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Enrichment Culture Media

Enrichment media are liquid media that also serves to inhibit commensals in the clinical specimen. Selenite F broth, tetrathionate broth and alkaline peptone water are used to recover pathogens from fecal specimens. Selectively allows certain organism to grow and inhibit others. Eg- Tetrathionate broth (S. Typhi), Alkaline peptone water broth (APW)- Vibrio, Selenite F broth-Shigella

Enrichment Media
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Basics:

  • Basic principle is to control the nutrients and culture conditions in such a way that it suits mainly to a specific species
    • temperature, air supply, light, pH
  • When we assume low amount of potential pathogens being present in the specimen, they have to be enriched first, to multiply up the low number – e.g. serum bouillon, dextrose bouillon, chopped meat bouillon
  • Promotes the growth of a particular organism by providing it with the essential nutrients, and rarely contains inhibitory substances to prevent the growth of normal competitors

Principle And Interpretation:

Enriched media contain the nutrients required to support the growth of a wide variety of organisms, including some of the more fastidious ones. They are commonly used to harvest as many different types of microbes as are present in the specimen(1). Enrichment medium is general purpose enrichment agar which can nourish and support the growth of gram-positive as well as gram-negative bacteria. It can also be supplemented with blood for enriched growth or study the haemolysis. This medium contains peptic digest of animal tissue and yeast extract which serves as source of nitrogen, carbon, amino acids, vitamins and growth factors for growth of bacteria. Dipotassium phosphate buffers the medium well. Agar is solidifying agent.

Enrichment Medium is a highly nutritive medium which can be used as a general purpose enrichment agar base.

Composition:

Ingredients Gms / Litre
Peptic digest of animal tissue 40.000
Yeast extract 6.000
Dipotassium phosphate 3.000
Agar 15.000
Final pH ( at 25°C) 7.0±0.2

Directions: Suspend 64 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.

Quality Control :

  • Appearance: Light yellow coloured homogeneous free flowing powder
  • Gelling: Firm, comparable with 1.5% Agar gel
  • Colour and Clarity of prepared medium: Light amber coloured clear gel forms in Petri plates.
  • Reaction: Reaction of 6.4% w/v aqueous solution at 25°C. pH : 7.0±0.2
  • pH: 6.80-7.20
  • Cultural Response: Cultural characteristics observed after an incubation at 35 – 37°C after 24 hours

Types of Culture Media :

There are some types of important culture or growth media used in microbiological laboratories:

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