ENRICHED CULTURE MEDIUM: CHARACTERISTICS, TYPES, USES - BIOLOGY - 2025

The enriched culture medium consists of the combination of a nutrient agar as a base plus an enriched substance or compound. The enriched media par excellence are blood agar and chocolate agar.

Both media can be prepared with a simple base of any nutrient agar, such as nutrient agar, trypticasein soy agar, or brain heart infusion agar, among others. Likewise, both media are supplemented with a highly enriched element that is blood, with the difference that in the first case it is used defibrinated and in the second case it is used heated.

Two blood agar plates (enriched culture medium) showing beta-hemolysis. Source: Pixabay.com

These media do not contain inhibitors so a wide variety of microorganisms can grow on them, including some nutritionally demanding strains. In those cases that are already extremely demanding require, in addition to blood, other special nutritional additives, which must be added to the medium in which it is intended to isolate.

On the other hand, if antibiotics or other inhibitory substances are added to an enriched medium, it immediately becomes a selective enriched medium. The latter are necessary when trying to isolate highly demanding microorganisms from an area rich in microbiota, such as Haemophilus sp and Neisseria meningitidis isolated from a sample of nasopharyngeal exudate.

characteristics

These media have a nutritional base made up of peptones, yeast extracts, pancreatic digest and sometimes glucose, among others. It also contains substances that balance pH, water and agar-agar.

On the other hand, defibrinated or heated blood can be added and in turn specific growth factors can be included, such as: vitamin complexes, biotin, para-amino benzoic acid, hemin, NAD, among others.

Differences between culture media and enrichment media

Importantly, Enriched Culture Media should not be confused with Enrichment Media. Although both contain nutrients and special nutritional requirements, the enrichment media are liquid and are used as a pre-treatment of samples where the presence of a type of pathogenic bacteria that is found to a lesser extent within a polymicrobial mixture is suspected.

The enrichment medium will inhibit microorganisms that do not belong to this group and will promote the growth of the pathogen.

In addition, they have the property of repairing damage to the cellular structure of the microorganism that is to be recovered, since it can normally be affected by previous treatments received and in this sense the medium acts increasing its viability.

Enriched agar types and uses

The most widely used enriched agar is blood agar, but this can be prepared with various nutritional bases and depending on it its usefulness changes.

On the other hand, there is also chocolate agar, which is an enriched medium par excellence. However, like blood agar it can be prepared with various nutritional bases, in addition to including other additives for the isolation of fastidious microorganisms.

Below is a summary of the most used agar bases to prepare blood agar and chocolate agar, as well as their usefulness in each case.

- Blood agar

Culture on blood agar of the facultative anaerobic bacterium Morganella morganii. Taken and edited from: Bacteria in Photos.

Prepared with:

Nutrient agar base

It is used for non-demanding bacteria, such as: Enterobacteriaceae, Pseudomonas sp, S. aureus, Bacillus sp, among others. It is not suitable for fastidious bacteria like Streptococcus.

Brain heart infusion agar base

Blood agar with this base is excellent for most bacteria, including Streptococcus sp, but it is not recommended for observing patterns of hemolysis. This combination with certain additives is useful for certain microorganisms. Examples:

If cystine and glucose are added to this medium, it is used to isolate Francisella tularensis. Whereas, if cystine tellurite is added, it is useful to isolate Corynebacterium diphteriae.

Likewise, this medium is used to isolate bacteria of the genus Haemophilus, but in this case it is necessary to add bacitracin, corn starch, horse blood and other enrichment supplements such as (IsoVitaleX).

Finally, if it is included in its preparation (chloramphenicol - gentamicin) or (penicillin - streptomycin) with horse blood, it is ideal for the isolation of Histoplasma capsulatum.

Trypticasein Soy Agar Base

It is excellent for promoting the growth of a wide variety of microorganisms, including Streptococcus sp, being the most recommended for observing hemolysis patterns.

If supplemented with cystine tellurite and lamb's blood it is ideal for Corynebacterium diphteriae.

Müeller Hinton Agar Base

Ideal for antibiograms of complicated microorganisms such as Streptococcus sp, as well as for the isolation of Legionella pneumophila.

Thayer Martin Agar Base

It is ideal for isolating the Neisseria meningitidis species.

Columbia agar base

It is special for the isolation of Helicobacter pylori and Gardnerella vaginalis. For greater success, different types of antibiotics are added to inhibit the accompanying microbiota.

Brucella agar base

The combination of lamb blood, vitamin K, and Brucella agar is excellent for cultivating anaerobic bacteria.

Campylobacter agar base

As its name implies, this base is used to isolate Campylobacter jejuni in stool samples. For this, it is supplemented with 5% of sheep's blood and cephalothin, amphotericin B, trimethoprim, polymyxin B and vancomycin.

- Chocolate agar

Chocolate agar

Prepared with:

Columbia agar base

The preparation of chocolate agar with this base is widely used to isolate bacteria of the Neisseria genus.

It is also recommended for the isolation of Brucella sp, but for this, vitamin K must be added and horse blood used.

GC base agar base

It is one of the recommended bases to make chocolate agar, especially for the isolation of gonococci.

Müeller Hinton Agar Base

Some fastidious microorganisms, such as Streptococcus pneumoniae, require this medium to be able to perform the antimicrobial susceptibility test.

It is also useful in the isolation of Neisserias and Haemophilus, with the exception that the genus Haemophilus prefers horse blood, since it is rich in factor X (hemin) and V (NAD).

Sometimes it is advisable to include antibiotics in its preparation to inhibit the accompanying microbiota.

Base with Thayer Martin agar

The use of lamb's blood is recommended to prepare the chocolate agar with this base. This medium is special for the isolation of Neisseria gonorrhoeae. Antibiotics are added to inhibit the accompanying microbiota.

Chocolate agar (enriched culture medium): Source: Pixinio.com

References

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