Bacteria

Bacteria (also: bacterium) are microscopic single-celled organisms belonging to the prokaryotes—meaning their genetic material (DNA) is not enclosed in a nucleus with a membrane, as is the case with animal and plant cells. Their size typically ranges from one to several micrometers, and they can only be seen with a microscope.

Bacteria live in nearly every environment: soil, water, air, deep oceans, and the bodies of other organisms. Many do not cause disease; some play roles in digestion, the nitrogen cycle, and the breakdown of organic matter. A small number cause diseases such as plague, cholera, and some types of fever.

Cell structure

A bacterial cell has a membrane and usually a cell wall made of peptidoglycan, which gives the cell its shape and strength. Inside are cytoplasm, ribosomes that synthesize proteins, and a single circular strand of DNA called the nucleoid. Some bacteria have plasmids—small, independent loops of DNA—and appendages such as flagella for movement.

Forms and classification

Common shapes include spherical (cocci), rod-shaped (bacilli), and spiral or curved (spirilla). The most widely used classification method is Gram staining, which divides bacteria into Gram-positive and Gram-negative based on cell wall structure. This classification affects which antibiotics will be effective.

Reproduction and growth

Bacteria reproduce through binary fission: a cell divides into two after the DNA replicates. Under favorable conditions, some species can divide within minutes, leading to rapid growth. Exchange of genetic material through horizontal gene transfer—such as the transfer of plasmids—allows them to acquire new traits rapidly, including antibiotic resistance.

Medical and industrial roles

Bacterial infections are treated with antibiotics, though overuse or improper use increases resistance. On the other hand, bacteria are used in producing fermented foods such as yogurt, cheese, and processed foods, and in biotechnology for producing insulin and enzymes.