CULTIVATION OF MICRO ORGANISMS

In health clinics and hospitals, it is necessary to detect microorganisms that are associated with disease. Selective and differential media are therefore used. 

Selective media are made to encourage the growth of some bacteria while inhibiting others. An example of this is bismuth sulphite agar. Bismuth sulfite agar is used to isolate Salmonella typhi from faecal matter. Salmonella typhi is a gram negative bacterium that causes salmonella. 

Differential media make it easy to distinguish colonies of desired organisms from non-desirable colonies growing on the same plate. Pure cultures of microorganisms have identifiable reactions with different media. An example is blood agar. Blood agar is a dark red/brown medium that contains red blood cells used to identify bacterial species that destroy red blood cells. An example of this type of bacterium is streptococcus pyogenes, the agent that causes strep throat.

MacConkey agar is both selective and differential. MacConkey agar contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria, and lactose, in which gram-negative bacteria can grow.

Enrichment cultures are usually liquids and provide nutrients and environmental conditions that provide for the growth of certain microorganisms, but not others.

PURE CULTURES

Infectious material or materials that contain pathogenic microorganisms can be located in pus, sputum, urine, feces, soil, water, and food. These infectious materials can contain several kinds if bacteria. If these materials are placed on a solid medium, colonies will form that are the exact copies of that same microorganism.

A colony arises from a single spore, vegetative cell, or a group of the same organism that attaches to others like it into clumps or chains. Microbial colonies have distinct appearances that distinguish one microorganism from another.

The streak plate method is the most common way to get pure cultures of bacteria. A device called an inoculating loop is sterilized and dipped into a culture of a microorganism or microorganisms and then is “streaked” in a pattern over a nutrient medium. As the pattern is made, bacteria are rubbed off from the loop onto the nutrient medium. The last cells that are rubbed off the loop onto the medium are far enough apart to allow isolation of separate colonies of the original culture.



GROWTH CYCLE OF BACTERIA

Bacteria reproduce by binary fission, a process by which one parent cell divides to form two progeny cells. Because one cell gives rise to two progeny cells, bacteria are said to undergo exponential growth (logarithmic growth). 

The doubling (generation) time of bacteria ranges from as little as 20 minutes for Escherichia coli to more than 24 hours for Mycobacterium tuberculosis. The exponential growth and the short doubling time of some organisms result in rapid production of very large numbers of bacteria. For example, 1 E. coli organism will produce over 1000 progeny in about 3 hours and over 1 million in about 7 hours. The doubling time varies not only with the species but also with the amount of nutrients, the temperature, the pH, and other environmental factors.

The growth cycle of bacteria has four major phases. If a small number of bacteria are inoculated into a liquid nutrient medium and the bacteria are counted at frequent intervals, the typical phases of a standard growth curve can be demonstrated (Figure 3–1).