Culture Media

Lab (2)

practical food microbiology

CULTURE MEDIA A culture medium is any nutrient liquid or solid that can be used in the laboratory for the growth of microorganisms. Such a medium may resemble the natural substrate on which the microorganisms usually grow (e.g. blood serum for animal pathogens, milk for milk microorganisms). Whatever the medium, it must include all the necessary requirements for growth, which vary according to the organism it is desired to grow but will include the following:-

A-Water The role of water in the culture media is to dissolve all the medium components. ™ Distilled water is preferred in use, that is because the tap water may contain calcium and magnesium salts which can react with phosphate found in peptone or beef extract and produce calcium phosphate and magnesium phosphate salts which are water insoluble which can inhibit the growth of microorganisms.

B-Nitrogen containing compounds (Peptides, proteins, amino acids, nitrogen-containing inorganic salts) in most cases we use peptone as nitrogen source, and it has another function in the medium that is buffering material. Because it is an amphoteric compound it can maintain the medium pH.

C- Energy sources (E.g. carbohydrates, peptides, amino acids and proteins) but in most cases we use carbohydrates like glucose (dextrose).

D- Accessory growth factors (Vitamins and mineral salts) usually we use beef extract as a source of the growth factors.

E- Solidifying agents These agents used only in the solid –reversible to liquid media. For example we use gelatin or Agar agar as solidifying agents.

1. Gelatin It is a protein which is added to the media in a concentration of 10-12 % of the medium. It melts at 25-30o c and solidifies at below 25o c. If it is heated to about 100o c or above it will melt but can not solidify again.

2. Agar agar It is a carbohydrate which is added in a concentration of 1.5 - 2% to the culture media. At 98o C it is liquid and solidify at 42- 45o C. Agar agar is the predominant solidifying agents, because: ™ Most bacterial strains can hydrolyze gelatin. ™ Gelatin can be molten at the incubation temperature.

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Lab (2)

practical food microbiology

Classification of culture media A- According to chemical composition: ™ Chemically defined media (Contains defined components and we can reconstruct it again in the same composition)

™ Non- chemically defined media (Contain plant and animal tissues, and it has unknown composition so, we can not reconstruct it. And usually it is a natural material like meat and potatoes).

B- According to texture: ™ Liquid media (Lactose broth)

™ Solid media (Potato slices) ]

™ reversible to liquid (Contain agar or gelatin as solidifying agents)

™ Semi solid media (Contain 1/4 agar quantity of solid –reversible to liquid)

C- According to special purposes: ™ Selective media Used to isolate one species of microorganisms and inhibit the growth of the other microorganisms (for example, when we want to isolate G+ from a mixed culture of G+ and G- bacteria, we use a medium containing crystal violet dye which inhibit the growth of G+ and not affect the growth of G- bacteria.

™ Differential media In which we add a natural or chemical compound to differentiate the microbial groups which may found in the same culture. Ex. Blood agar medium which contain blood to differentiate between the blood hydrolyzing and non blood hydrolyzing bacteria, the blood hydrolyzing bacteria which have colonies with clear zone around it.

™ Enriched media In which we need to add blood serum or plant tissues to enrich the growth of specific groups of microorganisms like heterophilic bacteria.

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