Microbiology is the study of organisms too small to be clearly seen
by the unaided eye (i.e., microorganisms); these include viruses, bacteria,
archaea, protozoa, algae, and fungi
Some microbes (e.g., algae and fungi) are large enough to be visible,
but are still included in the field of microbiology; it has been suggested
that microbiology be defined not only by the size of the organisms studied
but by techniques employed to study them (isolation, sterilization, culture
in artificial media)
The Discovery of Microorganisms
Invisible living creatures were thought to exist and were thought to
be responsible for disease long before they were observed
Antony van Leeuwenhoek (1632-1723) constructed microscopes and was the
first person to observe and describe microorganisms accurately
The Conflict over Spontaneous Generation
The proponents of the concept of spontaneous generation claimed that
living organisms could develop from nonliving or decomposing matter
Francesco Redi (1626-1697) challenged this concept by showing that maggots
on decaying meat came from fly eggs deposited on the meat, and not from
the meat itself
John Needham (1713-1781) showed that mutton broth boiled in flasks and
then sealed could still develop microorganisms, which supported the theory
of spontaneous generation
Lazzaro Spallanzani (1729-1799) showed that flasks sealed and then boiled
had no growth of microorganisms, and he proposed that air carried germs
to the culture medium; he also commented that external air might be needed
to support the growth of animals already in the medium; the latter concept
was appealing to supporters of spontaneous generation
Louis Pasteur (1822-1895) trapped airborne organisms in cotton; he also
heated the necks of flasks, drawing them out into long curves, sterilized
the media, and left the flasks open to the air; no growth was observed
because dust particles carrying organisms did not reach the medium, instead
they were trapped in the neck of the flask; if the necks were broken,
dust would settle and the organisms would grow; in this way Pasteur disproved
the theory of spontaneous generation
John Tyndall (1820-1893) demonstrated that dust did carry microbes and
that if dust was absent, the broth remained sterile-even if it was directly
exposed to air; Tyndall also provided evidence for the existence of heat-resistant
forms of bacteria
The Role of Microorganisms in Disease
Recognition of the relationship between microorganisms and disease
Agostino Bassi (1773-1856) showed that a silkworm disease was caused
by a fungus
M. J. Berkeley (ca. 1845) demonstrated that the Great Potato Blight
of Ireland was caused by a fungus
Louis Pasteur showed that the péine disease of silkworms was caused
by a protozoan parasite
Joseph Lister (1872-1912) developed a system of surgery designed to
prevent microorganisms from entering wounds; his patients had fewer
postoperative infections, thereby providing indirect evidence that microorganisms
were the causal agents of human disease; his published findings (1867)
transformed the practice of surgery
Robert Koch (1843-1910), using criteria developed by his teacher,
Jacob Henle (1809-1895), established the relationship between Bacillus
anthracis and anthrax; his criteria became known as Koch's Postulates
and are still used to establish the link between a particular microorganism
and a particular disease:
The microorganisms must be present in every case of the disease
but absent from healthy individuals
The suspected microorganisms must be isolated and grown in pure
culture
The same disease must result when the isolated microorganism is
inoculated into a healthy host
The same microorganism must be isolated again from the diseased
host
Koch's work was independently confirmed by Pasteur
The development of techniques for studying microbial pathogens
Koch and his associates developed techniques, reagents, and other
materials for culturing bacterial pathogens on solid growth media; these
enable microbiologists to isolate microbes in pure culture
Charles Chamberland (1851-1908) constructed a bacterial filter
that removed bacteria and larger microbes from specimens; this led to
the discovery of viruses as disease-causing agents
Immunological studies
Edward Jenner (ca. 1798) used a vaccination procedure to protect individuals
from smallpox
Louis Pasteur developed other vaccines including those for chicken
cholera, anthrax, and rabies
Emil von Behring (1854-1917) and Shibasaburo Kitasato (1852-1931)
induced the formation of diphtheria toxin antitoxins in rabbits; the
antitoxins were effectively used to treat humans and provided evidence
for humoral immunity
Elie Metchnikoff (1845-1916) demonstrated the existence of phagocytic
cells in the blood, thus demonstrating cell-mediated immunity
Industrial Microbiology and Microbial Ecology
Louis Pasteur demonstrated that alcoholic fermentations were the result
of microbial activity, that some organisms could decrease alcohol yield
and sour the product, and that some fermentations were aerobic and some
anaerobic; he also developed the process of pasteurization to preserve
wine during storage
Sergei Winogradsky (1856-1953) worked with soil bacteria and discovered
that they could oxidize iron, sulfur, and ammonia to obtain energy; he
also studied anaerobic nitrogen fixation and cellulose decomposition
Martinus Beijerinck (1851-1931) isolated aerobic nitrogen-fixing bacteria,
a root-nodule bacterium capable of fixing nitrogen, and sulfate reducing
bacteria
Beijerinck and Winogradsky pioneered the use of enrichment cultures
and selective media
The Members of the Microbial World
Procaryotes have a relatively simple morphology and lack a true membrane-delimited
nucleus
Eucaryotes are morphologically complex and have a true, membrane-enclosed
nucleus
In a commonly used classification scheme, organisms are divided into
five kingdoms: the Monera or Procaryotae, Protista, Fungi, Animalia, and
Plantae; microbiologists are concerned primarily with members of the first
three kingdoms and also with viruses, which are not classified with living
organisms
Recently a classification scheme consisting of three domains (Bacteria,
Archaea, and Eucarya) has become widely accepted; this scheme is followed
in this textbook
The Scope and Relevance of Microbiology
Microorganisms were the first living organisms on the planet, live everywhere
life is possible, are more numerous than any other kind of organism, and
probably constitute the largest component of the earth's biomass
The entire ecosystem depends on the activities of microorganisms, and
microorganisms influence human society in countless ways
Microbiology has an impact on many fields including medicine, agriculture,
food science, ecology, genetics, biochemistry, and molecular biology
Microbiologists may be interested in specific types of organisms:
Virologists-viruses
Bacteriologists-bacteria
Phycologists or Algologists-algae
Mycologists-fungi
Protozoologists-protozoa
Microbiologists may be interested in various characteristics or activities
of microorganisms:
Microbial morphology
Microbial cytology
Microbial physiology
Microbial ecology
Microbial genetics and molecular biology
Microbial taxonomy
Microbiologists may have a more applied focus:
Medical microbiology, including immunology
Food and dairy microbiology
Public health microbiology
Agricultural microbiology
Industrial microbiology
The Future of Microbiology
Microbiology has had and will continue to have a profound influence
on society.
In the future microbiologists will be:
Trying to better understand and control existing, emerging, and reemerging
infectious diseases
Studying the association between infectious agents and chronic diseases
Learning more about host defenses and host-pathogen interactions
Developing new uses for microbes in industry, agriculture, and environmental
control
Still discovering the many microbes that have not yet been identified
and cultured
Trying to better understand how microbes interact and communicate
Analyzing and interpreting the ever-increasing amount of data from
genome studies
Continuing to use microbes as model systems for answering fundamental
questions in biology
Assessing and communicating the potential impact of new discoveries
and technologies on society
