Bacterial Identification| 8 Methods & Tests

Bacteria are single-celled micro-organisms and are invisible to naked eye.

They are classified under plants due to similar resemblances.

But many bacteria infect humans and animals causing deadly diseases.

They can act as parasites in almost all the organs of human body.

Further they easily spread from one person to another by means of water, air, food, insects, sex etc.

So bacterial identification is a necessary part of disease diagnosis and cure.

Without the identification of causative bacteria, it is tough to provide effective treatment with available antibiotics.

Identification of bacteria:

Bacteria as mentioned before or so tiny that one cannot watch without a microscope.

So either we identify them using a microscope or by enhancing their population to a huge bulk or by biochemical methods.

Hence in identification of bacteria, we use microscopical, biochemical, spectrometrical & serological methods.

1.Based on size: Here bacteria are identified based on their physical size. Like small size or big one. 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.bacterial identification

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 equivalent 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 mediumwhile 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 Gram satin while Gram -ve bacteria don’t take up gram stain. 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 Polymerase chain reaction (PCR), gel electrophoresis, radio immuno asssay etc.

Identification of bacteria is necessary because

1. Identify the disease: It helps to know the type of infection and the disease caused in the individual. Ex. Tuberculosis can be identified by acid fast stain test for mycobacterium. So bacterial identification is important in health care.

2. Select suitable drug: Not all drugs (antibiotics) are  active against all the bacteria. So identification helps to target that specific bacteria with suitable choice of drugs. Example rifamycin is best suited for tuberculosis and cannot be controlled by other antibiotics.

3. Evaluation of treatment progress: 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.

During the treatment, the patient body samples are repeatedly checked during the course to see if there is recovery in the patient. This is ascertained by performing identification test for the causative bacteria. Indication of negative result in the test indicates, he is free from infection.

4. For industrial purposes: It also helps isolate specific strain of bacteria needed for research or other industrial productions like fermentation etc. Many vitamins (B12), hormones (insulin) and supplements are produced by rDNA technology by using the principles of biotechnology.

5. For storage: Some bacteria are specifically stored with an intention of future requirement. Identification is also necessary to store the bacteria in pure form without foreign contamination. These stored bacteria are supplied when required for research or industrial manufacture.

Because of above advantages, identification of bacteria is of most importance  in microbiology.

Unlike virus, bacteria are easily traceable by simple staining methods. Further by use of common microscope one can even see if they are moving (motile) or not moving, grouping etc. Virus needs to be seen only under electron microscope and tough to be identified.

Identification of bacteria is a part of diagnosis of the many diseases and hence health care profession has a good demand for microbiologists.

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