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Bacterias are very tiny orgnaisms which are existed almost every where. Bacterias are prokaryotes, they have only one cell, without nucleus. Their genome is only several hundred Kilo base pairs (Kb) to several Mega base pairs (Mb), compared to 3000 Mbs of human genome. Bacteria can be better observed under a microscope as they are typically several µm in diameter. Sometimes bacterias grow into a larger colony and it can then be seen by eye.

Bacterias are also presented in human body, especially in the gastrointestinal tract, respiratory tract such like nose, most of them are not harmful. Generally the human immune system has the power to kill invading bacterias, but not all the times. For example, the leukocyte in the blood can swallow the bacterias, although themself will die subsequenctly. In fact, a lot of bacterias can infect human and cause diseases such like diarrehha and cholera.

Disease	Bacteria Pathogen	Organs Affected	Transmission
Anthras	Bacillus Anthracis	Skin, Lung	Environment
Botulism	Clostridium Botulinum	Neural System	Food
Chlamydial Urethritis	Chlamydia Trachomatis	Cervix, Eye, Urethra	Sexual
Cholera	Vibrio Cholerae	Intestine	Food
Gonorrhea	Neisseria Gonorrhea	Urinary Tract	Contact, Sexual
Leprosy	Mycobacterium Leprae	Skin, Bones, Peripheral Nerves	Contact
Salmonellosis	Salmonella	Intestine	Food
Syphilis	Treponema Pallidum	Skin, Cardiovascular Organs	Contact
Tuberculosis	Mycobacterium Tuberculosis	Lung	Air
Typhus	Salmonella Typhi	Blood, Skin	Bugs, Contact

Most bacterias are sensitive to antibiotics. However, with the over use of antibiotics in recent years, some bacterias have gained antidrug ability and become more dangerous, such like tuberculosis. For example, E. coli can occasionally obtain DNA fragments such like those containing ampicillin resistance genes from the environment, and thus become resistant to ampicillin. This technique has been widely used in bacterial selective culturing in the labs.

With the new DNA sequencing technology, Scientists can now decipher the bacteria genomes rather easily. There are thousands of bacteria genomes have been sequenenced and made public to all researchers. Click here for the detailed bacteria sequencing project.