Growth and Colony Characteristics of Bacteria and Fungi
In the previous section we have learned the various types of media and specific purpose of each medium. Morphology is the basic criteria for the isolation, identification and classification of microorganisms. Colony characteristics are the basic tool in the field of taxonomy.
Bacteria grow in both solid and liquid medium, but identification will be easy on the solid medium. In solid medium bacteria form colonies. In liquid medium growth of bacteria are generally not distinctive because there is uniform turbidity or sediment at the bottom or pellicle is formed on the surface.
Some basic attributes such as shape, size, colour, pigmentation, texture, elevation and margin of the bacterial colony in the growth medium are explained below.
Shape: The shape of colony may be circular, irregular, filamentous, rhizoid.
Elevation: It is the side view of the colony. It may be flat, raised, umbonate (having a knobby protuberance) crateriform, convex pulvinate (cushion shaped)
Margin: The margin of the bacterial colony may be entire (smooth) irregular, undulate (ovary), lobate, curled, filiform. The irregular shape of the colony give irregular margin (Figure 5.13).
Colony Size: The diameter of the colony is measured in millimeter. It is described in relative terms such as pinpoint, small, medium and large.
Appearance of colony on the surface: The bacterial colonies are frequently shiny/ smooth in appearance. Colonies may be veined, rough, dull, wrinkled, or glistening.
Texture of the colony: Texture means consistency of the bacterial growth. It may be dry, moist, mucoid, brittle (dry breaks apart), viscid (sticks to loop, hard to get off), viscous, or butyrous (buttery).
Opacity of the bacterial Colony: Colonies may exhibit different optical density. It may be transparent (clear), opaque (not clear), translucent (almost clear), or iridescent (changing colour in reflected light).
Colony Odour: Some bacteria produce a characteristic smell, which sometimes helps in identifying the bacteria. Actinomycetes produce an earthy odour which is quite often experienced after rain. Many fungi produce fruity smell while Escherichia coli produce a faecal odour.
Colony Colour: Many bacteria develop colonies which are pigmented.(Table 5.3) Some bacteria produce and retain water insoluble pigments and the colonies appear coloured by taking the pigment intracellularly (Figure 5.14). But some bacteria produce water soluble pigment which diffuse into the surrounding agar. Example: Pyocyanin pigment of Pseudomonas aeruginosa is a water soluble pigment and give blue colour to the medium.
· If the entire broth appears milky and cloudy it is called turbid.
· If deposit of cells are present at the bottom of the tube, the term sediment is used.
· If the bacterial growth forms a continous or interrupted sheet over the broth it is called pellicle (Figure 5.15).
Fungi are eukaryotic organisms. They exist in both unicellular-yeast like form and in filamentous multicellular hyphae or mold form and some are dimorphic. Generally fungi prefer to grow in the acidic medium.
Sabourad Dextrose Agar (SDA) plates and Potato Agar plates are used for general cultivation of fungi. The acidic nature of SDA agar reduce the growth of bacteria.
The characters to be noticed in colony of fungi are colour of the surface and reverse of the colony, texture of the surface (powdery, granular, ecolly, cottony, velvety or glabrous), the topography (elevation, folding, margin) and the rate of growth.
Yeasts are grown on Sabourad Dextrose Agar aerobically. Yeasts grow as typical pasty colonies and give out yeasty odour. The colony morphology varies with different yeasts. Yeasts colonies generally have smooth texture and are larger than bacterial colonies on SDA medium (Figure 5.16a).
The genus Mucor is typically coloured white to brown or grey and is fast growing. Older colonies become grey to brown due to the development of spores. (Figure 5.16b).
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