sad but true: bacterial infections are a major cause of death especially for children and elderly people); or an equilibrium is reached in which host and bacteria live involuntarily together and damage is minimized.

Here follow some examples of virulence factors produced by certain bacteria, and the role they have in pathogenicity.

• Fimbriae, or pili. The hair-like structures on the surface of the bacterial body are called fimbriae or pili (Source: Todor's online Textbook of Bacteriology). These hairs are able to attach themselves to certain sites of our body, and in this way the bacteria cannot be washed away (removal of bacteria from sensitive sites is one of our defenses). For instance, uropathogenic E. coli produces fimbria that can attach to the epithelium lining of the urogenital tract. That is how they can cause a bladder infection without being flushed away. The first step in pathogenesis is for bacteria to reach the site of interest and to remain there. For many sites of the body the next challenge for pathogens is to compete with commensal flora (those bacteria that are good for you).
• Flagella. These are long tails with which the bacteria can swim. Have another look at the picture again You can see how flagella work (Source: Access Research Network). Here you can see bacteria swim (Source:Microbiology Video Library) . The motion helps bacteria reach the site where they can survive, and for that reason flagella can have a function in pathogenicity. A detailed description of the physics of bacterial motility (Source: American Institute of Physics).
• Toxins. Some bacteria produce toxic compounds that cause harm to their host. These factors are called toxins. Toxins can have all sorts of effects. They can induce vomiting and cause diarrhea, they can affect our nerve cells and paralyze us, or cause a muscular cramp, or cause severe pain, or fever, etc. Different bacteria produce different toxins, and sometimes they would be completely harmless without the toxins. In some cases bacteria produce toxins wherever they grow, and if we eat those bacterial products we get ill, even without being infected by any living bacteria. That is what happens during certain types of food poisening. Food on which certain bacteria have fed can contain toxins, and these bacterial products will make us sick. Clostridium bacteria are some nasty toxin-producers.
See our display on food safety in another subject category.
• Invasion. Some bacteria have learned to invade our cells. This is particularly important for pathogenic bacteria that colonize the gut. The outcome of this attack depends on the organism. For instance, Salmonella typhimurium will destroy the cells of our intestine to feed on the cell-content. The result is a severe diarrhea, but with the loss of stool the bacteria are removed as well. Mycobacterium tuberculosis, which often enters our body through the lungs, will remain in our body for the rest of our live. Our immune system cannot destroy them for they are hiding in our cells where there is plenty of food. In return they only cause disease in a limited number of infected people. So although these bacteria are not commensals or symbionts (they can cause disease) many infections occur unnoticed. Surviving the defense could be the summing up description of many ways in which pathogenic bacteria can hide, evade, or even destroy immunological reactions by the host, in order to cause infection. Since these strategies will help the bacteria in colonizing their host, and causing disease, these characteristics are also seen as pathogenic. The intricate relationship between host and pathogen is the subject of a lot of microbiology research. A better understanding of this aspect of pathogenicity can lead to better control, treatment and prevention of infectious diseases. It also gives insight in our general knowledge of biology.

The content of this page is copyright of the Virtual Museum of Bacteria and was written by Dr. T. M. Wassenaar (curator), with support of the Foundation for Bacteriology.

Choose another exhibit in this catagory or go to another catagory from the menue at the top of this page

Page last modified: 08 Mar 2007

Keep an eye on this site, we are reconstructing it and a new version will be online soon.