Microbiology, study of microorganisms, or microbes, a diverse group of generally minute simple life-forms that include bacteriaarchaeaalgaefungiprotozoa, and viruses. The field is concerned with the structure, function, and classification of such organisms and with ways of both exploiting and controlling their activities.

The 17th-century discovery of living forms existing invisible to the naked eye was a significant milestone in the history of science, for from the 13th century onward it had been postulated that “invisible” entities were responsible for decay and disease. The word microbe was coined in the last quarter of the 19th century to describe these organisms, all of which were thought to be related. As microbiology eventually developed into a specialized science, it was found that microbes are a very large group of extremely diverse organisms.

Daily life is interwoven inextricably with microorganisms. In addition to populating both the inner and outer surfaces of the human body, microbes abound in the soil, in the seas, and in the air. Abundant, although usually unnoticed, microorganisms provide ample evidence of their presence—sometimes unfavourably, as when they cause decay of materials or spread diseases, and sometimes favourably, as when they ferment sugar to wine and beer, cause bread to rise, flavour cheeses, and produce valued products such as antibiotics and insulin. Microorganisms are of incalculable value to Earth’s ecology, disintegrating animal and plant remain and converting them to simpler substances that can be recycled in other organisms.

Historical Background

Microbiology essentially began with the development of the microscope. Although others may have seen microbes before him, it was Antonie van Leeuwenhoek, a Dutch draper whose hobby was lens grinding and making microscopes, who was the first to provide proper documentation of his observations. His descriptions and drawings included protozoans from the guts of animals and bacteria from teeth scrapings. His records were excellent because he produced magnifying lenses of exceptional quality. Leeuwenhoek conveyed his findings in a series of letters to the British Royal Society during the mid-1670s. Although his observations stimulated much interest, no one made a serious attempt either to repeat or to extend them. Leeuwenhoek’s “animalcules,” as he called them, thus remained mere oddities of nature to the scientists of his day, and enthusiasm for the study of microbes grew slowly. It was only later, during the 18th-century revival of a long-standing controversy about whether life could develop out of nonliving material, that the significance of microorganisms in the scheme of nature and in the health and welfare of humans became evident.

Spontaneous generation versus biotic generation of life

The early Greeks believed that living things could originate from nonliving matter (abiogenesis) and that the goddess Gea could create life from stones. Aristotle discarded this notion, but he still held that animals could arise spontaneously from dissimilar organisms or from the soil. His influence regarding this concept of spontaneous generation was still felt as late as the 17th century, but toward the end of that century, a chain of observations, experiments, and arguments began that eventually refuted the idea. This advance in understanding was hard-fought, involving series of events, with forces of personality and individual will often obscuring the facts.

Although Francesco Redi, an Italian physician, disproved in 1668 that higher forms of life could originate spontaneously, proponents of the concept claimed that microbes were different and did indeed arise in this way. Such illustrious names as John Needham and Lazzaro Spallanzani were adversaries in this debate during the mid-1700s. In the early half of the 1800s, Franz Schulze and Theodor Schwann were major figures in the attempt to disprove theories of abiogenesis until Louis Pasteur finally announced the results of his conclusive experiments in 1864. In a series of masterful experiments, Pasteur proved that only preexisting microbes could give rise to other microbes (biogenesis). Modern and accurate knowledge of the forms of bacteria can be attributed to German botanist Ferdinand Cohn, whose chief results were published between 1853 and 1892. Cohn’s classification of bacteria, published in 1872 and extended in 1875, dominated the study of these organisms thereafter.

Types Of Microorganisms

The major groups of microorganisms—namely bacteria, archaea, fungi (yeasts and molds), algae, protozoa, and viruses—are summarized below. Links to the more detailed articles on each of the major groups are provided.

Bacteria (eubacteria and archaea)

Microbiology came into being largely through studies of bacteria. The experiments of Louis Pasteur in France, Robert Koch in Germany, and others in the late 1800s established the importance of microbes to humans. As stated in the Historical background section, the research of these scientists provided proof for the germ theory of disease and the germ theory of fermentation. It was in their laboratories that techniques were devised for the microscopic examination of specimens, culturing (growing) microbes in the laboratory, isolating pure cultures from mixed-culture populations, and many other laboratory manipulations. These techniques, originally used for studying bacteria, have been modified for the study of all microorganisms—hence the transition from bacteriology to microbiology.

The organisms that constitute the microbial world are characterized as either prokaryotes or eukaryotes; all bacteria are prokaryotic—that is, single-celled organisms without a membrane-bound nucleus. Their DNA (the genetic material of the cell), instead of being contained in the nucleus, exists as a long, folded thread with no specific location within the cell.

