Q: A bacterial species has a generation time of 30 minutes. You do a direct cell count using a hemocytometer and determine that you have 250 cells/ml in your culture. After 4 hours, how many cells should you have in culture? Use the formula $$N = N_0$ \times 2^n$$, where $$n is $$the number of generations.

$$N = 250 \times 2^8$ = 64,000$$ cells/ml

Question 1

You are studying a new species of bacteria and find that it grows slowly in the absence of oxygen and more quickly in its presence. Optimal growth conditions appear to be 60°C with a pH of 7.5. How would you classify this bacterium?

Accepted Answer

Facultative anaerobe and thermophile

Question 2

You have an obligate anaerobe and genetically engineer it so now it can produce superoxide dismutase enzyme. Do you think this will change its oxygen classification? If so, how?

Accepted Answer

Yes, it may now tolerate oxygen and become a facultative anaerobe

Question 3

Diagram the growth phases of a bacterial cell. Explain how the log phase is different than stationary phase even though the curve shows no change in cell numbers for both.

Accepted Answer

Log phase shows exponential growth, while stationary phase shows no net growth due to equal rates of cell division and death

Question 4

A bacterial species has a generation time of 30 minutes. You do a direct cell count using a hemocytometer and determine that you have 250 cells/ml in your culture. After 4 hours, how many cells should you have in culture? Use the formula $$N = N_0$ 	imes 2^n$$, where $$n is $$the number of generations.

Accepted Answer

$$N = 250 	imes 2^8$ = 64,000$$ cells/ml

Question 5

After you dilute your cells and grow them on nutrient agar, you obtain 50,000 CFU/ml. What is a likely reason for the difference between your calculated number and your actual number?

Accepted Answer

Some cells may have died or not formed colonies

Question 6

You did a viable cell count using the scheme below. Each tube contains 9 ml of sterile saline and you plate 0.1 ml from each onto petri plates. The plate from tube I has 550 CFU, tube II has 70 CFU and tube III has 5 CFU. How many CFU/ml did you have in your original sample?

Accepted Answer

5,500,000 CFU/ml

Question 7

Explain when you would want to use an antiseptic compared to a disinfectant.

Accepted Answer

Antiseptics are used on living tissue, disinfectants on inanimate objects

Question 8

List an example of a mechanical, a physical, and a chemical method to control growth of bacteria. For each example, state how it controls/kills the organisms and if it would be categorized as a sterilant, disinfectant, or antiseptic.

Accepted Answer

Filtration (mechanical, sterilant), heat (physical, sterilant), alcohol (chemical, antiseptic)

Question 9

How is sterilization different than disinfection?

Accepted Answer

Sterilization kills all forms of microbial life, while disinfection reduces the number of pathogenic organisms

Question 10

Aerobic cellular respiration in chemotrophs generates a maximum of 32 ATP per molecule of glucose. Explain how that number is calculated including the different pathways and total ATP and reducing power generated.

Accepted Answer

Glycolysis, Krebs cycle, and electron transport chain together produce 32 ATP per glucose

Question 11

Why do bacteria typically never make the full 32 ATP total?

Accepted Answer

Some energy is lost as heat and intermediates are used for biosynthesis

Question 12

Explain the steps of an electron transport chain.

Accepted Answer

Electrons are transferred through a series of carriers, creating a proton gradient used to generate ATP

Question 13

What is the difference between an anaerobic chemolithotroph and an anaerobic chemoorganotroph? Which would generate more ATP through oxidative phosphorylation?

Accepted Answer

Chemolithotrophs use inorganic molecules, chemoorganotrophs use organic molecules; chemoorganotrophs generally generate more ATP

Question 14

What is the difference between an anoxygenic phototroph and an oxygenic phototroph?

Accepted Answer

Oxygenic phototrophs produce oxygen as a byproduct, anoxygenic phototrophs do not

Question 15

Cyanobacteria are important because they are able to fix both carbon dioxide and nitrogen gas. What are these processes and why are they important?

Accepted Answer

Carbon fixation converts CO$$_2 to $$organic molecules; nitrogen fixation converts N$$_2 to $$ammonia, both are essential for biosynthesis

Question 16

An organism contains both photosystem I and II and the enzymes of the Calvin cycle. How would you classify this organism?

Accepted Answer

Oxygenic phototroph

Question 17

You are studying two organisms, one is running the ED pathway and the other is running the EM pathway. Which organism would be replicating at a faster rate? Why?

Accepted Answer

EM pathway produces more ATP per glucose, so the organism using EM would replicate faster

Question 18

Diagram the nitrogen cycle. Be sure to identify what types of microbes are involved with each stage.

Accepted Answer

Nitrogen-fixing bacteria convert N$$_2 to NH$_3$$, nitrifying bacteria convert NH$$_3 to NO$_2^-$$ and NO$$_3^-$$, denitrifying bacteria convert NO$$_3^- to N$_2$$

Microbial Growth, Classification, and Metabolism: Study Guide

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