Bacterial Identification | 8 Methods & Tests In Microbiology

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

They are classified under plants and not animals 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 the 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.

Bacterial Identification methods

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 the identification of bacteria, we use microscopical, medical, biochemical, & serological methods.

bacterial identification

Identification by morphology:

This method is to determine the individual bacterial physical appearance.

1. Based on size

Here bacteria are identified based on their physical size. Like small size or big one expressed in microns. For this bacteria are viewed under a microscope to view the size. One can apply some stain to the culture and use methods like glass slit or hanging drop method.

In the glass slit method, a layer of bacterial culture is applied, stained and viewed by a microscope above 40x magnification.

In the hanging drop method, a drop of culture is made to hang between the glass slide and slit and viewed under a microscope. The glass slide has a concave aperture on one side at the center where the glass slit with culture drop is placed over such that drop suspends in the concave aperture. The advantage of the hanging drop method is we can identify motile bacteria.

2. Based on the shape.

The structure of bacteria varies like spherical (cocus), bar or stick like (bacilli), chain-like (strepto), comma-shaped like virbrio-cholera bacteria.

Even the shape of the bacteria colony grown on the nutrition medium also differ for a different strain of bacteria. Hence under the microscope bacterial colony radius, the 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 the 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 flagellum, Polytrichous have many flagella. Even some bacteria have two flagella one on each side of the cell.

By Cultural characteristics

Here bacteria are identified as group or culture as a whole and not individual bacteria. Since most bacteria grow in colonies and also divide fast, they can be easily grown into a culture in suitable nutrition media. Based on the characteristics of the culture they can be identified as

a. The shape of culture: Circular, irregular, rhizoid, etc.

b. Size of culture or colonies in millimeters.

c. Type of elevation of culture like effuse, convex, concave, etc.

d. Margins of the colony like dentate, rough, smooth, etc.

e. The surface of the colony: Like smooth, wavy, papillate, etc.

f: Color of culture.

Based on resistance:

Antibiotics are specific to certain bacteria. Hence few bacteria develop resistance to a certain type of antibiotics. By adding the antibiotics to culture and measuring the resistance of microbe to the said antibiotic, the type of bacteria can be identified.

Ex: TB bacteria i.e. Mycobacterium tuberculosis is not affected by penicillin but killed by ciprofloxacin. So this differentiation can be used to identify a few bacteria.

Based on metabolism features:

Few bacteria are so specific for metabolic requirements. Based on this specificity, they can be identified.

Ex: Aerobic bacteria require oxygen for survival.

Anaerobic bacteria get killed when exposed to oxygen. Similarly, some require carbon dioxide, blood or other pigments for metabolism.

Identification of bacteria by biochemical tests.

This is one of the widely used methods and more important.

a. Sugar fermentation test:  Bacteria is grown in a sugar media.

b. Litmus milk test: When bacteria is grown in this medium, there may be the production of acids or alkali or even no change in pH.

c. Indole production test: Bacteria is grown in the peptone water culture. After 48 to 96hrs, incubation at 37°C, it is checked for the presence of red color. Red color indicates the production of indole from the amino acid tryptophan. The reagent used is called as Kovac’s reagent.

d. Methyl Red test: Bacteria is grown in glucose phosphate medium at 30°C for five days. Then few drops of 0.04% of methyl red are added and mixed to observe for color change. Red color indicates positive while yellow color indicates a negative test for glucose-fermenting bacteria.

e. Citrate utilization test: This test helps identify mostly intestinal gram-negative bacteria. The citrate medium also called as Koser’s medium is used here. Since the only carbon source is citrate here, its utilization is seen in terms of turbidity. If positive indicates the above-said microbes. See in detail on citrate test.

f. Voges Proskauer Test: This is a test used to detect bacteria like Enterobacter, Klebsiella. See more details on the VP test.

6. By differential staining: The identification depends on the 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 strains don’t take up this stain.

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

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

Identification of bacteria is necessary because

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

2. Select a suitable drug: Not all drugs (antibiotics) are active against all the bacteria. So identification helps to target that specific bacteria with a suitable choice of drugs. For 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 a recovery in the patient. This is ascertained by performing an identification test for the causative bacteria. An indication of a negative result in the test indicates, he is free from infection.

4. For industrial purposes: It also helps isolate a 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 the intention of future requirements. 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 the above advantages, the identification of bacteria is of most importance in microbiology.

Unlike a virus, bacteria are easily traceable by simple staining methods. Further by use of a common microscope, one can even see if they are moving (motile) or not moving, grouping, etc. Identification of bacteria is a part of the diagnosis of the many diseases and hence is a good demand for microbiologists.

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  1. I’ve been doing some reading about buffered peptone water, and I wanted to do a little more research. I’m glad you talked about how peptone water can grow bacteria. I’m going to have to look into some different buffered peptone water and see what I can find!


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