Bacteria may be classified by their shape, size, and aggregation properties when viewed under the microscope. The primary shape groups are rod-shaped called bacilli, spiral-shaped called spirilla, and spherical called cocci. Cocci can appear in pairs called diplococci, in chains, or in clusters such as that of staphylococci. They are composed of a cell membrane surrounded in some cases by a rigid cell wall. An exception to this is one of the smallest bacteria, mycoplasma, which is pleomorphic, lacking a rigid cell wall and without shape.
One of the principal methods of studying bacteria and being able to further divide them into groups is with the use of gram stain. When a special type of blue dye (crystal violet stain) is applied to bacteria, those that retain the blue stain are called gram positive. Those bacteria where the blue dye is able to be washed off by alcohol and appear red are called gram negative. A third group when stained with special dyes retains the stain even when treated with acid is called acid-fast bacteria.
Mycoplasma is an exception to the use of gram stain. Since it lacks a cell wall it is neither gram positive nor gram negative. One method used to examine very small unstained bacteria is Darkfield illumination.
Another method of studying bacteria is to do a culture. Bacteria are placed in a variety of culture media to see if or how they grow.
Bacteria vary widely in their need for nutrients, appropriate temperature, and pH requirements in order to multiply.
In addition, bacteria vary in their oxygen requirement needs. Many bacteria require the presence of oxygen for growth and are called aerobic bacteria. Those that are only able to grow in the absence of oxygen are called anaerobic bacteria. And, bacteria that are able to grow with or without the presence of oxygen are called facultative organisms.
Many pathogenic bacteria that invade the body produce toxins. The body defends itself by fighting back against the bacteria, rushing in leukocytes (white blood cells) and antitoxins to the location of the infection. Some of these white cells engulf the bacteria while the antitoxins neutralize the poisons. Extra blood supply in the area contributes to the inflammatory process. Fever and pain that can result also help by forcing the body to rest and conserve its resources to fight off the infection.
Different kinds of bacteria have a tendency to affect different organs and systems of the body, producing infectious diseases, each with its own group of symptoms or signs.
Understanding the differences in bacteria and the methods used to study them aid in providing diagnostic information that help with not only the selection of appropriate antibiotics for medical treatment, but also which antiseptic agents or disinfectants to use.
It is important to realize that the majority of bacteria coexist peacefully with the body. Some bacteria are helpful, such as those existing in the intestine that feed on other microscopic organisms that might actually harm the body. Some bacteria can also produce vitamins used by the body such as vitamin C and vitamin K.
In this section we are primarily concerned with those bacteria that are harmful and present an infectious state in the rat, along with suggested treatment and nursing care.