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, namely micro-animals, some fungi, and some algae. Microorganisms can have very different habitats, living everywhere from the poles to the 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 like tuberculosis and anthrax, leading to Koch's postulates for establishing disease causation. Microbes are essential in human culture and health, serving to ferment foods, treat sewage, and produce fuel and enzymes. They form the human microbiota, including essential gut flora, and are the target of hygiene measures against infectious diseases. Their diversity spans all domains of life, and they inhabit extreme environments from hot springs to deep sea, demonstrating remarkable 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. Microorganisms remain indispensable as model organisms in biology and have been used in biological warfare and bioterrorism.

Did You Know?

• The possible existence of unseen microbial life was suspected in 6th-century BC Jain literature.
• Anton van Leeuwenhoek first observed microorganisms under a microscope in the 1670s.
• Louis Pasteur debunked the theory of spontaneous generation by showing that microorganisms cause food spoilage.
• Robert Koch discovered that microorganisms cause tuberculosis, cholera, diphtheria, and anthrax in the 1880s.

Ancient Intuitions and the First Glimpses

The idea that invisible living things populate our world long predates any laboratory evidence. As early as the sixth century BCE, the Jain philosopher Mahavira spoke of unseen creatures dwelling in earth, water, air, and fire, and Jain scriptures described tiny organisms called nigodas that clustered in vast numbers and lived brief lives throughout the universe, even within plant and animal tissues. In ancient Rome, Marcus Terentius Varro cautioned against building near swamps, warning that invisible minute creatures floated in the air and entered the body through the mouth and nose to cause serious illness. Persian scholars added to this body of thought: Avicenna discussed microbial existence in his medical canon, Ibn Zuhr identified scabies mites, and Al-Razi provided what is recognized as the earliest description of smallpox. A tenth-century Taoist text similarly described countless microscopic worms resembling vegetable seeds. These scattered intuitions would not be confirmed visually for centuries.

Pioneers of the Microscopic World

The first confirmed visual encounters with microbial life arrived in the 1660s. The field then leapt forward in the nineteenth century. Ferdinand Cohn, a botanist studying algae and photosynthetic bacteria, described organisms including Bacillus and Beggiatoa, formulated the first taxonomic scheme for bacteria, and discovered endospores. Louis Pasteur disproved spontaneous generation through his famous experimental series, developed pasteurization, and created vaccines against anthrax, fowl cholera, and rabies. Robert Koch established the germ theory of disease and articulated what became known as Koch's postulates, proving that specific pathogens cause specific illnesses.

Taxonomic Diversity and the Boundaries of the Microbial World

Microbiology, a term drawn from the Greek words for small, life, and study of, encompasses an extraordinarily diverse collection of entities. The organisms it examines are broadly divided into prokaryotes and eukaryotes. Eukaryotic microorganisms possess membrane-bound organelles and include fungi and protists, while prokaryotic organisms, conventionally understood as lacking such organelles, comprise Bacteria and Archaea. The discipline branches into numerous sub-fields: virology, bacteriology, protistology, mycology, immunology, and parasitology. Yet the boundaries of what counts as a microorganism remain contested. Viruses occupy an ambiguous position, having been classified variously as very simple microorganisms or as very complex molecules. Prions, never formally regarded as microorganisms, have nonetheless been investigated by virologists because their clinical effects were originally presumed to stem from chronic viral infections; in the course of that search, researchers discovered what they termed infectious proteins. This taxonomic fluidity underscores that the microbial world resists neat categorization.

From Petri Dishes to DNA Sequences

For much of its history, microbiology depended on a relatively narrow toolkit: culturing organisms in isolation, applying staining techniques, and examining specimens under a microscope. These methods allowed scientists to isolate and identify microorganisms, but they carried a severe limitation. In common environments, fewer than one percent of the microorganisms present can actually be cultured in isolation using current techniques, meaning the vast majority of microbial life remained invisible to traditional laboratory practice. The emergence of biotechnology transformed this landscape. Microbiologists now rely heavily on molecular biology tools, particularly DNA sequence-based identification. A prominent example is the use of the 16S rRNA gene sequence to identify and classify bacteria, a method that bypasses the need for viable cultures entirely. This shift from phenotypic observation to genotypic analysis has opened access to the enormous, previously unculturable microbial diversity that inhabits virtually every environment on Earth.