Differential, selective, and enriched media.
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Selective media.
Isolates or is “selective for” certain groups/types of bacteria by incorporating chemical agents that inhibit the growth of certain organisms and promote the growth of other organisms. Selective media include (but not limited to):
Phenylethyl alcohol agar. Isolates Gram-positive organisms. Phenylethyl alcohol partially inhibits Gram-negative organisms.
Crystal violet agar. Selective for most Gram-negative organisms. Inhibits most Gram-positive organisms.
7.5% sodium chloride agar. Promotes halophilic organisms such as Staphylococcus, and is inhibitory for most others.
Differential/Selective media.
These types of growth media incorporate materials that aid in selection (promote/inhibit growth) and morphological differentiation. Examples are (but not limited to) MacConkey agar, Mannitol salt agar, and Eosin-methylene blue agar.
MacConkey agar. Contains bile salts and crystal violet which inhibit Gram-positive organisms, but promote the growth of Gram-negative organisms. MacConkey also contains lactose and pH indicator neutral red which distinguishes between lactose-fermentors (red) and non-lactose-fermentors (translucent). Enteric bacteria may be separated into lactose-fermentors and non-lactose-fermentors.
- Coliform bacilli: lactose fermentor which produces an acid by-product. Will appear red. E. coli will also turn the surrounds pink.
- Dysentery, typhoid, and paratyphoid: non lactose-fermentors. Will appear tan or transparent.
Mannitol salt agar. Promotes halophilic organisms (e.g. staphylococci) as this medium contains 7.5% NaCl (and inhibits most but not other organisms). The differential components are: mannitol which some staphylococci can ferment; pH indicator phenol red which detects acid produced from mannitol fermentation (creates a yellow-zone).
Eosin-methylene blue agar (Levine). Helps distinguish between enteric lactose-fermentors and non-lactose-fermentors and colon bacillus (E. coli). E. coli will appear blue-black with a green metallic sheen due to large amounts of acid by-products. E. aerogenes will make a thick mucous-looking pink colonies. Non-lactose-fermentors will appear transparent and unremarkable.
Enriched media.
Enriched media contains generous amounts of certain types of nutrients. For example, blood agar can contain 5% sheep blood to promote growth of fastidious organisms such as Streptococcus spp. Organisms that favor blood agar demonstrate hemolysis (breakdown of heme, blood).
Gamma hemolysis. No lysis of blood; no visible change in the medium.
Alpha hemolysis. Incomplete hemolysis resulting in a greenish halo surrounding the colonies.
Beta hemolysis. Complete hemolysis resulting in a clear zone around the colonies. Streptolysin O produces hemolysis by an antigenic, oxygenlabile enzyme. Streptolysin S is a nonantigenic oxygen-stable lysin.
TSA—tryptic soy agar; MSA—mannitol soy agar; MAC—MacConkey agar; PEA—phenylethyl alcohol agar; EMB—eosin methylene blue agar; BDA—blood agar.
Reference
Cappuccino, J. G., & Welsh, C. (2018). Microbiology: A laboratory manual.
