WORLD OF MICROBIOLOGY AND IMMUNOLOGY Mycology407•
be typical. Yet, little is still known about how people become
contaminated. One species, known as Mycobacterium mar-
inum, is found in swimming pool water, and can cause a skin
infection in fingers or toes upon contact with the skin of a
swimmer. Additionally, some evidence supports the transmis-
sion of atypical mycobacteria in aerosols (that is, as part of tiny
droplets that can drift through the air and become inhaled).
Contaminationwith atypical mycobacteria may be a
natural part of life. For the majority of people, whose immune
systems are functioning efficiently, the microbe does not
establish an infection. However, for those who immune sys-
tem is not operating well, the presence of the atypical
mycobacteria is a problem. Indeed, for those afflicted with
acquiredimmunodeficiencysyndrome (AIDS), infection with
atypical mycobacteria (typically with Mycobacterium avium
and Mycobacterium intracellulare) is almost universal.
Atypical mycobacteria tend to first establish a foothold
in the lungs. From there the bacteria can spread, via the blood-
stream, throughout the body. Infections in almost every organ
of the body can ensue. Examples of sites of infection include
the brain, lymph nodes, spleen, liver, bone marrow, and gas-
trointestinal tract. The overwhelming nature of the infections
can be fatal, especially to people already weakened by AIDS.
The spectrum of infection sites produces a wide range
of symptoms, which include a feeling of malaise, nausea,
worsening diarrhea, and, if the brain is affected, headaches,
blurred vision, and loss of balance.
Infrequently, those with healthy immune systems can
acquire an atypical mycobacterial infection. The result can be
a bone infection (osteomyelitis), a form of arthritis known as
septic arthritis, and localized infections known as abscesses.
The diagnosis of infection caused by atypical mycobac-
teria is complicated by the fact that the growth of the microor-
ganismson conventional laboratory agaris very difficult.
Specialized growth medium is required, which may not be
available or in stock in every clinical laboratory. The delay in
diagnosis can result in the explosive development of multi-
organ infections that are extremely difficult to treat.
Treatment of atypical mycobacteria is complicated by
the unusual cell wall possessed by the bacterium, relative to
other bacteria. The cell wall is made predominantly of lipids.
Partially as a result of their wall construction, atypical
mycobacteria are not particularly susceptible to antibiotic
therapy. As well, aggressive therapy is often not possible,
given the physical state of the AIDS patient being treated. A
prudent strategy for AIDS is the use of certain drugs as a
means of preventing infection, and to try to avoid those factors
that place the individual at risk for acquiring atypical
mycobacterial infections. Some risk factors that have been
identified include the avoidance of unwashed raw fruit and
vegetables. As well, contact with pigeons should be limited,
since these birds are known to harbor atypical mycobacteria in
their intestinal tracts.See alsoBacteria and bacterial infections; Immunodeficiency
diseasesMMycologyYCOLOGY
Mycology is the study of fungi, including molds and yeasts.
The study of mycology encompasses a huge number of
microorganisms. Indeed, just considering molds, the estimates
of the number of species ranges from the tens of thousands to
over 300,000.
Fungi are eukaryotic microorganisms (eukaryoteshave
their nucleic material contained within a membrane), which
can produce new daughter fungi by a process similar to bacte-
ria, where the nuclear material replicates and then the cell
splits to form two daughter cells, or via sexual reproduction,
where nuclear material from two fungi are mixed together and
the daughter cells inherit material from both parents. Growth
of fungi can occur either by the budding off of the new daugh-
ter cells from the parent or by the extension of the branch (or
hyphae) of a fungus.
The study of fungi can take varied forms. Discovery of
new fungi and their grouping with the existing fungi is one
aspect of mycology. Unraveling the chemical nature of the
fungal survival and growth is another aspect of mycology. For
example, some fungi produce antibioticssuch as penicillinas
part of their defensive strategies. This aspect of mycology has
proved to be extremely important for human health. The
adverse effects of fungi on human health and plants constitutes
yet another aspect of mycology. Still another aspect of mycol-
ogy, which can encompass some of the preceding, is con-
cerned with the economic impact, beneficial or not, of fungi.
For example, those fungi that are edible or which produce
antibiotics have a tremendous positive economic impact,
whereas fungi that cause damage to agricultural plants exact a
negative economic toll.
Some mycologists (scientists who study fungi) conduct
extensive research into the origin of fungi. The discovery of
fossilized fungi that resemble those from the four major
groups of modern fungi in rocks that date back 360–410 mil-
lion years indicate that fungi were already well-established
and diversifying even before other forms of life had made the
transition from the sea to the land.
Mycology has resulted in the classification of fungi into
four divisions. These divisions are the Chytridiomycota,
Zygomycota (which include the bread molds such as
Neurospora), Ascomycota (which include yeasts), and the
Basidiomycota. Lichens do not fit this classification, as
lichens are not single-celled fungi. Rather, they are a symbi-
otic association (an association that is beneficial for both par-
ticipants) between a fungus and an alga.
The health-oriented aspect of mycology is important,
particularly as the danger of fungal infections, especially to
those whose immune systemis compromised, has been recog-
nized since the identification of acquired immunodeficiency
syndrome in the 1970s.
For example, in those whose immune systems are func-
tioning properly, an infection with the mold known as
Aspergilluscan produce a mild allergic type of reaction.
However, in those people whose immune systems are not
operating efficiently, the mold can grow in the lungs, and can
produce a serious infection called bronchopulmonarywomi_M 5/7/03 7:53 AM Page 407