microorganisms

Microorganisms are microscopic organisms that include a broad range of life forms, some cellular and others acellular. Most are smaller than a millimeter, with many in the micrometer range; viruses can be even smaller. They inhabit nearly every environment on Earth, from oceans to soil to the human body.

The cellular forms are grouped into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes—cells without a nucleus. Eukaryotic microorganisms include fungi, protozoa, and certain algae. Viruses are acellular particles that replicate only inside host cells and are commonly considered microorganisms in a broad sense.

Microorganisms play essential ecological roles: decomposing organic matter, cycling nutrients such as carbon and nitrogen, and supporting photosynthesis in cyanobacteria and algae. They form symbiotic relationships with plants and animals, influence climate, and drive fermentation processes used in food and beverage production and biotechnology. In humans, a diverse microbiota protects against pathogens, aids digestion, and modulates immunity, while some species cause diseases.

Scientists study microorganisms using culture methods, microscopy, and molecular techniques such as DNA sequencing and metagenomics. Model organisms like Escherichia coli and Saccharomyces cerevisiae serve as tools to understand basic biology. Advances in microbiology have applications in medicine, industry, and environmental management, but they also require appropriate biosafety considerations.