commonly found in soil and water samples. This type of bacterium is especially
dangerous when it affects the wounds of seriously burned persons. Once established in
a burn site, Pseudomonas is difficult to remove. Neisseria gonorrhea is the
microorganism that causes gonorrhea. Salmonella is a type of bacterium that frequently
causes food poisoning. Salmonella typhi is the organism that causes typhoid fever.
This bacterium is especially prevalent in areas that have poor sanitation or in areas
affected by a disaster that has resulted in a contaminated water supply. Shigella is a
gram-negative bacterium that causes a type of food poisoning.
(3) Acid-fast bacteria. Two bacteria which are acid-fast (that is, stain a
particular color in this procedure) are medically important--or at least they were of great
concern until modern drugs were used against them. These bacteria are
Mycobacterium tuberculosis, the organism that causes tuberculosis and Mycobacterium
leprae, the organism that causes Hanssen's disease (leprosy).
(4) Other types of bacteria. Some bacteria are not easily identified by the
use of stains. In these instances, special techniques are used to make them visible
under the microscope. One of these bacteria is Treponema pallidum, the organism that
causes syphilis. This organism is identified by the dark field illumination technique.
a. Characteristics. Rickettsias (a type of bacteria) are the second smallest
microorganisms. That is, rickettsias are larger than viruses, but smaller than most other
bacteria. Specifically, the rickettsias range in size from 0.3 micron to 2.0 microns.
Rickettsias are found in the alimentary canals of blood-sucking arthropods such as
fleas, ticks, lice, and mites. The rickettsias do not form spores and are nonmotile.
Once the rickettsias enter the host through the bite of an arthropod, the rickettsias
reproduce. The rickettsias are obligate intracellular parasites.
b. Rickettsias and Disease. Rickettsias cause only a few types of diseases.
Rocky Mountain spotted fever is a well-known disease caused by a certain type of
rickettsia. At one time, Rocky Mountain spotted fever was found only in the Rocky
Mountain States. The disease has now spread throughout North America. This
particular disease is transmitted by ticks. Rocky Mountain spotted fever is
characterized by purple blotches over the entire body along with a high-grade fever.
Other diseases caused by rickettsias include typhus (both endemic and epidemic) that
is transmitted by lice, and Q (Query) fever, which is similar to pneumonia. The role of
ticks in the transmission of Q fever is not fully understood.