Explain facultative and obligate bacteria.

Hint: Bacteria are single-celled organisms that are neither plants nor animals. They usually measure a few micrometers in length and exist together in communities of millions. A gram of soil typically contains about 40 million bacterial cells. A milliliter of freshwater usually holds about one million bacterial cells.

Complete Answer:
- Bacteria are also called germs that are microscopic organisms not visible with the naked eye. Bacteria live everywhere that are both inside and outside of your body. Bacteria can live in an array of environments, from hot water to ice. Some bacteria are good for health, while others can make one sick.
- Bacteria are single-celled, or simple, organisms. Even If they are small, they are powerful and complex and can live on in extreme conditions too. Bacteria have a hard protective coating that boosts their resistance to white blood cells in the body.
- Some of the bacteria possess a tail, called a flagellum. The flagellum is the structure that helps a bacterium to move around. Some bacteria have sticky hair-like appendages that help them to fix to hard surfaces and also the human body cells.
- Bacteria which are described as facultative anaerobes can grow well in oxygen but can also continue growing in its absence without oxygen. While they particularly flourish in the presence of oxygen, they can also use processes such as fermentation to continue growing when oxygen is not available.
- Obligate bacteria are organisms which can live only in environments which lack oxygen. These organisms get poisoned by oxygen. Obligate anaerobes are characteristically bacteria and live in a variety of places in nature. Many obligate anaerobes to live inside the human body in spaces like the mouth and gastrointestinal tract where their oxygen levels are very low. Both of them cause benefits and harm to human beings.

Note: Bacteria are considered to have been the first organisms to appear on earth about 4 billion years ago. The oldest recognized fossils are of bacteria-like organisms.