Bacteria
Bacteria are single cells living nearly everywhere, including inside animals, and far more than the villains of the infection aisle.
By Greg Goodwin · Published November 4, 2026
Cheat Sheet
- Genome range: about 580,000 base pairs in Mycoplasma genitalium, 4.7 million in E. coli, and over 13 million in Sorangium cellulosum (Britannica).
- Packing job: an E. coli cell is about 0.001 mm long, yet its chromosome stretched out would be about 1.2 mm.
- Ribosomes: bacterial ones have three rRNA types and 55 proteins versus four and 78 in eukaryotes, and different antibiotics inhibit them.
- Reproduction: most prokaryotes divide by binary fission into two identical daughter cells.
- Strains: for many bacterial species there are thousands of serovars, strains told apart by antibody tests.
- US burden: CDC's 2026 AR Threats Report covers 19 bacterial and fungal pathogens, linked to 9.7 million infections and 50,800 deaths a year.
The 60-Second Version
Bacteria are microscopic, single-celled organisms that inhabit virtually all environments on Earth, including the bodies of multicellular animals. They lack a membrane-bound nucleus and other internal structures. Instead, their DNA sits as a long coil in the cell's interior, in many bacteria as a single circular chromosome, sometimes alongside small extra loops called plasmids. Most multiply by simply dividing in half. The reputation problem is that some bacteria cause infections, and CDC tracks drug-resistant ones as a serious public health threat. Yet Britannica's coverage also includes the importance of bacteria to humans in food, in industry and in medicine, a reminder that the full picture is broader than germs. The big idea for a newcomer is that bacteria are a huge, varied group, not one thing, so statements about what bacteria do usually mean particular kinds. This page is general information, not medical advice; for infections or symptoms, see a clinician.
The Long Version
The Bacterial Cell
Britannica describes bacteria as prokaryotes. Unlike the DNA in eukaryotic cells, which resides in the nucleus, DNA in bacterial cells is not sequestered in a membrane-bound organelle but appears as a long coil distributed through the cytoplasm. In many bacteria the DNA is a single circular chromosome, though some bacteria have two chromosomes and in some cases the DNA is linear. Smaller DNA molecules called plasmids can carry auxiliary information. The cytoplasm holds enzymes, metabolites and salts, and the cell's proteins are made on ribosomes scattered throughout it. Bacteria also keep storage granules that are never enclosed by a membrane, and many aquatic species make gas vacuoles that let them adjust their buoyancy.
Genomes Big and Small
Britannica reports that the DNA sequence has been determined for hundreds of bacteria, and the amount of DNA in bacterial chromosomes varies widely. It ranges from 580,000 base pairs in Mycoplasma genitalium to 4,700,000 base pairs in E. coli to roughly 9,450,000 in Myxococcus xanthus, and Sorangium cellulosum, a myxobacterium, has one of the largest bacterial genomes at more than 13 million base pairs. The size comparison to the cell itself is striking: the E. coli chromosome, stretched to its fullest extent, is about 1.2 mm, while the length of the cell is about 0.001 mm. That means the cell must coil its DNA into a very small space.
How Bacteria Multiply
According to Britannica, most prokaryotes reproduce by binary fission, in which the cell grows in volume until it divides in half to yield two identical daughter cells. Each daughter can keep growing at the same rate as its parent. The cell grows over its entire surface until division, when a new hemispherical pole forms at the division septum in the middle of the cell. In gram-positive bacteria the septum grows inward from the plasma membrane along the midpoint; in gram-negative bacteria the walls are more flexible, and the septum forms as the side walls pinch inward. Different wall-cross-linking enzymes must be active at the septum than elsewhere for the cell to split cleanly.
Variety Within a Species
Even within one species, bacteria vary. Britannica says the bewildering number of strains of bacteria that belong to the same species can be distinguished by serological tests, and it suggests that pathogenic bacteria constantly battling the host's immune system might account for the variety. Microbiologists often identify bacteria by specific molecules on the cell surface, detected with antibodies. For many species there are thousands of serovars, which differ mainly in the antigenic identity of their lipopolysaccharide, flagella or capsule. Different serovars of E. coli and Salmonella enterica are often associated with different host animals or different diseases, so the name of the species alone can hide a lot of difference.
Bacteria and Human Health
Some bacteria cause disease, and treating them depends on antibiotics, which is why resistance is a concern. CDC defines antimicrobial resistance as germs like bacteria and fungi defeating the drugs designed to kill them. CDC's first AR Threats Report in 2013 said at least 2 million people in the U.S. got a resistant infection each year and at least 23,000 died. Its 2019 report, recalculated with new data sources, said more than 2.6 million infections and nearly 44,000 deaths. The 2026 update lists 19 bacterial and fungal pathogens plus a six-pathogen Watch List and reports 9.7 million infections and 50,800 deaths a year, but CDC cautions that methods changed, so direct comparisons with earlier reports should not be made for most pathogens.
Why People Care
Bacteria are central to health, food and the environment, and the spread of drug resistance makes basic bacterial biology relevant to everyday news. A little background helps you read headlines about infections, antibiotics and the microbes around us with more context.
Glossary
- Prokaryote
- A cell without a membrane-bound nucleus; bacteria are prokaryotes.
- Plasmid
- A smaller, usually circular DNA molecule in a bacterial cell that can carry auxiliary information.
- Binary fission
- Reproduction in which a cell grows until it divides in half to yield two identical daughter cells.
- Serovar
- A strain of a bacterial species distinguished by antibody tests on molecules on its surface; short for serological variant.
- Antimicrobial resistance
- When germs such as bacteria and fungi can defeat the drugs designed to kill them.
Go Deeper
More to Explore
- Natural Selection
Natural selection is evolution's bookkeeper: it plans nothing and simply tallies who leaves more offspring.
- Mitochondria
Tiny former bacteria living inside nearly every cell you own, quietly paying the electric bill.
- Epigenetics
Same script, different performances: how the cells in your body read the same DNA and end up as brain, skin, or liver.
- DNA and RNA
DNA is the master recipe book; RNA is the photocopy that actually makes it to the kitchen.
- Genetic Inheritance
Pea plants taught Gregor Mendel the basic rules of inheritance in the 1860s, decades before anyone had identified DNA as the carrier.