Culture Media flow chart: Introduction,their composition

Culture Media Flow Chart: Introduction,Their Composition and Uses

Introduction of Culture Media

Media is plural while medium singular. Culture media require to grow the organisms from infected material to identify the causative agent. They are of different types on the basis of using purposes. Nutrient agar(NA) uses for the cultivation of non-fastidious bacteria like Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa whereas 5% of sheep blood agar (BAP) needs for Streptococcus species, Neisseria species. Chocolate agar(CHOC) is useful for the culture of Haemophilus influenzae. MacConkey agar (MAC) is a selective, differential, and indicator medium and uses for the cultivation of Gram-negative bacteria. All the organisms growing on nutrient agar, blood agar, MacConkey agar can easily grow on chocolate agar but not vice -versa. To remember the composition of culture media is really cumbersome but you have the idea about culture media flow chart from simple to complex, no doubt, then it will be easy. So, follow the flow charts as shown below.

Types of Media

Media are of flowing types.

Based on the physical State

I. Liquid media

II. Semisolid media ( Agar, 0.2-0.4% which enables motile bacteria to spread.)

III. Solid media

On the basis of the presence of molecular oxygen and reducing substances in the media

I . Aerobic media and

II. Anaerobic media

Based on nutritional factors

I. Simple media

II. Complex media

III. Synthetic media and

IV. Special media

Special media –

a )Enriched media

b) Enrichment media

c) Selective media

d) Differential media

e) Indicator media

f)Transport media and

g) Sugar media

Simple media

The nutrient broth is an example of a simple medium. It contains peptone water and meat extract 1%. When agar add is added to the nutrient broth, it becomes nutrient agar. This is the simplest and routinely employed medium in the laboratory for diagnostic purposes.

Complex media

All media other than simple media are complex media.

Synthetic media:

They prepare from pure chemicals and the exact composition of the medium is known. These uses for special studies such as metabolic requirements. Dubbo’s medium with tween 80 is an example of a synthetic medium.

Special Media

Enriched media : When basal medium is with addition of some nutrients such as blood, serum or egg. It becomes enriched medium. For e.g. in blood agar-there is addition of blood to nutrient agar. It may use for growing a number of bacteria but one specific example is Streptococcus that requires blood for its growth. In Loeffler’s serum slope, there is addition of serum for enriching the medium. This medium uses for growing Corynebacterium diphtheriae.

Enrichment media:

A fluid type of selective medium in which some substances incorporate that have either a stimulating effect on the bacteria to be grown or inhibits its competitors or both . This results in an absolute increase in the number of wanted bacteria related to other bacteria. Such medium is enrichment medium. Addition of tetrathionate in tetrathionate broth inhibits coliforms while allows typhoid-paratyphoid bacilli to grow. In selenite F broth, selenite has similar action as that of tetrathionate in tetrathionate broth.

Selective media:

Selective media contain substances that inhibit all but a few types of bacteria and facilitate the isolation of a particular species. These media use to isolate a particular bacteria from specimen where mixed bacterial flora has expected. Selective media are solid in contrast to enrichment media which are liquid. Example of selective media are deoxycholate citrate agar(DCA)-Addition of deoxycholate acts as selective agent for enteric bacilli (Salmonella, Shigella) and bile salt agar(BSA). Bile salt is a selective agent. It favors the growth of only Vibrio cholerae where as inhibits the growth of other intestinal organisms.

Differential media:

When a medium contains substances which help to distinguish differing characteristics of bacteria, is differential medium e.g. MacConkey’s medium, which contains peptone, lactose , agar, sodium taurocholate and neutral red. The lactose fermenters (LF) form pink-colored colonies where as non lactose fermenters (NLF) produce colorless or pale colonies.

Indicator Media :

These media contain an indicator that changes color when a bacterium grows in them. Salmonella enterica serotype Typhi grow as black colonies on Wilson and Blair’s medium containing sulfite. MacConkey ‘s medium is also an indicator medium. Due to fermentation of lactose , there is acidic pH which forms the pink colonies in the presence of neutral red indicator.

Transport media:

They use in the case of delicate organisms (e.g. gonococci) which may not survive the time taken for transit or may be overgrown by non pathogenic bacteria (e.g. cholera organisms). They maintain only viability. Examples of transport media are Stuart’s transport medium: is a non nutrient soft agar gel containing a reducing agent to prevent oxidation, and charcoal to neutralize bacterial inhibitors. It may be for organisms such as gonococci. Buffered glycerol saline transport medium for enteric bacilli.

Sugar media:

Sugar media help in the identification of bacteria. The term sugar in microbiology denotes any fermentable substance. Glucose, lactose, sucrose, and mannitol routinely employ for fermentation tests.

Anaerobic Media:

These use for cultivation of anaerobic bacteria e.g. Robertson’s Cooked meat medium and thioglycollate broth.

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