Why it's important
Bacteria that cause disease are called pathogenic bacteria. Bacteria
can cause diseases in humans, in other animals, and also in plants. Some
bacteria can only make one particular host ill; others cause trouble in
a number of hosts, depending on the host specificity of the bacteria. The
diseases caused by bacteria are almost as diverse as the bugs themselves
and include food poisoning,
ache, anthrax, even certain forms of cancer. It is impossible to sum up all bacterial diseases and it would be pretty boring.
The Infectious Diseases fact sheets (Source: New York State Department of Health) gives brief descriptions of diseases, including infectious diseases. Some diseases are named after
the organisms that cause them, or is it the other way round?
If you want to have a look at pathogenic bacteria under the microscope:
- View here pathogenic
bacteria under the microscope (Source: Des Moines Area
Community College) which also describes their properties, as part of a
course on medical microbiology. Some of the terms used here are explained
in our exhibit on pathogenicity.
More links to microscopic pictures can be found in the exhibit on Images of bacteria.
Some pathogenic bacteria have received disproportionate attention in the
press, e.g. the 'flesh-eating
bacteria' (Source: WebMD symtomchecker),
which in real life are called Streptococci. Indeed they can cause spectacular, but fortunately uncommon symptoms. In the
press, pathogenic bacteria are sometimes represented as (deadly) dangerous
enemies that lurk in the dark, unseen, ready to attack you. Although that
is exaggerated some bacteria can be life treatening, for example Legionella
pneumoniae, the causative of Legionnaire's disease. These bacteria
survive in moist places like air conditioners or hot-water pipes. Though
potentially life treatening, bacterial infections do not kill all their
victims. If they did, and if they spread fast, they would eradicate their
hosts completely. Although a popular theme for thrillers, this is a
'mission impossible' in real life. Some bacteria kill a high percentage
of people infected (they have a so-called a high mortality rate), but their
relatively inefficient rate of spreading makes up for that.
When an infectious disease spreads around
an area, and the cases of new infections reach a certain number, we call
it an epidemic. Visit the Epidemics Home
Site (National Library of Medicine). If, on the
other hand, a certain disease is always present in low numbers of cases
in a given area, that disease is said to be
endemic in that area.
Some epidemics become wide-spread and quickly reach distant parts of the
world: in modern times people travel fast and frequent, and our bacteria
travel with us. A classical example of such a pandemic (in this case caused by a virus) was a new
type of influenza ("Spanish 'flu") that reached continent after continent
early this century, killing thousands of people on it's way. Although only
a small proportion of the infected people died (the virus had a low mortality)
so many people got infected that even the small proportion of deaths amounted
to large numbers. An epidemic or pandemic can only occur if the population
is not immune to that disease. Read our special feature file on the history of infectious diseases about the times that pathogens could spread unlimited. Our exhibit on our
immune system explains how immunity
can prevent disease. So why do epidemics occur? Either because they are
caused by diseases that did not exist before, like AIDS, caused by the
HIV virus, or because new variants of bacteria (or viruses) arrive in an
area where they were not endemic before. This is why epidemics
of the common 'flu' occur frequently: the virus causing influenza is able
to change itself sufficiently to bypass our immunity built up from prior
infections. Although not caused by bacteria, this site on influenza is
very interesting if you want to understand epidemics of infectious diseases
(Source: Ultranet EJ Kimball). Whenever a new virus type arises, there is
the potential for a new epidemic. Fortunately, most of these new types
are not as vicious as the Spanish 'flu. The experts are currently keeping an eye on bird flu, a strain of influenza that has not (yet?) learned to jump from person-to-person (Source: CDC). If that would happen, a new pandemic could be the result.
An example of a bacterial disease
that caused successive epidemics, and even pandemics, in recent times is
Vibrio cholerae, the cause of cholera. Epidemiology is the
study to establish the cause of a disease. A classic epidemic in history was small pox raging through Europe in the 14th Century.
Bacteria have invented many different strategies to make us ill. These
strategies, called bacterial pathogenicity, are the subject of an
important division of medical microbiology. Understanding how certain bacteria
make us ill can result in better treatment, vaccination, or prevention of
that infectious disease. An example of such research is how treatment of
infections can be improved (Source: Montana State Uni).
In another exhibit some of the common mechanisms of bacterial
pathogenicity are explained. In order to keep this information balanced, now that you know what pathogenic bacteria
are, why not also check out how commensal
bacteria are good for you.
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.
Page last modified: 08 Mar 2007