Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Microorganisms

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ant to many antibiotics. As this strain increases its worldwide

distribution, Staphylococcus infections will become an

increasing problem.

Bacteria that normally live in the mouth are responsible

for the formation of dental plaqueon the surface of teeth.

Protected within the plaque, the bacteria produce acid that eats

away tooth enamel, leading to the development of a cavity.

A few examples of other clinically important bacteria

are Bacillus anthracis(anthrax), Clostridium tetani(tetanus),

Mycobacterium tuberculosis(tuberculosis), Corynebacterium

diphtheriae(diphtheria), various Rickettsias (Rocky Mountain

Spotted Fever, Q fever), Chlamydia trachomatis(chlamydia).

Fungi and yeastare also capable of causing infection.

For example, the fungal genus Tineacomprises species that

cause conditions commonly described as “jock itch” and “ath-

lete’s foot.” Scalp infections are also caused by some species

of fungus.

Virusesare also the cause of a variety of infections.

Inflammationof the coating of nerve cells (meningitis) and

brain tissue (encephalitis), and infections of tissues in the

mouth, bronchial tract, lungs and intestinal tract result from

infection by various viruses.

See alsoBlood borne infections; Cold, viruses; Laboratory

techniques in microbiology; Viruses and response to viral

infection; Yeast, infectious

MICROBIOLOGY, HISTORY OF•seeHISTORY

OF MICROBIOLOGY

MMicroorganismsICROORGANISMS

Microorganisms are minute organisms of microscopic dimen-

sions, too small to be seen by the eye alone. To be viewed,

microorganisms must be magnified by an optical or electron

microscope. The most common types of microorganisms are

viruses, bacteria, blue-green bacteria, some algae, some fungi,

yeasts, and protozoans.

Viruses, bacteria, and blue-green bacteria are all

prokaryotes, meaning that they do not have an organized cell

nucleusseparated from the protoplasm by a membrane-like

envelope. Viruses are the simplest of the prokaryotic life forms.

They are little more than simple genetic material, either DNA

(deoxyribonucleic acid) or RNA(ribonucleic acid), plus associ-

ated proteins of the viral shell (called a capsid) that together

comprise an infectious agent of cells. Viruses are not capable

of independent reproduction. They reproduce by penetrating a

host cell and diverting much of its metabolic and reproductive

physiology to the reproduction of copies of the virus.

The largest kingdom of prokaryotes is the Monera. In

this group, the genetic material is organized as a single strand

of DNA, neither meiosis nor mitosis occurs, and reproduction

is by asexual cellular division. Bacteria (a major division of

the Monera) are characterized by rigid or semi-rigid cell walls,

propagation by binary division of the cell, and a lack of mito-

sis. Blue-green bacteria or cyanobacteria (also in the Monera)

use chlorophylldispersed within the cytoplasmas the primary

light-capturing pigment for their photosynthesis.

Many microorganisms are eukaryotic organisms, hav-

ing their nuclear material organized within a nucleus bound by

an envelope. Eukaryotesalso have paired chromosomesof

DNA, which can be seen microscopically during mitosis and

meiosis. They also have a number of other discrete cellular

organelles.

Protists are a major kingdom of eukaryotes that

includes microscopic protozoans, some fungi, and some algae.

Protists have flagellated spores, and mitochondria and plastids

are often, but not always, present. Protozoans are single-celled

microorganisms that reproduce by binary fission and are often

motile, usually using cilia or flagellae for propulsion; some

protozoans are colonial.

Fungi are heterotrophic organisms with chitinous cell

walls, and they lack flagella. Some fungi are unicellular

microorganisms, but others are larger and have thread-like

hyphaethat form a more complex mycelium, which take the

form of mushrooms in the most highly developed species.

Yeasts are a group of single-celled fungi that reproduce by

budding or by cellular fission.

Algae are photosynthetic, non-vascular organisms,

many of which are unicellular, or are found in colonies of sev-

eral cells; these kinds of algae are microscopic.

In summary, microorganisms comprise a wide range of

diverse but unrelated groups of tiny organisms, characterized

only by their size. As a group, microorganisms are extremely

important ecologically as primary producers, and as agents of

decay of dead organisms and recycling of the nutrients con-

tained in their biomass. Some species of microorganisms are

also important as parasites and as other disease-causing

agents in humans and other organisms.

See alsoBacteria and bacterial infection; Genetic identifica-

tion of microorganisms; Viruses and responses to viral infec-

tion; Microbial flora of the skin; Microbial genetics; Microbial

symbiosis; Microbial taxonomy; Microscope and microscopy

A lichen growing on wood.

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