They are prokaryotic in nature meaning they are the earliest forms of the cell without a nucleus.
These characters make them immortal, omnipresent and also very versatile.
They are useful to nature, man, and life on earth, Though some of them can cause diseases to humans.
Unique & Distinguishing Characteristics of bacteria
1. No nucleus: Unlike another eukaryotic cell, the bacterial cell is the only cell which lacks a prominent nucleus within. This feature has compelled the scientists to consider bacteria as primitive organisms, i.e., earliest forms of life on earth.
2. Presence of Cell wall: Bacteria has a cell wall which is distinct from other cells having a cell wall. The wall is made of different substances like glycoproteins, lipopolysaccharides and lipoproteins.
3. Cell membrane: This is present immediately below the cell wall. This has a small variation with that of plants and animals cell membranes. In plants and animals, it is mostly a single or double layer (bilayer) of lipids. But bacteria especially the gram-negative ones have an extra membrane, i.e., an outer membrane called periplasm which lies bellow cell wall but above the cell membrane.
4. Pilli: These are small and tube-like projections from the bacterial surface. They are the organelles of sexual reproduction and are involved in the exchange of genetic material as part of reproduction between two bacteria.
5. Food dependence: All the organisms either live on their own or other resources. But bacteria have different modes of food procurement. Some of them synthesize their food like plants by use of sunlight.
Many bacteria are saprophytes which means, they live on dead and decaying matter.
While others are symbiotic. They live in a give and take relationships with other animals. Ex: rhizobium in roots, E-coli in the intestine.
6. Respiration: Bacteria do not have mitochondria like a typical animal and plant cells. However, they derive energy by aerobic respiration. In the higher cells, aerobic respiration occurs due to the glycolysis and oxidative phosphorylation pathway in the mitochondria.
7. Reproduction: Most organisms have one or two methods of reproduction. But bacteria have many.
a. Asexual reproduction: The bacteria cell undergoes to produce two bacteria by Binary fission and also by endospores.
b) Sexual reproduction: It is also by two distinct methods like transformation, conjugation, and transduction. In transformation, one bacteria release naked DNA strand into the surroundings. The other bacteria take it up and incorporate into their genome. Transduction happens due to a virus.
Read how do bacteria reproduce for more details.
8. Spore formation: When the environmental conditions around are harsh, the bacteria convert to a hard spore form. This spore is highly resistant to heat, chemicals and drought conditions.
When the conditions are favorable, these spores break open and release the bacteria.
9. DNA Gyrase: Gyrases are the enzymes involved in DNA transcription. They perform, nick and joint function in the DNA chains. They cut the strands, induce negative super-coils and also join the cut ends after the process. In Humans and other mammals, instead of DNA gyrase, there is DNA topoisomerase-II for a similar function.
10. Ribosomes: The protein-making machinery has 70S ribosome which is of two sub-units as 50S and 30S. While in other animals and plants, it is 80S ribosome consisting of two sub-units 60S and 40S.
11. The absence of Cell organelles: Other cell organelles like mitochondria, Golgi bodies, endoplasmic reticulum are absent in a bacterial cell.
12. Flagella presence: Some of the bacteria are motile. They, especially the bacilli type have flagella. These are the organs of locomotion. They are long filamentous organs originating from the cell membrane. You can learn more about flagella at bacteria classification. They are classified based on the number and arrangement of flagella on the cell.
13. Bacteria shape: The shape of bacteria is very characteristic to them. It is also easy for the identification of bacteria by recognition of shapes. These bacterial structures have a specific shape like a stick (bacilli), spherical (cocci), coma (cholera bact.), spiral (spirochete), in a bunch or isolated, etc.
14. Cosmopolitan distribution: Bacteria are the only organisms which show cosmopolitan distribution. They are present in air, water, soil, snow, etc. They are present at very cold (minus degree) temperature regions and also very high-temperature regions. They can live in an acidic environment (Helicobacter pyroli) and even alkaline environment (iron bacteria).
15. Resistance and tolerance: Some of the bacteria are highly resistant to adverse situations. Even they aquire a tolerance to harsh chemical and other stuff which destroy them. Hence irrational use of antibiotics. THis has become so severe that the World Health Organization warns of the rampant prevalence of resistant bacteria which can be incurable by currently available drugs.