Culturing Microorganisms

There are various culture media. Mixed culture, pure culture and in vitro ( cultured in containers), in vivo (grown in live animal and plant tissue).Culture medium basically requires a carbon source, nitrogen source, macronutrients, micronutrients, growth factors, gelling agents, antibiotics and dyes. There are two gelling agents agar and gelatin.

Agar is made out of red algae which have a melting point of 85 Celsius and a complex mixture of polysaccharides extracted from red algae. It is insoluble in cold water and sets between 32-39 Celsius. Gelatin is made out of hooves of animals. Thus high in protein. Melting point 30-35 celsius and readily digests by some bacteria.

Antibiotics are generally added before pouring into plates to inhibit a specific group of microorganisms. Since streptomycin is heat resistant it is added before heating whereas penicillin is heat sensitive thus added after heating.

Natural media

First used by Leeuwenhoek. This medium is prepared using peppercorns in water, hay in water and also meat in water. These are called infusions. Since the composition is not known it is difficult to reproduce.Eg; PDA, NA, Malt extract.

Artificial media

First used by pasture. Can be reproduced.Eg; Capex-dox nutrient.

• NaNO3  2g
• K2HPO4  1g
• MgSO4.7H2O  0.5g
• KCl  0.5g
• FeSO4  0.01g
• Sucrose 30g
• Agar 20g
• Distilled water 1L  

Basal media

Both natural and synthetic media.This suite for broad ranges of nutritionally undemanding groups of microorganisms.Eg; Cook’s no 2.

Selective media

A media that favor the growth of one type of specific microorganism while inhibiting the growth of undesirable microorganisms. By adding or removing of certain nutritional substances can separate one organism from another.

Eg; Coliform bacteria, Salmonella typhi.

Differential media

A culture media that distinguish different types of microorganisms by their metabolism of media components and colony appearance.Eg; Eosin Methylene blue agar, Yeast mannitol agar.

Suppressive media

Suppression of bacteria and encouragement of fungal growth.

Eg; Martin’s medium. This medium contains the antibiotic streptomycin and the dye rose Bengal. Rose Bengal suppresses bacterial growth as a result fungi grow.

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