Microorganism

Lore & Background

The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in Jain literature authored in 6th-century BC India. The scientific study of microorganisms began with their observation under the microscope in the 1670s by Anton van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused tuberculosis, cholera, diphtheria, and anthrax. Microorganisms are extremely diverse, representing most unicellular organisms in all three domains of life: Archaea and Bacteria only contain microorganisms, while Eukaryota includes multicellular organisms and many unicellular protists and protozoans. Some protists are related to animals and some to green plants. Many multicellular organisms are also microscopic, including micro-animals, some fungi, and some algae. Microorganisms can have very different habitats, living everywhere from poles to equator, in deserts, geysers, rocks, and the deep sea. Some are adapted to extremes such as very hot or very cold conditions, high pressure, or high radiation. Microbes are a vital component of fertile soil and make up the microbiota found in and on all multicellular organisms. There is evidence that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest direct evidence of life on Earth.

Reader's Guide

Microorganisms are fundamental to understanding life on Earth. They were the first forms of life, appearing approximately 3.5 billion years ago, and for about 3 billion years all organisms were microorganisms. Their study revolutionized medicine: Pasteur's work refuted spontaneous generation and established that microbes cause food spoilage, while Koch's discoveries linked specific microbes to diseases such as tuberculosis and anthrax, leading to Koch's postulates for establishing causation. Microbes are essential tools in biology as model organisms and are used to ferment foods, treat sewage, and produce fuel and enzymes. They also constitute the human microbiota, including gut flora, and are the target of hygiene measures against infectious diseases. The diversity of microbial habitats—from deep-sea vents to high-radiation environments—highlights their adaptability. The discovery of viruses by Beijerinck and the concept of chemolithotrophy by Winogradsky expanded microbiology beyond medical contexts, revealing microbes' roles in geochemical cycles. Despite their small size, microorganisms have shaped human culture, health, and the planet's biogeochemistry.

Did You Know?

• The earliest direct evidence of life on Earth is from 3.45-billion-year-old Australian rocks that once contained microorganisms.
• Anton van Leeuwenhoek first observed microorganisms under a microscope in the 1670s.
• Louis Pasteur debunked the theory of spontaneous generation by showing that microorganisms caused food spoilage.
• Robert Koch discovered that microorganisms cause tuberculosis, cholera, diphtheria, and anthrax.

The Vast Diversity of the Microbial World

Microbiology, a term rooted in Greek words meaning the study of small life, encompasses an extraordinarily broad spectrum of entities. The organisms it examines range from unicellular forms to multicellular complexes and even acellular entities that lack cellular structure entirely. At the most fundamental taxonomic divide, microorganisms are sorted into two major camps: eukaryotes, which harbor membrane-bound organelles and include fungi and protists, and prokaryotes, which conventionally lack such organelles and encompass Bacteria and Archaea. Beyond these, viruses occupy a contested boundary, having been described variously as the simplest microorganisms or as extraordinarily complex molecules. Prions, never formally classified as microorganisms, were nonetheless investigated by virologists because their clinical effects were initially mistaken for chronic viral infections, leading to the unexpected discovery of infectious proteins. The field's breadth is reflected in its numerous sub-disciplines, including virology, bacteriology, protistology, mycology, immunology, and parasitology, each addressing a distinct slice of this invisible world.

Centuries of Intuition Before the Lens

Long before any glass lens revealed the invisible, thinkers across civilizations intuited that unseen creatures governed disease and decay. In sixth-century BCE India, the Jain philosopher Mahavira asserted that sub-microscopic beings called nigodas swarmed in vast clusters throughout earth, water, air, and fire, even within plant tissues and animal flesh, though their lifespans were exceedingly brief. In ancient Rome, Marcus Terentius Varro cautioned against building near swamps, warning that minute, invisible creatures floated in the air and entered the body through the mouth and nose to cause serious illness. Persian scholars contributed their own observations: Avicenna discussed the topic in his Canon of Medicine, Ibn Zuhr identified scabies mites, and Al-Razi produced the earliest known description of smallpox in his work al-Hawi. A tenth-century Taoist text, the Baoshengjing, described countless micro organic worms resembling vegetable seeds, a passage that led Dutch sinologist Kristofer Schipper to conclude that harmful bacteria were known to Chinese thinkers of that era.

The First Glimpses Through Glass

The transition from philosophical speculation to empirical observation unfolded in the mid-seventeenth century, and the question of who truly first saw a microorganism remains debated. Yet evidence suggests the Jesuit priest Athanasius Kircher may have preceded him. His simple yet powerful instruments allowed him to see a world no one had previously documented, earning him the title of father of microbiology.

Forging a Discipline in the Nineteenth Century

The nineteenth century transformed microbiology from a collection of curious observations into a rigorous biological science. Ferdinand Cohn, a botanist whose research on algae and photosynthetic bacteria led him to describe organisms including Bacillus and Beggiatoa, is credited with founding bacteriology. He devised the first taxonomic classification scheme for bacteria and discovered endospores. Working alongside Cohn, Louis Pasteur conducted a landmark series of experiments that dismantled the long-held belief in spontaneous generation, thereby anchoring microbiology firmly within the biological sciences. Pasteur also developed practical methods for food preservation through pasteurization and created vaccines against anthrax, fowl cholera, and rabies. His student Adrien Certes went on to establish marine microbiology as a distinct field. Robert Koch, meanwhile, advanced medical microbiology by proving that specific diseases were caused by specific pathogenic microorganisms, a principle formalized in what became known as Koch's postulates. Together, these three figures laid the intellectual and practical foundations upon which all modern microbiology rests.