The BHI agar Brain Heart Infusion Agar is a solid nutritious culture medium. In Spanish we refer to it as brain heart infusion agar. It is a non-selective culture medium, which means that all types of Gram positive and Gram negative bacteria can develop, as well as some yeast and filamentous fungi..
It is composed of an infusion of calf brain and heart, peptic hydrolyzate from animal tissues, pancreatic hydrolyzate of casein, sodium chloride, glucose, disodium phosphate and agar..
It should be noted that BHI agar is one of the most frequently used culture media in bacteriology laboratories. Can be used without supplements as primary culture, subculture of colonies obtained on other selective media or for maintenance of strains in the laboratory.
On the other hand, it is an ideal medium to be used as a base in the preparation of enriched media, such as blood agar and chocolate agar. Both ideal for isolating demanding microorganisms from a nutritional point of view. However, it should be noted that as it contains glucose it is not suitable for observing hemolysis patterns..
Likewise, BHI agar can be used for the preparation of special media for the isolation of pathogenic microorganisms that are difficult to grow in common media, including: Haemophilus sp, Francisella tularensis, Corynebacterium diphtheriae and Histoplasma capsulatum.
With the antibiotic additive, BHI agar becomes a selective medium for the isolation of fungi.
Article index
It is a nutritious culture medium to isolate moderately demanding microorganisms, and its enrichment can be increased with the addition of blood and other nutritional supplements..
It is a non-selective culture medium, therefore it allows the growth of most Gram positive and Gram negative bacteria, as well as some fungi. However, it can become selective with the addition of antibiotics..
The medium contains beef brain and heart infusion, animal tissue peptic hydrolyzate and pancreatic casein hydrolyzate; all of these compounds act as sources of vitamins, amino acids, nitrogen, and carbon.
Glucose is a carbohydrate that provides energy to microorganisms once they have fermented it. Meanwhile, sodium chloride and disodium phosphate maintain osmotic balance and provide a pH close to neutrality. Finally, the agar gives the solid consistency to the medium..
Weigh 52 grams of the dehydrated medium and dissolve in one liter of distilled water. Bring the mixture to a heat source until boiling, stirring frequently during the dissolution process..
BHI agar plates or wedges can be prepared without additives.
For the preparation of wedges, serve the preparation until half of each tube is filled, cover and sterilize in an autoclave at 121 ° C for 15 minutes, when leaving, lay on a base until they solidify. Later store in the refrigerator until use.
The dissolved mixture is autoclaved at 121 ° C for 15 minutes, when leaving it, it is allowed to cool to 50 ° C and 20 ml of the medium are served in sterile Petri dishes. They are allowed to solidify, are inverted and stored in the refrigerator until use. Allow plates to come to room temperature before seeding.
The pH of the medium must remain at 7.4 ± 0.2.
Raw medium is beige in color and prepared is light amber in color.
After sterilizing the medium, cool to a temperature of approximately 45 to 50 ° C, then add the blood (50 ml), mix gently to homogenize and aseptically serve 20 ml in each Petri dish. If bubbles form on the plate, the lighter flame must be passed quickly over the bubbles to eliminate them..
Likewise, special media can be prepared by adding the corresponding additives when the mixture reaches a temperature of 45 to 50 ° C..
The middle is cherry red.
BHI agar without additives is useful as a primary culture and for sowing pure strains of low or medium demanding microorganisms for their subsequent identification..
As it is a light-colored medium, it is ideal for observing pigments and, as it does not contain interfering substances, some biochemical tests, such as oxidase and catalase, can be performed on it, or other biochemical tests can be mounted from colonies from this agar.
Likewise, BHI agar wedges are widely used for the maintenance of strains for a certain time in the laboratory (bacterioteca).
Plates or wedges seeded by surface with bacterial strains are incubated at 37 ° C for 24 to 48 hours. Whereas, in fungi, the temperature and incubation time will depend on the type of fungus that is being sought..
With this base you can prepare enriched and selective media.
Its main function is to serve as a basis in the preparation of blood agar for routine use in microbiology laboratories. Especially, the BHI base is conducive to the isolation of strains of Streptococcus sp. However, it has the disadvantage of not being suitable for the observation of hemolysis patterns because it contains glucose..
It is also used in the preparation of rabbit or horse blood agar for the isolation of Haemophilus sp. For better results, an enrichment supplement (IsoVitaleX) can be added.
If the samples come from the respiratory tract to the agar, bacitracin can be added to inhibit the accompanying flora and increase the probability of recovery of the strains of Haemophilus sp.
On the other hand, blood agar (lamb or human) with cystine tellurite can be prepared to isolate Corynebacterium diphtheriae. Likewise, it is useful to prepare rabbit blood agar, with the addition of cystine and glucose for the isolation of Francisella tularensis.
The seeding of the blood agar plates is done by exhaustion and they are incubated at 35-37 ° C for 24-48 hours in microaerophilicity (5-10% COtwo).
This medium with the addition of antibiotics can replace Sabouraud agar for the isolation of fungi..
The combination of BHI agar with chloramphenicol - gentamicin or penicillin -, streptomycin and horse blood is ideal for the isolation of Histoplasma capsulatum.
Depending on the microorganism to be isolated, incubation at 35-37 ° C or at room temperature in aerobiosis is recommended. Sometimes incubation is necessary in both temperature ranges, using 2 plates for this..
Some mushrooms like Trichophyton mentagrophytes should be incubated at room temperature for up to 7 days.
From each batch prepared, it is recommended to incubate 1 plate or wedge at 37 ° C for 24 hours and verify that there is no growth; especially important when preparing blood agar, as it is an easily contaminated medium.
On the other hand, the quality of the medium can be evaluated by inoculating known or certified standard strains and observing their development..
In this sense, to evaluate BHI agar without additives, strains of Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 or Candida albicans ATCC 10231. Incubate at 37 ° C in aerobiosis for 24 to 48 hours. Satisfactory growth is expected in all cases.
To evaluate blood agar plates, strains of Streptococcus pyogenes ATCC 19615, Streptococcus pneumoniae ATCC 6305 or Trichophyton mentagrophytes ATCC 9533.
Bacterial strains are incubated at 37 ° C in microaerophilicity for 24 hours, while the fungus is incubated at room temperature in a humid chamber for up to 7 days. Satisfactory growth is expected in all cases.
Yet No Comments