Bacteriology (continued)
all other cells (both plant and animal). Unlike animal and plant
cells, bacteria have no discrete, membrane-bound nucleus and
no special intracellular membrane-bound organelles. Bacteria
were called prokaryotic cells, meaning primitive or primordial;
other, more advanced cells were termed eukaryotic.
Bacteria and disease
One of the most important events in the history of civiliza-
tion was the discovery that bacteria cause disease (Fig. 2).
Initially, the discovery of bacteria was made by Antonie van
Leeuwenhoek in 1676. However, scientists did not realize how
these microorganisms might be involved in disease for 200
years. In 1876, Robert Koch provided convincing proof that
anthrax, an important disease of cattle and sometimes of hu-
mans, was caused by bacteria. Soon after Koch's monumental
discovery, Pasteur, while studying cholera in chickens, discov-
ered the basis for acquired immunity to disease. He found that
old cells of the disease-causing bacteria became attenuated,
or changed in such a way that the disease did not occur when
they were inoculated into a healthy chicken. Moreover, injec-
tion of young virulent bacteria no longer caused the disease;
the chicken had become immune to cholera. These discoveries
by Koch and Pasteur were the impetus for a subsequent golden
era of study of the cause and control of bacterial disease of
humans and of immunization as a means to combat diseases.
Models for biochemistry and genetics
At the molecular level, the basic physiological processes as-
sociated with metabolism and growth are nearly the same for
all forms of life. Because of their rapid growth rate (some bac-
teria divide every 20–30 min), and the ease of obtaining large
amounts of material in a controlled atmosphere in a short time,
bacteria were favorite subjects for studies of biochemistry and
cell physiology. Much of the current knowledge of the proper-
ties and activities of enzymes, the mechanism and control of
degradation of foodstuffs, and the biosynthesis of proteins,
nucleic acids, carbohydrates, and lipids has been gained from
studies of bacteria.
The same properties that made bacteria useful for biochemi-
cal studies made them ideal subjects for studies of genetics.
The ability to obtain population increases from a few hundred
to millions of cells overnight and in a test tube made studies
of mutation and selection feasible. It was studies of behavior
of populations of bacteria that led researchers to the proof
that deoxyribonucleic acid (DNA), and not protein, is the cell
macromolecule of which genes are made. In addition, bacteria
play a key role in genetic engineering studies. In particular,
bacteria possess small pieces of DNA called plasmids, which
can be isolated, recombined, and then reinserted into a bacte-
rium. Therefore, individual genes can be inserted into specific
sites on plasmids in cell cultures and the recombinant plasmid
thus formed can be introduced into a living cell by the process
of bacterial transformation. For example, the gene for insulin
production has been transferred in this way from animal cells
into Escherichia coli bacteria, and insulin is being produced on a
commercial scale by using the bacteria.
Biotechnology
Bacteria are the basis of many important industrial processes
and play a key role in many biotechnological applications. They
are involved in the production of cheese and fermented food
products (for example, sauerkraut, pickles, and sausage), as
well as in the formation of methane gas from sewage and other
wastes. Most medically important antibiotics are produced by
bacteria. Exploitation of bacteria for detoxification of envi-
ronmental pollutants and for production of useful materials
promises to increase in the future.
Fig. 2: A digitally colorized scanning electron micrograph (SEM) depicting a number of
mustard-colored, spheroid-shaped Staphylococcus aureus bacteria that were in the process
of escaping their destruction by blue-colored human white blood cells. Antibiotic-resistant
strains of Staphylococcus aureus are of growing concern to physicians. (Credit: Frank De-
Leo/National Institute of Allergy and Infectious Diseases)
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