Fastidious organisms having exacting nutritional requirement can be cultivated on infusion media. Meat infusions were included in the first media used for the cultivation of . bacteria Huntoon prepared a medium using fresh beef heart and peptone which became known as Heart Infusion Agar. He demonstrated it could be used to support the growth of nutritionally fastidious microorganisms without the addition of enrichment, such as animal blood. Heart Infusion Agar is a general-purpose growth medium. It is recommended for the cultivation of nutritionally fastidious microorganisms and as a basal medium with a variety of applications.
Composition of Heart Infusion Agar
Ingredients Gms/liter Beef heart, infusion form 500.0 Sodium Chloride 5.00 Tryptose 10.00 Agar 15.00 Demineralized Water 1000.0
Final pH (at 25°C): 7.4 ± 0.2
Principle of Heart Infusion Agar
Tryptose and beef heart infusion provide nutritional requirements for the pathogenic bacteria. Beef heart infusion supply nitrogenous compounds and amino acids necessary for the growth of nutritionally fastidious bacteria. Sodium chloride provides essential electrolytes and maintains osmotic equilibrium. Agar is a solidifying agent.
Preparation and Method of Use of Heart Infusion Agar
Suspend 40 grams in 1000 ml of distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. If desired 5% v/v sterile defibrinated blood may be added. Mix well and dispense as desired. Inoculate and streak the specimen as soon as possible after it is received in the laboratory. If the material is being cultured directly from a swab, roll the swab over a small area of the agar surface and streak for isolation. Incubate aerobically or in 5-10% CO2 at 33-37°C for 18-24 hours. Examine for typical colony morphology
Result Interpretation on Heart Infusion Agar
Organisms Growth Escherichia coli luxuriant growth; beta hemolysis Staphylococcus aureus Good-luxuriant growth; beta hemolysis Neisseria meningitides Luxuriant growth; no hemolysis Streptococcus pneumoniae Good growth; alpha hemolysis Streptococcus pyogenes Good growth; alpha hemolysis; beta hemolysis
Uses of Heart Infusion Agar
It is recommended for use in qualitative procedures for isolation of a wide variety of fastidious microorganisms. It can also be used for the cultivation of Vibrio and Streptococci species. It is used for mass cultivation of bacteria required in vaccine preparation. On the supplementation of blood, Heart Infusion Agar can be used to study hemolytic reactions. It can also be supplemented with glucose, horse serum and antibiotics for the cultivation a wide variety of organisms. This medium was used for isolation and enumeration of hemolytic Streptococci in milk. The deep agar fill can be used as an oxygen gradient to determine the oxygen requirements of the inoculum by observing wherein the tube the growth is prominent.
It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification. Due to the various nutritional requirements of some organisms, occasional isolates may be encountered which fail to grow or grow poorly on this medium.
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