Identification of Bacteria: By Staining & other methods


Bacteria are single-celled micro-organisms and are invisible to naked eye. They are classified under plant cells due to similar resemblances. But many bacteria infect animals and humans causing deadly diseases and infections. They cause diseases in almost all the parts of the body and spread from one person to anther or from one animal to another by easy means like water, air, food, sex etc. So microbe identification is necessary to ascertain the cause of disease when one is infected.These bacterial infections help in giving effective treatment with available range of medicines (antibiotics).

# Identification of bacteria.

Identification of bacteria is necessary because:

1.  It helps to know the type of infection and the disease caused in the individual. Ex. TB, cholera, syphilis etc.

2. Not all drugs (antibiotics) are  active against all the bacteria. So identification helps to target that specific bacteria with suitable choice of drugs.

3. Identification of bacteria is also necessary to know how far the drug is effective and if the patient is freed from the said bacteria by the treatment.

4. It also helps isolate specific strain of bacteria needed for research or other industrial productions like fermentation etc.

5. Identification is also necessary to store the bacteria without other contamination.

Identification of bacteria:

This is done by microscopical, biochemical, spectrometrical, serological methods etc.

1.Based on size: Here bacteria are identified by size. Like small or big etc. Here bacteria are viewed under microscope to view the size. One can apply some stain to the culture use methods like glass slit or hanging drop method. In glass slit method a layer of bacterial culture is applied, stained and viewed by microscope above 40x magnification. In hanging drop method a drop of culture is made to hang between glass slide and slit and viewed under microscope. The glass slide has a concave aperture on one side at center where the glass slit with culture drop is placed over such that drop suspends in the concave aperture. The advantage of hanging drop method is we can identify motile bacteria.

2. Based on shape.The structure of bacteria varies like spherical (cocus), bar or stick like (bacilli), chain like (strepto), comma shaped like cholera bacteria etc. Even the shape of bacteria colony grown on the nutrition medium also differ for different strain of bacteria. Hence under microscope bacterial colony radius, shape can be viewed and strain of bacteria be identified.

3. By extra-cellular characters: Here some extra-cellular characters are used in identification of bacteria. Some bacteria have flagella for motion. So they can be identified based on the number of flagella and the arrangement of flagella on the bacterial surface. Ex: Atrichous bacteria have no flagella, Monotrichous have one flagella, Polytrichous have many flagella. Even some bacteria have two flagella one on each side of the cell.

4. By flow cytometry.Here bacteria flow through a minute pore being in a solution.Hence when bacteria passes the pore, it also displaces volume of solution equalent to its size and this volume displaced is function of bacteria size and to some extent shape and there are corresponding changes in conductivity or electric parameters etc. which are measured for identification of bacteria.

5. By differential nutrient media: This is based on the fact that some bacteria grow in certain media exponentially while other strain can’t Ex: Aerobic bacteria grow in aerobic medium and not in anaerobic medium. Some bacteria grow in glucose or salt rich medium but others can’t. Similarly some bacteria grow in acidic medium while others grow well in neutral culture media.  Bacteria examples include. Halophilic bacteria grow in high amount of salt media. Tuberculosis bacteria grow in acidic media.

6. By differential staining: The identification depends on staining of bacteria. And most bacteria can be stained by specific stain ex: Gram +ve bacteria are stained by Gramsatin while Gram -ve bacteria don’t take up gramstain. Tuberculous bacteria can be stained by acid-fast stain specifically and other strain don’t take up this stain.

7. Serological methods. here identification of bacteria is done by use of antibodies and antigens which are specific against the suspected bacteria. Antigens and antibodies are very specific and bind to single type of bacteria.

8. By protein and nucleotide analysis. the cellular constituents like protein content, nucleotide sequence in DNA are used for identification of bacteria. This requires methods like Ploymerase chain reaction (PCR), gel electrophoresis etc.

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