MODULE 4 – GROWTH, ISOLATION AND CULTIVATION OF MICROORGANISMS

Microorganisms use chemicals called nutrients for growth and development. They need these nutrients to build molecules and cellular structures. The most important nutrients are carbon, hydrogen, nitrogen, and oxygen. Microorganisms get their nutrients from sources in their environment. When these microorganisms obtain their nutrients by living on or in other organisms, they can cause disease in that organism by interfering with their host’s nutrition, metabolism, and, thus disrupting their host’s homeostasis, the steady state of an organism.

Organisms can be classified in two groups depending on how they feed themselves. Organisms that use carbon dioxide (CO2) as their source of carbon are called autotrophs. These organisms “feed themselves,” auto- meaning “self” and -troph meaning “nutrition.” Organisms that obtain carbon from organic nutrients like proteins, carbohydrate, amino acids, and fatty acids are called heterotrophs. Heterotrophic organisms acquire or feed on organic compounds from other organisms.

CULTURE MEDIA

A culture medium is nutrient material prepared in the laboratory for the growth of microorganisms. Microorganisms that grow in size and number on a culture medium are referred to as a culture.

In order to use a culture medium, it must be sterile, meaning that it contains no living organisms. This is important because we only want microorganisms that we add to grow and reproduce, not others. We must have the proper nutrients, pH, moisture, and oxygen levels (or no oxygen) for a specific microorganism to grow.

Many culture media are available for microbial growth. Media are constantly being developed for the use of identification and isolation of bacteria in the research of food, water, and microbiology studies.

The most popular and widely used medium used in microbiology laboratories is the solidifying agent agar. Agar is a complex polysaccharide derived from red algae. Very few microorganisms can degrade agar, so it usually remains in a solid form. 

Agar media are usually contained in test tubes or Petri dishes. The test tubes are held at a slant and are allowed to solidify on an angle, called a slant. A slant increases the surface area for organism growth. When they solidify in a vertical tube it is called a deep. 

The shallow dishes with lids to prevent contamination are called Petri dishes. Petri dishes are named after their inventor, Julius Petri, who in 1887 first poured agar into glass dishes.

For a medium to support microbial growth, it must provide an energy source, as well as carbon, nitrogen, sulphur, phosphorous, and any other organic growth factors that the organism cannot make itself, source for the microorganisms to utilize.

A chemically defined medium is one whose exact chemical composition is known. Chemically defined media must contain growth factors that serve as a source of energy and carbon. Chemically defined media are used for the growth of autotrophic bacteria. Heterotrophic bacteria and fungi are normally grown on complex media, which are made up of nutrients, such as yeasts, meat, plants, or proteins.

Different vitamins and organic growth factors can be provided by meat and yeast extracts. If a complex medium is in a liquid form it is called a nutrient broth. If agar is added, it is called a nutrient agar. Agar is not a nutrient; it is a solidifying agent.