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Short Overview

June 2002

Microorganisms serve important functions in nature. Microbes are part of the biosphere and some have adapted to extremes of temperature, pressure, and oxidation state of their surroundings. Many breakdown wastes, incorporate nitrogen from the air into organic compounds, and participate in photosynthesis which generates food and oxygen. A small minority of microbes are disease producing.

Microbes are classed according to their degree, and complexity, of cellular makeup.

Virus particles are not cellular. Their cores are either DNA or RNA which is covered with a coat of protein which may have a lipid (water insoluble) envelope. A virus can only reproduce inside the cells of a host organism. They are the smallest infectious agents and can readily pass through filters that stop all known bacteria.

Bacteria (and Archaea) are one-celled organisms where their genetic material is not enclosed in a special nuclear membrane. They may be thought of as pre-nucleus. In appearance there are three varieties, rod like (bacillus), spherical or ovoid (coccus), and curved or corkscrew (spirochete). Each variety may be harmless or may cause disease. Anthrax is a bacillus, strep throat is a coccus, and syphilis is a spirochete.

Fungi contain their DNA within a nucleus surrounded by a nuclear membrane. They may be unicellular or multicellular. Yeasts, used in so many ways, are unicellular fungi larger than bacteria.

Algae are unicellular photosynthetic microbes that need light and air for growth.

Protozoa are unicellular but have means for locomotion by which they are described.

Helminths are not technically microbes, but they require some of the same identification procedures used for microbes. Flatworms and round worms are examples.

Brief chronological history

In 1798, Edward Jenner showed that milkmaids who contracted Cowpox were immune to Smallpox. Some 180 years later, the earth was rid of natural smallpox infections by judicious use of Cowpox vaccinations around each point of outbreak.

In 1835, Agostino Bassi made the first association between a microorganism and disease by noting that a silkworm disease was caused by a fungus.

In the 1840s Ignaz Semmelweis showed that patients of physicians who disinfected their hands experienced minimal infections during childbirth procedures.

Louis Pasteur discovered that another silkworm disease was caused by a protozoan in 1865. Louis Pasteur later found that heating beer or wine just enough to kill bacteria prevented their spoilage and this procedure also comes down to us today in pasteurized milk and other food products. He was also the first person to develop an effective vaccine against rabies.

Robert Koch put the germ theory of disease on solid footing in 1876 when he demonstrated that a certain bacteria found in the blood of cattle that died of Anthrax, could be grown in culture and used to induce Anthrax in healthy animals from which the same bacteria could be extracted from the blood of the infected animals. This procedure became the standard proof relating specific pathogens to specific diseases in plant and animals.

In 1910, Paul Ehrlich discovered that salvarsan, an arsenic compound, was effective against syphilis. Quinine had been used much earlier to treat malaria, but it is not known who first made the discovery.

The first true antibiotic was discovered by Alexander Fleming in 1928. He noted that the mold Penicillium notatum inhibited growth of bacteria. Penicillin was developed for antibacterial use in the 1940s.

In the 1930s, sulfa drugs were discovered to have positive chemotherapy effects. Fleming's growth inhibitor was mass-produced and clinically tested in the 1940s. His discovery became known as Penicillin.

Microbiology as a science exploded after World War II with numerous specialties arising.

Requirements for Microbial Growth

Temperature must be within a range that allows any specific species to grow. Water is needed to transfer nutrients to the microbe. Relative acidity must also be in a range the microbe can stand. Most bacterial grow within a range of 6.5 to 7.5 on the pH scalemost rivers and lakes are near or within this range. Some microbes can grow below a pH of 4.0.

Nutrients include soluble forms of carbon, nitrogen, sulfur, and phosphorus. Oxygen is a common requirement, but some microbes thrive in its absence. They belong to the anaerobic group. Like people, microbes need vitamins.

Mutation Changes in genetic material

Since microbes can multiply so rapidly by cell division, genetic events can also. Microbes are identified by their genotype (genetic makeup) as expressed by their actual physical appearance (phenotype). In the most sophisticated procedures, their DNA is analyzed and mapped.

Genetic alterations may be caused in a variety of ways. The natural background of cosmic radiation is one important source. Any artificial radiation source has similar effects. Certain chemicals can also damage DNA in ways that result in mutations.

A gene is a segment of DNA that codes for a functional product. When one of its base elements is replaced by a different base, it is likely that the code will change and affect the product. Most such changes degrade the viability of the mutant organism. On a rare occasion, the mutation will enhance the mutated organism. Some number of mutations combined can give rise to a new species.


A vaccine is a suspension of microbes, or some part or product of them, that will induce immunity instead of infection when administered to the host.

Incubation Period

This is the period between exposure and onset of symptoms.

Sites for Countering Bioterror Threats

* Health Alert Network
* Immunization
* Quarantine & Migration


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