Selective media<\/strong>.<\/h3>\nIsolates 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):<\/p>\n
Phenylethyl alcohol agar<\/strong>. Isolates Gram-positive organisms. Phenylethyl alcohol partially inhibits Gram-negative organisms.<\/p>\nCrystal violet agar<\/strong>. Selective for most Gram-negative organisms. Inhibits most Gram-positive organisms.<\/p>\n7.5% sodium chloride agar<\/strong>. Promotes halophilic organisms such as Staphylococcus<\/em>, and is inhibitory for most others.<\/p>\nDifferential\/Selective media<\/strong>.<\/h3>\nThese types of growth media incorporate materials that aid in selection (promote\/inhibit growth) and morphological differentiation.\u00a0 Examples are (but not limited to) MacConkey agar, Mannitol salt agar, and Eosin-methylene blue agar.<\/p>\n
MacConkey agar<\/strong>. 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.<\/p>\n\n- Coliform bacilli<\/strong>: lactose fermentor which produces an acid by-product. Will appear red. E. coli<\/em> will also turn the surrounds pink.<\/li>\n
- Dysentery, typhoid, and paratyphoid<\/strong>: non lactose-fermentors. Will appear tan or transparent.<\/li>\n<\/ul>\n
Mannitol salt agar<\/strong>. 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\u00a0staphylococci can ferment; pH indicator phenol red which detects acid produced from mannitol fermentation (creates a yellow-zone).<\/p>\nEosin-methylene blue agar<\/strong> (Levine). Helps distinguish between enteric lactose-fermentors and non-lactose-fermentors and colon bacillus (E. coli<\/em>). E. coli<\/em> will appear blue-black with a green metallic sheen due to large amounts of acid by-products. E. aerogenes<\/em> will make a thick mucous-looking pink colonies. Non-lactose-fermentors will appear transparent and unremarkable.<\/p>\nEnriched media.<\/h3>\n
Enriched media contains generous amounts of certain types of nutrients. For example, blood agar<\/strong> can contain 5% sheep blood to promote growth of fastidious organisms such as Streptococcus<\/em> spp. Organisms that favor blood agar demonstrate hemolysis (breakdown of heme, blood).<\/p>\nGamma hemolysis<\/strong>. No lysis of blood; no visible change in the medium.<\/p>\nAlpha hemolysis<\/strong>. Incomplete hemolysis resulting in a greenish halo surrounding the colonies.<\/p>\nBeta hemolysis<\/strong>. Complete hemolysis resulting in a clear zone around the colonies. Streptolysin O<\/strong> produces hemolysis by an antigenic, oxygenlabile enzyme. Streptolysin S<\/strong> is a nonantigenic oxygen-stable lysin.<\/p>\nTSA\u2014tryptic soy agar; MSA\u2014mannitol soy agar; MAC\u2014MacConkey agar; PEA\u2014phenylethyl alcohol agar; EMB\u2014eosin methylene blue agar; BDA\u2014blood agar.<\/p>\n
Crystal violet agar<\/strong>. Selective for most Gram-negative organisms. Inhibits most Gram-positive organisms.<\/p>\n 7.5% sodium chloride agar<\/strong>. Promotes halophilic organisms such as Staphylococcus<\/em>, and is inhibitory for most others.<\/p>\n These types of growth media incorporate materials that aid in selection (promote\/inhibit growth) and morphological differentiation.\u00a0 Examples are (but not limited to) MacConkey agar, Mannitol salt agar, and Eosin-methylene blue agar.<\/p>\n MacConkey agar<\/strong>. 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.<\/p>\n Mannitol salt agar<\/strong>. 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\u00a0staphylococci can ferment; pH indicator phenol red which detects acid produced from mannitol fermentation (creates a yellow-zone).<\/p>\n Eosin-methylene blue agar<\/strong> (Levine). Helps distinguish between enteric lactose-fermentors and non-lactose-fermentors and colon bacillus (E. coli<\/em>). E. coli<\/em> will appear blue-black with a green metallic sheen due to large amounts of acid by-products. E. aerogenes<\/em> will make a thick mucous-looking pink colonies. Non-lactose-fermentors will appear transparent and unremarkable.<\/p>\n Enriched media contains generous amounts of certain types of nutrients. For example, blood agar<\/strong> can contain 5% sheep blood to promote growth of fastidious organisms such as Streptococcus<\/em> spp. Organisms that favor blood agar demonstrate hemolysis (breakdown of heme, blood).<\/p>\n Gamma hemolysis<\/strong>. No lysis of blood; no visible change in the medium.<\/p>\n Alpha hemolysis<\/strong>. Incomplete hemolysis resulting in a greenish halo surrounding the colonies.<\/p>\n Beta hemolysis<\/strong>. Complete hemolysis resulting in a clear zone around the colonies. Streptolysin O<\/strong> produces hemolysis by an antigenic, oxygenlabile enzyme. Streptolysin S<\/strong> is a nonantigenic oxygen-stable lysin.<\/p>\n TSA\u2014tryptic soy agar; MSA\u2014mannitol soy agar; MAC\u2014MacConkey agar; PEA\u2014phenylethyl alcohol agar; EMB\u2014eosin methylene blue agar; BDA\u2014blood agar.<\/p>\nDifferential\/Selective media<\/strong>.<\/h3>\n
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Enriched media.<\/h3>\n
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