A microbiological culture medium is a substance formulated to support the growth of microorganisms in a laboratory. Among the various types, an enriched medium is specifically designed to cultivate fastidious organisms. These are microbes with complex nutritional needs that cannot synthesize certain nutrients on their own and would not grow on simpler formulas.
Core Properties of an Enriched Medium
Enriched media begin with a nutrient-rich base, often a nutrient broth or agar, which contains basic sources of carbon and nitrogen like peptone or beef extract. This base is then supplemented with additional substances that provide the specific growth factors required by fastidious organisms.
Common enriching substances include whole blood, blood serum, yeast extract, and egg yolks. Blood and serum are rich in proteins and other compounds, while yeast extract provides a source of B vitamins and amino acids. These additives supply essential molecules that allow fastidious bacteria to multiply when they otherwise could not.
Practical Examples in the Laboratory
Blood agar is a widely used enriched medium that incorporates 5-10% sheep or horse blood into a nutrient agar base. This enrichment supports the growth of many fastidious species, including various types of Streptococcus, which are responsible for conditions ranging from strep throat to pneumonia.
Another common example is chocolate agar, which contains no chocolate. It is an enriched medium made by adding blood to a heated agar base, which causes the red blood cells to lyse, or break open. This process makes growth factors like hemin and NAD (nicotinamide adenine dinucleotide) readily available, making it effective for cultivating fastidious respiratory bacteria like Haemophilus influenzae and Neisseria gonorrhoeae.
Enriched Media vs. Other Culture Media
Enriched media differ from selective media, which are formulated to inhibit the growth of unwanted microorganisms. Selective media often contain substances like high concentrations of salt or antibiotics that permit only certain types of bacteria to grow.
A medium can also be differential, meaning it contains indicators that cause different bacteria to appear visually distinct. Some enriched media are also differential. Blood agar, for instance, is enriched but also allows technicians to differentiate bacteria based on how they break down red blood cells, a process called hemolysis.
Finally, enriched media are a type of complex, or undefined, medium. This is because the exact chemical composition of enriching substances like blood or yeast extract is not precisely known. In contrast, a defined medium is made from pure chemical ingredients in exact known quantities.