Colonial Morphology of Bacteria

Microorganisms produce definite patterns on culture media plates as they grow and divide. These specific patterns (inclusive of shapes and sizes) also aid in the preliminary identification of the organisms for further characterization. Colonial morphology is the size, shape, colour, texture and the general structure of an individual colony of a particular microorganism (in this case bacterium or fungus) on a culture media plate that supports its growth. These specific features (as outlined in Table 1) which are unique to bacteria and fungi are used for the preliminary identification of a bacterium in the microbiology laboratory.

Colonial morphology which can also be referred to as cultural characteristics pertains to the macroscopic appearance of a bacterium on different kinds of growth culture media. A microscope is usually not required to establish the cultural characteristics of a bacterium, rather the visible eyes is mostly used for the identification. However, microscopical examination of colonial characteristics of a bacterium may be necessary, for example when examining the margin/edge of a bacterium in order to decipher if it is actually smooth or filamentous.

A colony is a macroscopically visible population of cells growing on solid media, and that arises from a single cell; and microorganisms (particularly bacteria and fungi) are presumptively recognized on solid agar media or plates based on some unique growth features which they express on such growth media (Figure 1). Microorganisms (bacteria and fungi) grow as colonies on solid culture media. The growth of a bacterium or fungus in or on a solid growth media is thus taken into consideration when characterizing the organism. The colony of an organism is visibly studied when establishing the cultural characteristics of a bacterium.

The appearance of bacterial of fungal colonies on a culture media plate is generally species-specific and can be very helpful in identifying isolates on individual basis. The size of a bacterium can be measured using a meter rule to determine its diameter. Bacterial colonies can vary from large colonies to tiny colonies that are less than 1 mm. A bacterium that is less than 1 mm in size is generally called punctiform bacteria (i.e., pin-point bacteria). The appearances of observed colonial growth of a bacterium on a solid culture media plate are as outlined in the table below. Bacterial nutrition and growth as well as the different types of culture media used for microbiological experimentation (especially as it relates to fungi and bacteria) shall be highlighted in this unit with illustrations.

Table 1: Schematic illustrations of the colonial/cultural characteristics of bacteria on solid media

Figure 1: A culture of Klebsiella pneumoniae growing on MacConkey agar. The organism produces mucoid and roundish colony on agar medium. MicroDok


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