Lab Quiz Prep

  1. Catalase test: Catalase is an enzyme produced by bacteria to detoxify hydrogen
    peroxide (H2O 2) to H2O and O 2 gas. Rapid bubbling due to the production of O2 gas indicates a positive test. No bubbling indicates a negative result. (differentiates staphylococcus and streptococcus species).
    and the starch polymer.

  2. Citrate utilization test: Simmons citrate agar is a selective media that tests the
    ability of organisms to utilize citrate as a sole carbon source. The ammonium
    compounds produced during citrate utilization change the pH of the medium from
    neutral to alkaline, producing a deep blue color. A positive test is indicated by
    bacterial growth accompanied by this deep blue color. At a neutral pH, bromothymol
    blue is green in color (note the color of the uninoculated medium). Little to no growth
    without any change in the color of the medium indicates a negative test. (produces ammonium)

  3. Motility test: Motility agar is inoculated by stabbing the agar with an inoculating needle, it is important to keep the needle in the same line it entered as it is removed from the medium. A positive test for motility is indicated by red, diffuse growth radiating from the stab line. A negative test is indicated by red growth only along the stab line.

  4. Triple Sugar Iron Agar (TSIA): Triple sugar iron agar is a differential medium that
    differentiates organisms based on their ability to reduce sulfur and ferment
    carbohydrates. The medium contains the sugars lactose, sucrose, and glucose, and the pH indicator phenol red (if yellow appears barbs have been fermented). Additionally, sodium thiosulfate is used as a source of reducible sulfur. The iron in ferrous sulfate is the hydrogen sulfide indicator. The medium is prepared as a shallow agar slant with a deep butt, thereby providing both aerobic and anaerobic growth environments. It is inoculated by a stab in the agar butt, followed by a streak of the slant. (Differtial, test its ability to reduce sulfur and ferment carbose

    • Yellow slant/ Yellow butt: Aerobic and Anaerobic, Glucose and Lactose and/or Sucrose, and no sulfur reduction

    • Yellow slant/ Black butt: Aerobic and Anaerobic, Glucose and Lactose and/or Sucrose, and includes sulfur reduction

    • Red slant/ Yellow butt: Aerobic and Anaerobic, Glucose and peptone, and no sulfur reduction

    • Red slant/ Black butt: Aerobic and Anaerobic, Glucose and peptone, and includes sulfur reduction

    • Red slant/ Red butt: Aerobic and Anaerobic, Only peptone, and no sulfur reduction

    • Red slant/ No change: Aerobic only, Only peptone, and no sulfur reduction

  5. Carbohydrate fermentation: differential test that determines the ability of organisms to ferment carbohydrates. The test uses phenol red broth. An inverted Durham tube is placed in the broth to capture any gas produced during fermentation. Phenol red turns yellow below a pH of 6.8 and pink above a pH of 7.4. If the organism inoculated is capable of fermentation, the acidic by-products produced (for example: lactic acid, and acetic acid) will turn the medium yellow. In addition to the acidic products, some organisms also produce gas (CO2). Any gas produced is trapped in the inverted Durham tube. If on the other hand, the organism is incapable of fermentation, the medium will turn pink due to the alkaline by-products produced by utilization of the peptone. No gas bubble will be trapped in the Durham tube for non-carbohydrate fermenting organisms.

  6. Urease detection test: Urease degrades urea to ammonia and carbon dioxide. Yeast extract (carbon source), a buffer, and the pH indicator phenol red which turns yellow in an acidic environment and bright pink in an alkaline environment. If the urea in the medium is degraded and ammonia is produced, an alkaline environment is created and the media turns pink, indicating a positive result. Growth accompanied by an orange or yellow-colored medium is negative for urease.

  7. Methyl red test: The methyl red (MR) test is a differential test. The MR test is used to identify mixed acid fermenters’.The pH indicator is methyl red. if acidic end products are present (positive result), the broth will stay red. Otherwise, the broth will return to its clear/yellow color, indicating a negative result. Yellow at or above pH 6.2 and red at or below pH 4.4. Phenol red turns yellow below a pH of 6.8. If you get these two pH indicators confused, you will have difficulty interpreting test results.

  8. Nitrate reduction test: Nitrate broth is a differential media used to determine the ability of an organism to reduce nitrate (NO3) to nitrite (NO2) during anaerobic respiration. In some cases, organisms reduce NO2 fully to N2 gas. This reacts with the α-naphthylamine to form a red-colored compound indicating a positive test.