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Microbiology Lab Exam Two Review

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  • Which protocols use phenol red as a pH indicator?

    Triple Sugar Iron (TSI) agar, carbohydrate fermentation (glucose, lactose, sucrose), urea utilization, and Mannitol Salt Agar (MSA).

  • What does phenol red indicate in urea utilization tests?

    It detects ammonia production, turning fuchsia in basic conditions due to urease activity breaking down urea.

  • Which protocols use 'clearing' as an indicator of enzyme synthesis?

    DNase test, starch hydrolysis, casein hydrolysis, blood agar (beta hemolysis), and lipid hydrolysis.

  • What does clearing indicate in DNase and starch hydrolysis tests?

    Clearing shows bacteria produce enzymes that degrade DNA (DNase) or starch (amylase) into smaller molecules.

  • Which protocols involve fermentation?

    Carbohydrate fermentation, Mannitol Salt Agar, Triple Sugar Iron agar, Litmus milk reactions, Methyl Red, and Voges-Proskauer tests.

  • How does litmus milk indicate fermentation?

    Acid fermentation turns litmus pink and may cause a yogurt-like protein precipitate (casein coagulation).

  • What is the difference between Methyl Red and Voges-Proskauer tests?

    Methyl Red detects mixed acid fermentation; Voges-Proskauer detects butanediol fermentation by detecting acetoin.

  • Name two selective media and what they select for.

    Mannitol Salt Agar selects for Gram-positive cocci; Eosin Methylene Blue agar selects for Gram-negative coliforms.

  • How is zinc used in the nitrate reduction test?

    Zinc is added after nitrate reagents if no color change occurs; it detects unreacted nitrate by producing a color change.

  • Compare catalase activity to oxidase activity.

    Catalase converts hydrogen peroxide to water and oxygen gas; oxidase transfers electrons to oxygen to form water in aerobic respiration.

  • What does casein hydrolysis test detect?

    It detects caseinase enzyme that breaks down casein into amino acids, indicated by clearing around colonies.

  • How does litmus milk reaction differ from casein hydrolysis?

    Litmus milk uses pH indicator; acid fermentation causes coagulation (white yogurt-like mass), while casein hydrolysis causes clearing (amber supernatant).

  • What does a white band at the bottom of a litmus milk tube indicate?

    Reduction of litmus, indicating anaerobic conditions or reduction reactions by bacteria.

  • What pH indicator is used in citrate utilization tests?

    Bromthymol blue, which turns blue at basic pH indicating citrate utilization.

  • How does citrate utilization change the media pH?

    Carbon dioxide reacts with carbonate in the media, making it alkaline and turning the indicator blue.

  • What is the role of mannitol in Mannitol Salt Agar?

    Mannitol is fermented by some bacteria, producing acid that lowers pH and changes phenol red indicator color.

  • What does alpha, beta, and gamma hemolysis indicate on blood agar?

    Alpha: partial discoloration; Beta: clearing (complete lysis); Gamma: no change.

  • Which enzyme is detected by the catalase test?

    Catalase enzyme that breaks down hydrogen peroxide into water and oxygen.

  • What does the Voges-Proskauer test detect?

    Production of acetoin, a metabolite in butanediol fermentation.

  • What is the significance of clearing in lipid hydrolysis tests?

    Indicates production of lipase that breaks triglycerides into fatty acids and glycerol.