LAB 21: Selective and Differential Media Quiz

Selective and Differential Agar

• Selective media encourages the growth of certain bacteria types while inhibiting the growth of others.
• Differential media differentiates between bacterial types growing on the same agar.
• Examples of enrichment media include antibiotics, nutrients, salt, pH, and oxygen.
• Selective media uses inhibitors that suppress the growth of certain bacteria types.
• Differential media usually has a pH indicator that changes color.

Snyder Test

• The Snyder test measures susceptibility to dental caries using a selective and differential agar.
• The medium selects for acidiphiles by inhibiting mesophile growth at low pH.
• The medium differentiates the amount of acidic producing bacteria by the rate of color change in the agar.
• The Snyder agar contains a high percentage of glucose and a low percentage of agar.
• Uninoculated Snyder agar is initially green in color.
• Bacteria ferment glucose, producing acid that lowers the pH, turning the agar yellow.
• The rate of color change indicates the susceptibility to dental caries: no color change indicates no dental caries, 24 hours indicates high susceptibility, 48 hours indicates moderate susceptibility, and 72 hours indicates low susceptibility.

MSA (Mannitol Salt Agar)

• MSA is a selective and differential medium that differentiates pathogenic Staph.aureus from non-pathogenic resident skin bacteria.
• Staph.aureus is a mannitol fermenter that produces acid during fermentation, turning the agar yellow.
• MSA selects for Gram-positive halophiles, as the salt inhibits Gram-negative bacteria.
• MSA agar contains mannitol and peptides.
• The pH indicator in the agar turns yellow due to acidic mannitol fermentation.
• Colonies with red agar indicate no mannitol fermentation, suggesting bacteria like S.epidermidis or M.luteus.
• Colonies with yellow agar possibly contain S.aureus.

Coagulase Test

• The coagulase test helps identify pathogenic species of Staphylococcus.
• Coagulase is an enzyme produced by certain bacteria that activates prothrombin to thrombin.
• Thrombin then activates fibrinogen to fibrin, forming a clot.
• Staph.aureus produces coagulase, allowing it to hide within blood clots from phagocytes.
• A positive coagulase test results in solidification when inoculated serum.
• A positive catalase test differentiates staphylococcus from other Gram-positive bacteria.
• Staph.aureus is catalase positive, producing oxygen bubbles when hydrogen peroxide is dripped on it.

SS (Salmonella/Shigella) Agar

• SS agar is a selective and differential agar used to differentiate pathogenic Shigella, Salmonella typhimiurium, and E.coli.
• Salmonella can metabolize the amino acid cysteine.
• E.coli can metabolize lactose.
• Shigella cannot metabolize either cysteine or lactose.
• SS agar selects for enteric bacteria, as bile salts inhibit other bacteria.
• SS agar contains lactose and cysteine peptides.
• Acidic lactose fermentation causes the pH indicator in the agar to precipitate, producing purple colonies, indicating the presence of E.coli.
• Cysteine fermentation produces hydrogen sulfide, which reacts with iron in the media, forming iron sulfide and black colonies, indicating the presence of Salmonella typhimiurium.
• White colonies on SS agar indicate the presence of Shigella, as they are negative for both lactose and cysteine metabolism.

Test your knowledge on selective and differential agar, including the principles behind enrichment media and their applications like the Snyder test. Understand how different media types promote or inhibit bacterial growth and how they can indicate susceptibility to dental caries.