The following points highlight the nine main types of culture of microorganisms. The types are: 1. Streak Culture 2. Stab or Stick Culture 3. Stock Cultures 4. Starter Culture 5. Enrichment Culture 6. Batch Culture 7. Continuous Culture 8. Synchronous Culture 9. Fed-Batch Culture.
Type # 1. Streak Culture:
Streak culture (Fig. 16.1) of a microorganism is that which has developed by drawing an inoculated needle in a straight line over the surface of a medium.
Type # 2. Stab or Stick Culture:
These cultures (Fig. 16.2) are prepared in solidified (by agar or gelatin) or liquid media by inserting straight an inoculation needle for some distance. The main points to note in these cultures are the growth of microorganisms along the line of puncture (insertion) and the changes occurring in the medium.
Type # 3. Stock Cultures:
Stock cultures are, in fact, pure cultures of known species of microorganisms which are maintained in viable conditions in laboratory for a longer duration of time.
Actually, these cultures are the stocks of particular microbial species or their strains, and one can take samples from them for study. There are many stock culture collection centres established throughout the world to help microbiologists obtaining cultures various studies.
Type # 4. Starter Culture:
Many microorganisms have been identified which act upon specific substances and result in the production of specific end-products. Many commercial laboratories prepare cultures, either of known species or of combination of known species and supply to the manufacturers to use to start fermentative processes. These cultures are called starter culture.
For instance, several commercial laboratories specialize in preparing starter culture for the dairy industry; the principal microorganisms involved are the species of Lactobacillus, Streptococcus, Leuconostoc which are responsible for the desirable changes in manufacturing fermented milk-products.
Type # 5. Enrichment Culture:
Enrichment culture is that in which the growth of a particular microorganism is favoured as against a mixed population by adjusting the nutritional requirements and environmental factors (Table 16.1).
For example, we have to isolate the microorganism ‘W’ from a mixture of microorganisms ‘WXYZ’, the nutritional requirements are so modified that the growth of ‘W’ is favoured. Now the microorganisms ‘W’ can be transferred to other medium and a pure culture of it can be obtained.
Besides nutritional requirements, growth of a specific microorganisms can be obtained in an enrichment culture by adjusting environmental factors such as temperature, pH, and so forth. The temperature adjusted at 5°C favours the growth of microorganisms that live at low temperatures, or at 37°C in order to obtain microbes capable of growth at human body temperature.
Type # 6. Batch Culture:
Growth of microorganisms in a limited volume of culture medium is referred to as a batch culture. A flask containing a constant volume of culture medium inoculated with a bacterium (e.g., E. coli) is an example of a batch culture.
Since the volume of the culture medium is constant in batch culture as no fresh nutrients are supplied in it, its nutrients are expended as a result of their use by growing microbial cells and, metabolic products of microbes accumulate in it. Batch culture is used to study the growth characteristics mainly of bacteria.
Type # 7. Continuous Culture:
Unlike batch culture, a continuous culture is that in which fresh nutrients are supplied and the end-products continuously removed through a siphon-flow. A continuous culture, therefore, helps maintaining exponential growth of bacterial population at a constant rate for over a long period of time.
Type # 8. Synchronous Culture:
A synchronous culture is that in which all microbial cells growing are physiologically identical and are in the same stage of division-cycle. A synchronous culture can be obtained cither by manipulating environmental conditions or by physical separation of cells.
Type # 9. Fed-Batch Culture:
The cultures like batch, continuous, and synchronous are most frequently used in laboratory to study various growth and physiological characteristics of microorganisms. These cultures are not useful in industrial fermentation processes. These are the fed-batch cultures which are used in such processes.
Fed-batch culture is a hybrid of batch culture and continuous culture, and involves ordinary batch growth of microorganisms until the medium present in the fermentation vessel is fully utilized, followed by regular additions of fresh medium and removal of used medium at intervals. This helps maintaining the continuity of the fermentation process resulting in continuous formation of the desired fermentation product.