When bacteria are grown on culture media, they show some specific set of colonies, on a controlled condition of temperature and nutrients these characters can be repeated with every culture of that bacteria. These can be treated as signature of that particular bacteria and can be known for the identification of that bacteria. Because bacteria are so small to observe by naked eyes, characters of entire colony is noted and studied.
Cultural characteristics are one of the important features in identification of particular bacterial colony from other bacterial colonies.
Cultural characteristics provide additional information for the identification of the bacterium. Same bacteria may show different cultural characteristics on different type of media. While studying colonies on solid media, the following features are noted:
- Shape of colony – circular, irregular, or rhizoid
- Size in millimeters
- Elevation – effuse, elevated, convex, concave, umbonate or embilicate
- Margins – beveled or otherwise
- Surface – smooth, wavy, rough, granular, papillae or glistening
- Edges – entire, undulate, created, fimbriate or curled
- Structure – opaque, translucent or transparent
- Consistency – membranous, friable, butyrous or visvid
- Whether they are differetiated into a central and a peripheral portion.
While studying a stock culture of bacteria, following characters are noted –
- The degree of growth – scanty, moderate, or profuse
- Their nature – discrete or confluent, fill form, spreading or rhizoid
- Their elevation, surface, edges, color, structure, odor, emulsifiability, consistency and changes in the medium.
In a fluid medium, the degree of growth, presence of turbidity and its nature, presence of deposit of turbidity and its nature, presence of deposit and its character, nature of surface growth such as pellicle and its quality and ease of disintegration, and of our are noted.
While identification and confirming the bacteria, its resistance to heat and resistance to disinfectants is tested, both for vegetative and spore forms. The resistance of S. facialis heat at 60 C for half an hour and of clostridia spores to boiling for various periods are example. Resistance to antibiotic and chemotherapeutic agents and bacteriocins would also help in differentiation and identification.
The requirement of oxygen, the need for carbon dioxide, the capacity to form pigments, and the production of hemolysis help in classification of bacteria, and its identification as well.