Bacteria are one-celled animals. In terms of sheer numbers, they are easily the dominant type of life on Earth. You carry 10 times more bacteria than cells of your own body. Most of them are concentrated on your skin and in your digestive system. Most of the weight of your excrement consists of bacteria.

They are only a few micrometers in length, and are found growing in almost every environment: in the soil, in water, in acidic hot springs, in radioactive waste and deep in the Earth’s crust. Some bacteria are even anaerobic, meaning they don’t use oxygen to live on.

bacteriaandyourimmunesystem For the first billion years of life on earth, all life were one-celled bacteria. A fossilized bacterial cell was found in Western Australia that’s 3.5 million years old — the oldest evidence of life on Earth. Scientists believe that one-celled blue-green algae are the oldest type of true living organisms on Earth.

Unlike viruses, bacteria are alive. Tiny and only one-cell, but alive. They must eat to survive, they reproduce and they die.

They’re small pieces of cytoplasm with a thin membrane and containing the nucleis acids DNA and RNA. Some are essentially plants, because they use photosynthesis to gain energy directly from sunlight. Some are essentially animals because they feed on something which they use enzymes to digest.

Life on this planet could not exist without bacteria. They bring in nitrogen from the atmosphere and decompose the bodies of dead plants and animals so that their material can be reused. They are used in the processing of sewage, and in producing cheese and yogurt — and the antibiotics used to kill harmful bacteria.

Technically, bacteria are prokaryotes. That means that they don’t have a nucleus and don’t usually have membrane-bound organelles. They were discovered in 1676 by Antoni van Leeuwenhoek, the first person to observe bacteria using a microscope.

They come in a wide variety of shapes (called their morphologies) — spherical, rod-shaped, comma-shaped, spiral-shaped, coiled and even cubes and tetrahedrons.

Some types of bacteria can form themselves into strong, virtually indestructible balls called endospores, and in that way go dormant yet still survive for millions of years. They get through extreme heat, UV light, gamma radiation, detergents, disinfectants, pressure and lack of water. One type that can do this is anthrax. Another is tetanus.

Bacteria grow to a fixed size and then reproduce. They divide themselves using asexual reproduction, binary fission. Bacteria can grow and divide extremely rapidly, and bacterial populations can double every 9.8 minutes.

Bacteria cause bacterial infections, which are subject to control by antibiotics, though of course some antibiotics are better at killing off some types of bacteria than others. Plus, they do continually evolve to resist antibiotics and antibacterial agents such as special soaps, deodorants, cleansers and so on.

Because they’re relatively large and exist independently, they’re easy for your body’s macrophages to detect when they gain entry into your body. Your T-cells can then destroy them. Bacteria are subject to cell-mediated immunity, your natural or innate immune system.