Types of culture media-classification
Types of culture media
- The food material or substances required for growing microorganisms in vitro (outside the body) is called culture medium.
- It is important to grow microorganisms outside the body for the following purposes:
1. to identify the cause of infection from the clinical sample, so that proper treatment can be given.
2. to study the characteristics or properties of microorganisms.
3. to prepare biological products like vaccines, toxoides, antigens…etc.
Types of culture media
I. Classification based on physical state
a) solid medium b) semi solid medium c) liquid medium
II. Classification based on the ingredients
a) simple medium b) complex medium c) synthetic or defined medium d) Special media
III-Classification based on physical state
- Solid medium agar is the most commonly used solidifying agent.
- Semi-solid media Such media are soft and are useful in demonstrating bacterial motility and separating motile from non- motile strains .
- Liquid media are sometimes referred as “ broth “. bacteria grow uniformly producing general turbidity eg. Nutrient broth
IV-Classification based on the ingredients
- Simple media – eg: Nutrient broth, N. agar – NB consists of peptone, meat extract, NaCl, – NB + 2% agar = Nutrient agar
- Special media :- Enriched media • Selective media • Differential media • Transport media • Anaerobic media
a) Enriched media • Substances like blood, serum, egg are added to the simple medium. • Used to grow bacteria that are exacting in their nutritional needs. • eg: Blood agar, Chocolate agar, Blood agar BAP contains mammalian blood(usually sheep or horse) typically at a concentration of 5-10%, used to isolate fastidious organisms and detect hemolysis. Chocolate agar contain red blood cells that have been lysed by slowly heating to 80 c .and it used for growing fastidious bacteria, such as Haemophilus influenzae.
b) Selective media • The inhibitory substance is added to a solid media to inhibit commensal or contaminating bacteria such as : • Antibiotics • Dyes • Chemicals • Alteration of pH
Examples
i) Thayer Martin medium selective for Neisseria gonorrhoeae → It usually contains the following combination of antibiotics:
- Vancomycin: which is able to kill most Gram-positive organisms.
- Colistin,: which is added to kill most Gram-negative organisms except Neisseria.
- Nystatin,: which can kill most fungi
- Trimethoprim: which inhibits Gram-negative organisms, especially swarming Proteus.
ii) Eosin methylene blue → selective for gram negative bacteria • The dye methylene blue in the medium inhibit the growth of gram positive bacteria.
ii) Campylobacter agar → Is used for isolation of Campylobacter jejuni from fecal or rectal swab. • Contain Bacteriological charcoal , Cefoperazone and Amphotericin B.
iii) Lowenstein –Jenson medium → is solid medium used for Mycobacterium tuberculosis. • contain penicillin, nalidixic acid and malachite green to inhibit growth of gram positive and gram negative bacteria, in order to limit growth to Mycobacteria species only.
c) Differential media → these are designed in such a way that different bacteria can be recognized on the basis of their colony color. • Dyes and metabolic substrates are incorporated so that those bacteria that utilize them appear as differently colored colonies. Examples: • MacConkey agar • CLED agar • TCBS agar • XLD agar
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MacConkey medium • Distinguish between lactose fermenters & non lactose fermenters. • Lactose fermenters – Pink colonies • Non lactose fermenters – colorless colonies
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Thiosulfate-citrate-bile salts-sucrose agar(TCBS) • highly selective for the isolation of V. cholerae and V. parahaemolyticus Yellow coloured (sucrose fermenting) colonies of Vibrio cholerae on TCBS agar.
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Cysteine Lactose Electrolyte Deficient Agar(CLED) • For cultivation of pathogen from urine specimen , inhibit swarming of proteus sp. CLED, serratia dfd
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Xylose Lysine Deoxycholate Agar(XLD) • Used for the recovery of Salmonella and Shigella species.
d) Transport media –> • Media used for transporting the samples. • Delicate organisms may not survive the time taken for transporting the specimen without a transport media. • Eg: – Stuart’s medium – Buffered glycerol saline
e) Anaerobic media • These media are used to grow anaerobic organisms. Eg: • Robertson’s cooked meat medium. • Thioglycolate broth medium.
Exam Important
- Simple media – eg: Nutrient broth, N. agar – NB consists of peptone, meat extract, NaCl, – NB + 2% agar = Nutrient agar
- Special media :- Enriched media • Selective media • Differential media • Transport media • Anaerobic media
- Thayer Martin medium selective for Neisseria gonorrhoeae
- Thiosulfate-citrate-bile salts-sucrose agar(TCBS) • highly selective for the isolation of V. cholerae and V. parahaemolyticus Yellow coloured (sucrose fermenting) colonies of Vibrio cholerae on TCBS agar.
- Xylose Lysine Deoxycholate Agar(XLD) • Used for the recovery of Salmonella and Shigella species.
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