Bacteria are prokaryotes, which means they are unicellular and have no nucleus. They come in various shapes: rodlike, spherical or spiral. Bacteria are very small. A typical one is only several micrometres in size. 1 billion could fit on 1 square centimeter of space on the human gums, and 1 gram of digested food has 10 billion bacteria.
Because they are such small organisms, light microscopes can only let us see their shape, and how they move. Under an electron microscope, we can see the details of their external parts and perhaps a blurred center due to their genetic material not being within a nucleus.
Bacteria have a cell wall that can be either thick or thin. The chemical test, the Gram Staining technique, can help a researcher to determine this characteristic for keying out and idenficaton purposes. A thin-walled bacteria will appear to be purple for a positive Gram-staining, while bacteria will be pink if they are Gram-negative.
Bacteria can have external locomotive structures called flagella or pilli. Locomotive structures are those that allow the organisms to move around in its environment.
Under ideal conditions, one bacterium duplicates itself in about 20 mins, i.e. every 20 minutes, the number of bacteria doubles. In one day, from one bacterium, it can reproduce to 2^72 new organisms (which is 4,722,366,482,869,645,213,696). In one 43 hours, their population is about 2^130 organisms. One bacterium weighs about 10-12g. 2130 bacteria weighs over 6 x 1024 kg, which is the weight of the Earth!
Because they can reproduce so fast, their populations can adapt to new environments; they evolve to meet the demands of any new localized stressors. They can change so that they can use other substances as their food and can develop a resistance to antibiotics. An important note to the above calculation is it is often that those bacteria don't have enough food to reach that population. They also have a lot of enemies. They die as fast as they duplicate.
Bacteria belong to domain Eubacteria