The planet earth is full of bacteria and they play an important role in the different functions whether they are related to humans, animals or plants. All the bacteria are unicellular. They do not have the proper cell structure and cell organelles are scattered in the cytoplasm including the genetic material because they also lack nucleus. There is only a thick cell wall which encloses all the organelles and gives the proper shape to the bacteria. The cell wall is covered with a structure called as capsule.

Bacteria are considered living organisms because they divide by binary fission unlike viruses which need a host cell to multiply and usually cause infection in the host cell. Bacteria are the part of the environment and ecosystem. Almost all types of bacteria are helpful to mankind except few species which cause diseases in the human body. There are various functions and characteristics of bacteria which work either against or for the human beings.

Beneficial Bacteria:-
Some species of bacteria live in the human intestines where they help in digestion of food by releasing certain components. They also produce some vitamins which are helpful for the human body. E. coli is the specie which resides in the digestive tract of humans and help in breaking down the lactose. The new born babies do not suffer or have fewer chances of suffering from diarrhea if they are administered with the bacteria Lactobacillus reuteri or Bifidobacterium.

The process of fermentation takes place with the help of bacteria. If the milk is fermented and contains live bacteria Lactobacillus casei, it removes the harmful bacteria Helicobacter pylori from children's body. Similarly, Saccharomyces boulardii also helps to reduce the risk of antibiotic associated diarrhea from the children. Some bacterial species live on our skin and protect it from getting fungal infections. If the DNA molecule undergoes some internal damage in its structure or function, six strains or bacteria help to reduce the damage.

It can be said that human body is just like a home to bacteria. They live within almost every part of the body as well as outside the body. There are many bacteria which live inside mouth, throat, nose and intestines and perform various useful functions. They also do not let the harmful microorganisms live inside or on the human body and in return provide benefit to the human body. Various species of bacteria live in the humans' intestine and work with the immune system to protect the body against various diseases. Some intestinal bacteria are able to produce Vitamin A and K. the main function of bacteria residing in the stomach is to maintain the ph and acidity level in the stomach.

Harmful Bacteria:-
Along with the beneficial importance of bacteria, they also have some harmful effect on the human body. Some bacterial species are the source of causing diseases like typhoid fever, tuberculosis, cholera, syphilis and food borne illness. Streptococcus bacterium causes throat infections and some streptococcus species are even fatal. Though bacteria are mostly beneficial for the human body but if the person is weak and his immune system does not fight against diseases then these beneficial bacteria can also cause infections in the body. For example, AIDS is a disease in which person's immune system becomes very weak and all the viruses and bacteria start causing infections in the body.

Not all the bacteria which live in the human intestines are beneficial but some of them which enter the intestines through mouth cause infections and can be fatal sometimes. Bacteria present in food are the source of causing botulism. In this disease paralysis can occur and it can be fatal if one millionth of a bacterium is ingested through mouth.

To kill harmful bacteria, antibiotics are used. Some precautionary measures should also be taken so that they do not cause infections in the body, like washing hands before meal will kill the bacteria. Eating and drinking from unhealthy places causes bacterial infections. A person should himself take care of his health by staying clean, frequently washing hands and taking bath regularly.

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