CLINICAL LAB SCIENEC

CHAPTER 15: MICROBIOLOGY 387
After the organisms are fairly well defined, another type of medium is used to
grow a pure culture from the isolated colonies on the original Petri dish. Antibiotics
that are effective toward eliminating the infectious organism are determined
based on the body site or type of specimen from which the organism was isolated,
as the antibiotic must be able to reach the site of the infection. The organism is
finally grown on a special medium where paper discs impregnated with a variety
of antibiotics are dropped onto the surface of the inoculated plate. Failure of the
bacteria to grow within a certain distance from the edge of the discs indicates
sensitivity to the antibiotic, and often the physician then has a choice between
several antibiotics. A rather recent phenomenon related to indiscriminate use of
antibiotics has been the increasing incidents of culturing organisms that are resistant
to almost all and, in some cases, all of the antibiotics available.
Diagnosis and Treatment
To determine if antibiotic therapy is appropriate, the organism must be identified
and, in many cases, evaluated as to the antibiotic that will be effective against
it. Effective antibiotic therapy depends on the site of the infection, as not all
antibiotics will reach the tissues or the body site where an infection is present.
In addition, depending on the species of bacterial organism present, the antibiotic
must be chosen from among a large arsenal to combat the infection. Most
importantly, antibiotics are developed for bacteria, and not for viruses. There are
antivirals that chiefly prevent the reproduction of a virus. Antivirals work best
if administered before signs and symptoms appear in those with a known viral
infection. Other microorganisms that are isolated, identified, and treated through
actions of the microbiology department are parasites, of which there are a number
that commonly infect people from around the world; fungi; yeasts; and molds.
There are two types of tests for determining the presence of microorganisms
that may be infecting a patient. Normally, viral infections are determined by the
indirect method. Usually, a sample of blood is collected, and the sample is tested
for antibodies that the immune system has formed against an infectious agent that
has invaded the body. But there is also a direct method for determining some viral
infections, which entails growing the viruses on cell cultures, as viruses can only
survive by inhabiting living cells. In some procedures, these cells can then be stained
or tagged with material that fluoresces and becomes visible microscopically, allowing
the determination of the presence of colonies of organisms such as fungi. The
direct method includes a group of tests requiring the collection of a sample of the
body fluids or tissue that may be infected by bacteria. Complicating this method of
collection is possible contamination of the sample with organisms normally found
on the skin, in the nose and mouth, or in the intestine. Even blood collected for
bacteriological agents may be contaminated with skin contaminants and will yield
erroneous results. Another method, enzyme-linked immunosorbent assay (ELISA),
may detect both antibodies and antigens (the infecting organism itself).
The term fungi relates to a group of organisms that also includes yeasts and
molds. Molds, yeasts, and fungi have minor differences but may be considered
as somewhat related. Molds, unlike certain yeasts and other fungi, seldom pose a
problem as infective organisms. Many fungal and yeast infections can be traced to
heavy and often unwarranted use of antibiotics that eliminates normally occurring
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