Until the late 1970s, it was generally accepted that all bacteria are closely related to evolutionary development. This concept was challenged in 1977 by Carl R. Woese and co-investigators at the University of Illinois, whose research on ribosomal RNA from a broad spectrum of living organisms established that two groups of bacteria evolved by separate pathways from a common and ancient ancestral form. This discovery resulted in the establishment of new terminology to identify the major distinct groups of microbes—namely, the eubacteria (the traditional or “true” bacteria), the archaea (bacteria that diverged from other bacteria at an early stage of evolution and are distinct from the eubacteria), and the eukarya (the eukaryotes). Today the eubacteria are known simply as the true bacteria (or the bacteria) and form the domain Bacteria. The evolutionary relationships between various members of these three groups, however, have become uncertain, as comparisons between the DNA sequences of various microbes have revealed many puzzling similarities. As a result, the precise ancestry of today’s microbes is very difficult to resolve. Even traits thought to be characteristic of distinct taxonomic groups have unexpectedly been observed in other microbes. For example, an anaerobic ammonia-oxidizer—the “missing link” in the global nitrogen cycle—was isolated for the first time in 1999. This bacterium (an aberrant member of the order Planctomycetales) was found to have internal structures similar to eukaryotes, a cell wall with archaean traits, and a form of reproduction (budding) similar to that of yeast cells.

Bacteria have a variety of shapes, including spheres, rods, and spirals. Individual cells generally range in width from 0.5 to 5 micrometers (ÎŒm; millionths of a meter). Although unicellular, bacteria often appear in pairs, chains, tetrads (groups of four), or clusters. Some have flagella, external whiplike structures that propel the organism through liquid media; some have capsules, an external coating of the cell; some produce spores—reproductive bodies that function much as seeds do among plants. One of the major characteristics of bacteria is their reaction to the Gram stain. Depending upon the chemical and structural composition of the cell wall, some bacteria are gram-positive, taking on the stain’s purple color, whereas others are gram-negative.

bacterial cell

bacterial cell Schematic drawing of the structure of a generalized bacterium.

Through a microscope, the archaea look much like bacteria, but there are important differences in their chemical composition, biochemical activities, and environments. The cell walls of all true bacteria contain the chemical substance peptidoglycan, whereas the cell walls of archaeans lack this substance. Many archaeans are noted for their ability to survive unusually harsh surroundings, such as high levels of salt or acid or high temperatures. These microbes, called extremophiles, live in such places as salt flats, thermal pools, and deep-sea vents. Some are capable of a unique chemical activity—the production of methane gas from carbon dioxide and hydrogen. Methane-producing archaea live only in environments with no oxygen, such as swamp mud or the intestines of ruminants such as cattle and sheep. Collectively, this group of microorganisms exhibits tremendous diversity in the chemical changes that it brings to its environments.


The cells of eukaryotic microbes are similar to plant and animal cells in that their DNA is enclosed within a nuclear membrane, forming the nucleus. Eukaryotic microorganisms include algae, protozoa, and fungi. Collectively algae, protozoa, and some lower fungi are frequently referred to as protists (Kingdom Protista, also called Protoctista); some are unicellular and others are multicellular.

Unlike bacteria, algae are eukaryotes and, like plants, contain the green pigment chlorophyll, carry out photosynthesis, and have rigid cell walls. They normally occur in moist soil and aquatic environments. These eukaryotes may be unicellular and microscopic in size or multicellular and up to 120 meters (nearly 400 feet) in length. Algae as a group also exhibit a variety of shapes. Single-celled species may be spherical, rod-shaped, club-shaped, or spindle-shaped. Some are motile. Algae that are multicellular appear in a variety of forms and degrees of complexity. Some are organized as filaments of cells attached end to end; in some species, these filaments intertwine into macroscopic, plantlike bodies. Algae also occur in colonies, some of which are simple aggregations of single cells, while others contain different cell types with special functions.

Representative algae. EncyclopĂŠdia Britannica, Inc.


Fungi are eukaryotic organisms that, like algae, have rigid cell walls and may be either unicellular or multicellular. Some may be microscopic in size, while others form much larger structures, such as mushrooms and bracket fungi that grow in soil or on damp logs. Unlike algae, fungi do not contain chlorophyll and thus cannot carry out photosynthesis. Fungi do not ingest food but must absorb dissolved nutrients from the environment. Of the fungi classified as microorganisms, those that are multicellular and produce filamentous, microscopic structures are frequently called molds, whereas yeasts are unicellular fungi.

In molds, cells are cylindrical in shape and are attached end to end to form threadlike filaments (hyphae) that may bear spores. Individually, hyphae are microscopic in size. However, when large numbers of hyphae accumulate—for example, on a slice of bread or fruit jelly—they form a fuzzy mass called a mycelium that is visible to the naked eye.

The unicellular yeasts have many forms, from spherical to egg-shaped to filamentous. Yeasts are noted for their ability to ferment carbohydrates, producing alcohol and carbon dioxide in products such as wine and bread.

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