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Culture Media Susmita Chakrabarty

These are classified into six types:

  1. Basal media,
  2. Enriched media,
  3. Selective media,
  4. Indicator media,
  5. Transport media, and
  6. Storage media.

1. BASAL MEDIA - Basal media are those that may be used for growth (culture) of bacteria that do not need enrichment of the media. Examples: Nutrient broth, nutrient agar and peptone water. Staphylococcus and Enterobacteriaceae grow in these media.

Basal Media

2. ENRICHED MEDIA - The media are enriched usually by adding blood, serum or egg. Examples: Enriched media are blood agar and Lowenstein-Jensen media. Streptococci grow in blood agar media.

Lowenstein-Jensen media

3. SELECTIVE MEDIA - These media favour the growth of a particular bacterium by inhibiting the growth of undesired bacteria and allowing growth of desirable bacteria. Examples: MacConkey agar, Lowenstein-Jensen media, tellurite media (Tellurite inhibits the growth of most of the throat organisms except diphtheria bacilli). Antibiotic may be added to a medium for inhibition.

Escherichia coli under MacConkey agar

4. INDICATOR (DIFFERENTIAL) MEDIA - An indicator is included in the medium. A particular organism causes change in the indicator, e.g. blood, neutral red, tellurite. Examples: Blood agar and MacConkey agar are indicator media.

Blood Agar is an indicator (differential) media

5. TRANSPORT MEDIA - These media are used when specie-men cannot be cultured soon after collection. Examples: Cary-Blair medium, Amies medium, Stuart medium.

Cary-Blair medium

6. STORAGE MEDIA - Media used for storing the bacteria for a long period of time. Examples: Egg saline medium, chalk cooked meat broth fermented, i.e. the bacterium is non-lactose fermenter, e.g. Salmonella. Shigella, Vibrio.

Egg yolk agar

COMMON MEDIA IN ROUTINE USE

Nutrient Broth - 500 g meat, e.g. ox heart is minced and mixed with 1 litre water. 10 g peptone and 5 g sodium chloride are added, pH is adjusted to 7.3.

Nutrient Broth

Uses:

  1. As a basal media for the preparation of other media,
  2. To study soluble products of bacteria.

Nutrient Agar - It is solid at 37°C. 2.5% agar is added in nutrient broth. It is heated at 100°C to melt the agar and then cooled.

Nutrient Agar

Peptone Water - Peptone 1% and sodium chloride 0.5%. It is used as base for sugar media and to test indole formation.

Peptone Water, 500 Grams (RPI P20240-100.0 Peptone, Powder, 100g)

Blood Agar - Most commonly used medium. 5- 10% defibrinated sheep or horse blood is added to melted agar at 45-50°C. Blood acts as an enrichment material and also as an indicator. Certain bacteria when grown in blood agar produce haemolysis around their colonies. Certain bacteria produce no haemolysis. Types of changes : (a) beta (p) haemolysis. The colony is surrounded by a clear zone of complete haemolysis, e.g. Streptococcus pyogenes is a beta haemolytic streptococci, (b) Alpha (a) haemolysis. The colony is surrounded by a zone of greenish discolouration due to formation of biliverdin, e.g. Viridans streptococci, (c) Gamma (y) haemolysis, or, No haemolysis. There is no change in the medium surrounding the colony,

Blood Agar

Chocolate Agar or Heated Blood agar - Prepared by heating blood agar. It is used for culture of pneumococcus, gonococcus, meningococcus and Haemophilus. Heating the blood inactivates inhibitor of growths.

Chocolate Agar

MacConkey Agar - Most commonly used for enterobac-teriaceae. It contains agar, peptone, sodium chloride, bile salt, lactose and neutral red.

MacConkey Agar

It is a selective and indicator medium :

  1. Selective as bile salt does not inhibit the growth of enterobactericeae but inhibits growth of many other bacteria.
  2. Indicator medium as the colonies of bacteria that ferment lactose take a pink colour due to production of acid. Acid turns the indicator neutral red to pink. These bacteria are called 'lactose fermenter', e.g. Escherichia coll. Colourless colony indicates that lactose is not secondary bacteria without an inhibitory effect on diphtheria bacilli. It is also an indicator medium as the diphtheria bacilli produce black colonies. Tellurite metabolized to tellbrism, which has black colour.

EMB (Eosin-methylene blue) Agar - A selective and differential medium for enteric Gramnegative rods. Lactose-fermenting colonies are coloured and nonlactose-fermenting colonies are nonpigmented. Selects against gram positive bacteria.

EMB (Eosin-methylene blue) Agar

XLD (Xylose Lysine Deoxychoiate) - It is used to isolate Salmonella and Shigella species from stool specimens. This is a selective media.

XLD (Xylose Lysine Deoxychoiate)

SS (Salmonella-Shigella) Agar - It is a selective medium used to isolate Salmonella and Shigella species. SS Agar with additional bile salt is used if Yersinia enterocolitica is suspected.

SS (Salmonella-Shigella) Agar

DCA (Desoxycholate Citrate Agar) - It is used for isolation of Salmonella and Shigella. The other enteric bacteria are mostly inhibited (a selective medium). It is also a differential (indicator) medium due to presence of lactose and neutral red.

DCA (Desoxycholate Citrate Agar)

Tetrathionate Broth - This medium is used for isolating Salmonella from stool. It acts as a selective medium. It inhibits normal intestinal bacteria and permits multiplication of Salmonella.

Tetrathionate Broth

Selenite F Broth - Uses and functions are same as that of tetrathionate broth. Thiosulphate-Citrate-Bile-Sucrose (TCBS) Agar. TCBS agar is a selective medium used to isolate Vibrio cholerae and other Vibrio species from stool.

Selenite F Broth

Charcoal-yeast agar - Used for Legionella pneumophila. Increased concentration of iron and cysteine allows growth.

Charcoal-yeast agar

Tellurite-Gelatin Agar Medium (TGAM) - It may be used as transport, selective and indicator medium.

Campylobacter Medium - This selective medium is used to isolate Campylobacter jejuni and Campylobacter coli from stool.

Cary-Blair Medium - It is used as a transport medium for faeces that may contain Salmonella, Shigella, Vibrio or Cam-pylobacter species.

Cary-Blair Medium

Amies medium is used for gonococci and other pathogens.

Peptone Water Sugar Media - These indicator media are used to study 'Sugar fermentation'. 1 % solution of a sugar (lactose, glucose, mannitol etc.) is added to peptone water containing Andrade's indicator in a test tube. A small inverted Durham tube is placed in the medium. The media are colourless. After culture, change of a medium to red colour indicates acid production. Gas, if produced collects in Durham tube.

Motility Indole Urea (MIU) Medium - This is used to differentiate enterobacteria species by their motility, urease, and indole reactions.

KIA (Kligler Iron Agar) - This is a differential slope medium used in the identification of enteric bacteria. The reactions are based on the fermentation of lactose and glucose and the production of hydrogen sulphide.

KIA (Kligler Iron Agar)

Christensen's Urea Medium - This is used to identify urea splitting organisms, e.g. Proteus. A purple pink colour indicates urea splitting.

Christensen's Urea Medium

Bordet-Gengou Medium - This medium is used for culture of Bordetella pertussis. Increased concentration of blood allows growth. It contains agar, potato, sodium chloride, glycerol, peptone and 50% horse blood. Penicillin may be added to it.

BORDET GENGOU AGAR (Sheep Blood 15%)

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