bacteria
Meaning of bacteria: Vastly more of them live on a person harmlessly than will ever cause illness, which is why killing them indiscriminately has costs.
Definition of bacteria
Bacteria are single-celled organisms that lack a nucleus, keeping their genetic material loose within the cell instead. They are among the most numerous living things on Earth, found in soil, water, air and on and inside the body. Most are harmless or useful to people; a minority cause infection. The singular form is bacterium.
How is a bacterial cell built?
A bacterium has no nucleus and none of the membrane-wrapped compartments found in animal and plant cells, which is what places it in a separate group from them. Its DNA sits as a single circular loop in the body of the cell, often accompanied by small extra rings called plasmids that can be passed between bacteria and that frequently carry resistance genes. Around the cell is a wall, and the structure of that wall is what the Gram stain distinguishes: bacteria with a thick outer layer hold the dye and are called Gram positive, while those with a thin layer behind an outer membrane do not and are called Gram negative. Shape adds a second description, whether round, rod-shaped or spiral. Reproduction is by simple division, which under good conditions can occur within minutes.
Which bacteria are harmful?
| Relationship | What it means |
|---|---|
| Beneficial | Residents of the digestive tract and skin that aid digestion, make vitamins and crowd out invaders |
| Harmless | Present without effect either way |
| Opportunistic | Harmless in their usual place, but causing disease if they reach tissue where they do not belong |
| Pathogenic | Capable of causing disease in a healthy person |
The great majority of the bacteria a person carries fall into the first two rows. Illness follows when an organism reaches somewhere it should not be, when it multiplies beyond what the immune system can contain, or when it produces a toxin, which is how some bacteria cause harm at a distance from where they are growing.
Why does antibiotic resistance develop?
An antibiotic kills bacteria or stops them multiplying, but any that already carry a means of surviving it are left behind to multiply in the space cleared. Resistance genes also spread directly between bacteria on plasmids, including between different species, so a trait that arises in one organism does not stay there. Antibiotics have no effect on viruses, so they do nothing for colds or influenza, and using them where they cannot help exposes a person's harmless bacteria to selection for no benefit.
Also known as
Used in a sentence
Cultures were obtained to identify the specific bacteria responsible for the infection.
Good to know
Most bacteria a person carries do not cause disease and some are beneficial, and antibiotics act only on bacteria, having no effect on infections caused by viruses or other organisms.
Word origin
Where “bacteria” comes from
From Greek bakterion, a diminutive of baktron, meaning a staff or rod, describing the shape of the first bacteria observed under a microscope.