Microbiology Lab Practical Exam 2 Questions and Answers 100% Pass

Microbiology Lab Practical Exam 2 Questions and Answers 100% Pass Oxidase Test - *Positive Test:* -Turns blue within 20-30 seconds *Negative Test* -No color change within thirty seconds *Reagent used:* -Oxidase What is the purpose of the oxidase test? - -The tests is used to identify bacteria containing the respiratory enzyme cytochrome c oxidase as an electron acceptor in the ETC. How is the oxidase test performed? - 1. Transfer bacterium to the paper 2. Add drops of complex A & B to bacterium 3. Observe for color change within 30 seconds Why should you read the results of the oxidase test within 30 seconds? - -The reagents of this test are unstable, therefore they may oxidize independently shortly after becoming moist. Therefore it is important to read within 30 seconds. Catalase Test - *Positive* -Visible Bubble production *Negative* 2 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE -No Bubbles *Reagent Used:* -Hydrogen Peroxide Why is the Catalase Test Used? - -This test is used to identify organisms that produce the enzyme catalase, and its ability to break down hydrogen peroxide. How is the catalase test performed? - 1. Transfer a large amount of growth to slide 2. Place one to two drops of hydrogen peroxide on bacteria 3. Observe for bubbles MacConkey Test - *Poor Growth or No growth:* -Organism is inhibited by CV and/or bile, therefore is Gram-positive *Good Growth:* -Organism is not inhibited by CV and/or bile, therefore Gram-Negative *Pink to red growth with or without bile precipitate:* -Organism produces acid from lactose fermentation therefore is a probable coliform *Growth is "colorless" (not red or pink):* -Organism does not ferment lactose, therefore is a noncoliform What does the bacteria look like on a MacConkey Media? - *Negative Growth:* -Streak lines visible, but no colonies (Gram-positive) *Positive For Growth, but colorless:* 3 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE -Gram-negative, but no lactose fermentation (noncoliform) *Positive for growth and Lactose fermentation:* -Pink growth and good growth indicates a Gram-negative bacteria and Lactose fermentation (coliform) What does the MacConkey Test Do? - The test isolates and differentiates members of the *Enterobacteriaceae* based on the ability to ferment lactose and their ability to grow in the presence of bile salts and crystal violet. Is the MacConkey test Selective or Differential? - The MacConkey Agar is both selective and differential for it is selective for only Gram-negative bacterial growth, and differential for it differentiates between lactose fermenters and lactose nonfermenters Malonate Test - *Positive:* -Color change in medium from green to blue (malonate is utilized) *Negative* -No color change from uninoculated tube, remains green (malonate is not utilized) Why is the malonate test used? - Used to assess a bacteria's ability to use malonate as its sole carbon source. What is the chemical reaction that takes place during the Malonate Test? - In one portion of the Krebs cycle, which requires the enzyme succinate dehydrogenase, the coenzyme FAD is reduced to FADH2. Malonate, when added to the media, is similar enough to succinate to replace it as the substrate in the reaction. When combined with subsequent build up of succinate in the cell, shuts down the Krebs cycle and will kill the organism unless it can ferment malonate as its sole remaining carbon source. What causes the color change in the malonate test? - -Bromthymol blue dye, which is green when in uninoculated media, is added to indicate any shift in the pH. If an organism cannot utilze malonate but 4 | P a g e Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE manages to ferment a small amount of glucose, it may turn yellow or have n o color change at all. (Negative result) *If the organism utilizes malonate, it will alkalize the medium and change the indicator from green to deep blue.* Simmon's Citrate Test - *Positive Result:* -Blue (even the slightest) and/or growth. This means citrate is utilized. *Negative Result* -No change in color and no growth, no change from uninoculated tube. This means that citrate is not utilized. Why is the Citrate Test Used? - Determines the ability of an organism to use citrate as its sole source of carbon. Determine the chemical reaction that takes place during the Citrate Test... - In a medium containing citrate as the only available carbon source, bacteria that possess citrate-permease can transport molecules into the cell and enzymatically convert it to pyruvate. Pyruvate then can be converted to a variety of products, depending on pH of the environment. What cause the color change in the Citrate Test? - Bromthymol bl