Final answer:
A selective medium is a type of growth medium in microbiology designed to support the growth of specific microorganisms while suppressing others, such as MacConkey agar for gram-negative bacteria. Enriched media provide additional nutrients for fastidious organisms, while chemically defined media contain only known chemical components.
Step-by-step explanation:
A nutrient medium designed to favor the growth of certain microorganisms while inhibiting others is known as a selective medium. Selective media work by providing essential nutrients and conditions that support the growth of the desired organisms and suppress the competitors. MacConkey agar is a classic example of a selective medium that is biased towards gram-negative bacteria, especially the Enterobacteriaceae family, by incorporating ingredients like bile salts and crystal violet which hinder the growth of many gram-positive bacteria.
Another type is enriched media, which includes additional nutrients like vitamins and growth factors to cater to the needs of fastidious organisms, or those with complex nutritional requirements. To identify bacteria, special media like differential media are also used, which can indicate different bacterial strains by the color changes they produce on the medium.
When the exact chemical composition of a medium is known, it is termed a chemically defined medium. In contrast, complex media contain various extracts and digest whose precise chemical make-up is unknown, making them suitable for a broader range of bacterial growth.