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Several factors are usually taken into consideration when isolating and characterizing or identifying bacteria from any environment. The isolation, purification and identification of bacteria from either clinical or non-clinical samples are usually the first steps to take in any bacteriological studies. Bacterial isolation is done to obtain a pure bacterial culture of the bacteria/bacterium of interest from a given sample. In isolation, the sample is cultured in a broth or liquid medium and incubated overnight at a certain temperature, and it is from this broth that loopfuls of the turbid suspension is transferred onto solid culture media using inoculating loop fof the isolation of pure cultures. Several sub-culturing can be done on freshly prepared solid culture plate; and all this are aimed at getting a pure bacterial culture – which is an important aspect to identifying the organism of interest. Pure bacterial culture is essential in the study of the morphology, physiology, and biochemical characteristics of the bacteria. It is only pure bacterial cultures that can be used to undertake antimicrobial susceptibility studies in the microbiology laboratory.

Pure cultures of bacteria can be obtained through the streak plate method, spread plate method or pour plate method. However, the streak plate method is commonly used in the laboratory to obtain pure bacterial cultures. After getting a pure culture, it is important to store the organism for future study. Bacterial cultures can be stored over a given period of time and even for years using freeze-drying technique which can also be called lyophilization. Bacterial cultures can also be stored in solid medium in a tube over a short period of time through sub-culturing. Stored cultures should be properly labeled before storage and storage is usually done in the refrigerator at ambient temperature.  

However, pure cultures of microorganisms can also be obtained from culture collection and preservation organizations or institutes from around the world. The main responsibility of these culture collection centers is to maintain culture collections of different microbes for the purpose of commercialization. The American Type Culture Collection (ATCC) located in Rockville, Maryland, U.S.A; the National Culture Collection (NCC) in the UK; Commonwealth Mycological Institute (CMI) located in Kew, Surrey, England; the Fermentation Research Institute (FERM) located in Tokyo, Japan; and the Research Institute for Antibiotics (RIA) located in Moscow, Russia are some examples of culture collection organizations found around the world. These organizations are constant suppliers of pure cultures of microbes required for various industrial, medical and commercial purposes; and they serve as repositories of important microbes at their pure forms.  

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Goldman E and Green L.H (2008). Practical Handbook of Microbiology, Second Edition. CRC Press, Taylor and Francis Group, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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