Identification Of Bacteria: Morphology And Staining Reactions

Since we are studying bacteria. It become important to identify its type, species or strain to continue our documentation or research procedure. There are several method obtained to identify bacteria. As we know bacteria are microscopic organisms. They cannot be confirmed by their shape and size. To make sure about bacterial strain or species we study certain set of its characters starting from its morphology, staining procedures, biochemical characteristics. After doing all we combine the result and confirm from the already collected records. Here we will discuss about pure culture morphology of bacteria and its staining pattern to confirm the type of microorganism.


Morphology of the bacteria cultured on artificial media, depend on a number of factors such as strain studied, nature of culture media, temperature and time of incubation, age of the culture and the number of subcultures it has undergone.

While confirming the bacterial strain artificially cultured we should mention and look for all of the above said parameters. So that it can be understood better.

After getting the pure culture of bacteria by following all standardization, we mainly focus on characters like shape, size, arrangement, motility, flagella, spores and capsules. Obviously all of these cannot be made out in a single medium.

The shape may be spherical, filament ours, rod shaped, comma shaped or spiral. The axis of the organism may be straight or curved. The length and breadth may vary. The sides of the organism may be parallel, convex, concave or irregular. The ends may be cut straight, rounded or tapering. Considerable variations in shape and size leading to club, navicular and swollen or shadow or giant forms may be seen. Bacteria over culture medium may be arranged in singly, in pairs, in tetrads or in packets of eight, or in chains, short or long, in case of cocci, bacilli may be arranged at random, in short or long chains, in Chinese letter patterns, as palisades or in bundles, vibrios may be single or in S or spiral forms.

They may be non motile, sluggishly motile, actively motile or may exhibit darting motility. They may be without flagella, that is atrichate, or monotrichate, lophotrichate, amphitrichate or peritrichate.

The spores when present may be oval or spherical or ellipsoidal and may be of the same width or wider than that o the bacillary body. The spores may be equatorial, subterminal or terminal. Capsules may or may not be present.

Staining Reactions

Staining is the next step towards identification of bacteria. Once we have gone through the morphology of bacterial culture, we are intended to use most appropriate staining method to identify bacterial species.

The age of the culture is important to perform staining procedure. In older cultures, staining characteristics either vary or are not brought out well. Simple staining bring out the morphology best. Differential and special stains are necessary to bring out characteristics like flagella, capsules, spores and meta chromatic granules.

The Gram stain divides bacteria into Gram positive and Gram negative, the Ziehl – Nielsen stain into acid fast and non – acid fast. The fluorescent antibody technique enables one to identify them according to their surface antigens.

The study of morphology and staining characteristics helps in preliminary identification of the isolate.

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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