It was suggested that bacteria thrive in the Mariana Trench, which with a depth of up to 11 kilometres is the deepest known part of the oceans.The famous notion that bacterial cells in the human body outnumber human cells by a factor of 10:1 has been debunked.

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This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants.

Bacteria display a wide diversity of shapes and sizes, called morphologies.

Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometres in length.

However, a few species are visible to the unaided eye—for example, Thiomargarita namibiensis is up to half a millimetre long Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes.

There are approximately 5×10 Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere.

The nutrient cycle includes the decomposition of dead bodies and bacteria are responsible for the putrefaction stage in this process.

The most common fatal bacterial diseases are respiratory infections, with tuberculosis alone killing about 2 million people per year, mostly in sub-Saharan Africa.

In developed countries, antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem.

A small number of other unusual shapes have been described, such as star-shaped bacteria.