Exercise 3-4: Learning in Action 1. With MacConkey agar, what would be the possible consequence of: a. replacing the lactose with glucose? Replacing the lactose w/ glucose would defeat the ability to differentiate between fermenting gram negative organisms and those that can't ferment lactose b. replacing the neutral red with phenol red (yellow when acidic; red or pink when alkaline)? 2. How would removing the crystal violet from MacConkey agar alter the sensitivity and specificity of the medium? 3. MacConkey agar is a selective medium. Is it also a defined or an undefined medium? Why is that formulation desirable? 4. Do Gram-positive bacteria grow on MacConkey agar? If not, what prevents their growth?
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Replacing lactose with glucose: MacConkey agar is used to differentiate lactose-fermenting bacteria from non-lactose fermenting bacteria. If lactose is replaced with glucose, the agar would lose its ability to differentiate these bacteria, as most bacteria can Show more…
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You inoculated a MacConkey agar plate with a gram-positive and a gram-negative bacterial strain. When you observe the plate after incubation, you notice that only the gram-negative bacteria formed colonies. In addition, you notice there is a color change in the media (red to yellow), indicating that this gram-negative bacterium is capable of lactose fermentation. What type of media is this? a) selective b) Differential c) All of the above 2. Fannie Hesse developed the idea of replacing gelatin with agar. What are the benefits of using agar over gelatin? a) Agar is cheap and a good solidifying agent b) Gelatin melts at a lower temperature (40 degrees Celsius), whereas agar does not. c) Agar is a great nutrient source for most bacteria. d) Answer A and B are correct 3. You have been instructed to measure an overnight culture for bacterial density. Which of the following methods would you use if you do not care about distinguishing dead from live bacterial cells? a) Membrane filtration method b) Turbidity c) Most probable number d) Plate count using the spread plate method e) Plate count using the pour plate method.
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3. MacConkey Agar is selective against what type of bacteria?
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