Microbiology is the branch of biology that studies microorganisms and other biological entities that require microscopic methods or specialised techniques for their investigation. The field traditionally includes bacteria, archaea, microscopic fungi, protozoa and many algae, while viruses and other acellular infectious agents are also studied extensively within microbiology even though they are not generally considered living cells. Microbiology examines the structure, genetics, metabolism, reproduction, evolution, ecology and interactions of microorganisms.
The term combines Greek roots associated with smallness and the study of life. Microorganisms occur almost everywhere, including soil, water, air, sediments, food, plants, animals and the human body. They can be beneficial, harmless or harmful depending on their species, strain, environment and relationship with a host. Microbiology therefore encompasses basic science and practical disciplines concerned with health, agriculture, industry, environmental processes, biotechnology and food production.
Microbial diversity is enormous, and microorganisms differ substantially in cellular organisation and metabolism. Bacteria and archaea are generally prokaryotic organisms, meaning their genetic material is not enclosed within a membrane bound nucleus. Eukaryotic microorganisms, including many fungi, algae and protozoa, possess nuclei and other membrane bound organelles. Viruses differ fundamentally because they consist of genetic material enclosed in protein and sometimes lipid structures and depend on host cells for replication.
Microbial cells may obtain energy through photosynthesis, respiration, fermentation, oxidation of inorganic compounds or other metabolic strategies. Some microorganisms require oxygen, others are harmed by oxygen, and some can switch between metabolic modes depending on environmental conditions. This metabolic diversity enables microorganisms to inhabit extreme environments such as acidic waters, deep sediments, high salt environments and hot springs as well as ordinary environments associated with plants and animals.
Microbiology became a distinct scientific discipline through advances in microscopy, experimental methods and germ theory. Early microscopic observations revealed previously unknown forms of life, while later researchers established that microorganisms could cause fermentation, decomposition and disease. Developments associated with figures such as Antonie van Leeuwenhoek, Louis Pasteur and Robert Koch helped establish foundational concepts, although microbiology developed through contributions from many scientists and institutions. The recognition that specific microorganisms could cause particular diseases transformed medicine and public health. Sterilisation, antisepsis, vaccination, laboratory culture and diagnostic microbiology subsequently became important applications. Modern microbiology has expanded far beyond organisms that can readily be grown in laboratory culture because molecular sequencing has revealed enormous microbial diversity that cannot be captured by traditional cultivation alone.
Medical microbiology studies microorganisms associated with human disease, including bacterial, viral, fungal and parasitic infections. Researchers investigate how pathogens enter hosts, attach to tissues, evade immune responses, obtain nutrients, reproduce and cause cellular or systemic damage. Diagnostic microbiology identifies infectious agents using microscopy, culture, biochemical tests, antigen detection, nucleic acid amplification, sequencing and other methods. Antimicrobial susceptibility testing can help determine which drugs are likely to inhibit particular bacterial infections, although resistance can complicate treatment.
Microbiology is also essential for infection prevention because understanding transmission routes informs hygiene, sterilisation, isolation and environmental control. Importantly, not all microorganisms are pathogenic. The human microbiota includes vast communities of bacteria, archaea, fungi and other microorganisms associated with skin, mouth, intestine and other body sites, and these communities can influence nutrition, metabolism, immune development and resistance to colonisation by some pathogens.
Environmental microbiology examines microorganisms in natural and engineered ecosystems. Microbes are major participants in carbon, nitrogen, sulphur and phosphorus cycles and contribute to decomposition, nutrient transformation and ecosystem functioning. Certain bacteria and archaea perform specialised processes such as nitrogen fixation, nitrification, denitrification and methanogenesis. Microorganisms can break down organic pollutants and are therefore used in bioremediation. Wastewater treatment relies heavily on microbial communities to remove organic matter and nutrients.
Soil microorganisms influence plant nutrition, decomposition and soil structure, while marine microorganisms contribute substantially to oceanic nutrient cycling and carbon processing. Microbial ecology increasingly uses metagenomics and other molecular methods to study entire communities without requiring each organism to be isolated individually. These approaches have revealed that microbial communities are complex networks of interacting organisms rather than simple collections of independent species.
Industrial and food microbiology apply microorganisms or microbial processes to useful purposes. Yeasts are used in bread making and alcoholic fermentation, while bacteria are important in yoghurt, cheese, vinegar and other fermented foods. Industrial microorganisms can produce enzymes, organic acids, vitamins, amino acids, antibiotics and other commercially valuable substances. Biotechnology can use genetically modified microorganisms as biological factories for producing specific proteins or chemicals.
Microbial fermentation can occur under controlled conditions in bioreactors where temperature, oxygen, pH, nutrients and other variables are monitored. Food microbiology also investigates spoilage organisms and pathogens to improve preservation and safety. Some microorganisms are deliberately selected for desirable metabolic properties, whereas others must be controlled because they cause contamination or disease.
Microbial genetics and molecular microbiology investigate how microorganisms store, express and exchange genetic information. Bacterial genomes may include a chromosome and additional DNA molecules called plasmids, some of which carry genes providing traits such as antibiotic resistance. Horizontal gene transfer can occur through mechanisms including transformation, transduction and conjugation, allowing genetic information to move between cells and potentially accelerate adaptation. Microbial communities can evolve rapidly under selective pressures because of short generation times, large population sizes and genetic variation. Molecular techniques such as polymerase chain reaction, genome sequencing, transcriptomics and proteomics allow researchers to investigate microbial functions at increasingly fine scales. CRISPR based systems, originally discovered as microbial defence mechanisms against genetic elements, have also become important biotechnology tools.
Contemporary microbiology is therefore an interdisciplinary science connecting cell biology, genetics, chemistry, medicine, ecology, environmental science and biotechnology. A microorganism cannot be defined simply as a harmful germ because the microbial world includes beneficial symbionts, environmental decomposers, industrial organisms, pathogens and organisms whose roles remain incompletely understood. The distinction between microorganisms and larger organisms is also based largely on scale and practical observation rather than a single biological boundary.
Advances in sequencing, imaging, single cell analysis and computational biology are revealing previously inaccessible microbial communities and functions. Microbiology is important because microorganisms affect disease, food, agriculture, environmental cycles, industrial production and global ecosystems. Understanding them requires considering both individual microbial species and the communities and molecular systems in which they operate.
