Opening the book…
1. You are studying a bacterium found in the normal intestinal flora of humans and most other mammals. You have inoculated five nutrient broths, and are incubating one broth at each of the following temperatures: 5°C, 25°C, 38°C, 42°C, and 55°C. At which temperature would you predict to see the optimum growth rate?
- a) 42°C
- b) 38°C
- c) 25°C
- d) 55°C
- e) 5°C
Organisms of the normal intestinal flora are mesophiles, with optimal growth temperatures between approximately 20°C and 45°C. Human body temperature is 37°C, so growth is optimal nearest that temperature.
2. Which of the following processes removes or destroys endospores?
- a) Sterilization
- b) Disinfection
- c) Antisepsis
- d) None of the above
Endospores are the most resistant form of microorganism. Methods capable of destroying endospores are known as sterilizing methods, because all forms of microorganisms have been killed.
3. What would the colony growth on a nutrient agar plate look like if you were to take a psychrophilic bacteria culture and put it in a 35°C incubator?
- a) Some growth with small colonies
- b) Large colonies in high numbers
- c) More information would be needed to make a conclusion
- d) No growth
Psychrophiles have a maximum growth temperature around 15°C and are not capable of growth above 20°C, so no growth would occur at 35°C.
4. Why can temperature kill microorganisms?
- a) Reduced temperatures reduce proteins, inhibiting enzymes
- b) Exposure to heat dissolves cell membranes
- c) Exposure to heat denatures proteins, inhibiting enzymes
- d) Increased temperatures disintegrate lipids
Proteins, many of which act as enzymes that facilitate cellular metabolism, depend on their structure to function. Heat denatures those proteins, inhibiting the enzymes and halting metabolism.
5. What are the three possible outcomes for each antibiotic and microorganism pair in a Kirby-Bauer test?
- a) Similar, reversion, indeterminate
- b) Susceptible, resistant, indeterminate
- c) Stunted, robust, intermediate
- d) Susceptible, resistant, intermediate
The measurements indicate that the microorganism is susceptible (S), intermediate (I), or resistant (R) to the drug.
6. A Staphylococcus species is found to grow best at 30°C, and will not grow at temperatures below 25°C or above 38°C. Which temperature group best describes this species?
- a) Psychrophile
- b) Thermophile
- c) Psychrotroph
- d) Mesophile
Mesophiles have optimal growth temperatures between approximately 20°C and 45°C. A species growing best at 30°C within a 25°C to 38°C range falls in this group.
7. You perform a Kirby-Bauer test on Staphylococcus aureus and measure the zone of inhibition around a tetracycline disk as 13 mm. How would you report the susceptibility of the organism to tetracycline? (Refer to the zone of inhibition diameter standards tables.)
- a) Susceptible
- b) Intermediate
- c) Resistant
- d) Cannot be determined with the provided information
For Staphylococcus aureus, tetracycline breakpoints are S at 19 mm or greater, I at 15 - 18 mm, and R at 14 mm or less. A 13 mm zone falls in the resistant range.
8. You perform a Kirby-Bauer test on Staphylococcus aureus and measure the zone of inhibition around a rifampin disk as 21 mm. How would you report the susceptibility of the organism to rifampin? (Refer to the zone of inhibition diameter standards tables.)
- a) Susceptible
- b) Intermediate
- c) Resistant
- d) Cannot be determined with the provided information
For Staphylococcus aureus, rifampin breakpoints are S at 20 mm or greater, I at 17 - 19 mm, and R at 16 mm or less. A 21 mm zone falls in the susceptible range.
9. You perform a Kirby-Bauer test on Staphylococcus aureus and measure the zone of inhibition around an erythromycin disk as 11 mm. How would you report the susceptibility of this organism to erythromycin? (Refer to the zone of inhibition diameter standards tables.)
- a) Susceptible
- b) Intermediate
- c) Resistant
- d) Cannot be determined with the provided information
For Staphylococcus aureus, erythromycin breakpoints are S at 23 mm or greater, I at 14 - 22 mm, and R at 13 mm or less. An 11 mm zone falls in the resistant range.
10. Psychrotrophs are a particular concern for food-borne illness because they:
- a) Tolerate growth temperatures from around 0°C - 5°C, so they continue to grow in refrigerated food, yet also grow at temperatures up to around 35°C
- b) Are not capable of growth above 20°C, so they survive only in frozen food
- c) Require temperatures above 80°C, so they survive cooking
- d) Grow only at a pH less than 5.5, the pH of most preserved foods
Psychrotrophs tolerate growth temperatures around 0°C - 5°C up to around 35°C, meaning they can grow at refrigeration temperatures and also at temperatures found in and on the human body.
11. On the pH scale, as the concentration of hydrogen ions increases, the measurement moves toward:
- a) 0, and the sample is said to be acidic
- b) 14, and the sample is said to be acidic
- c) 0, and the sample is said to be alkaline
- d) 14, and the sample is said to be alkaline
As hydrogen ion concentrations increase, the measurement moves toward 0 on the pH scale and is said to be acidic. When hydrogen ions decrease, the measurement moves toward 14 and is alkaline.
12. In what ways can a change in pH damage a bacterial cell? Select all that apply.
- a) The hydrogen bonding that holds the DNA double helix together may be disrupted
- b) Proteins may denature permanently, which can halt metabolism
- c) The hydrogen ion gradient across the cell membrane that ATP synthase requires may be affected
- d) The cell wall is converted into a capsule
Changes in pH can disrupt the hydrogen bonding that holds DNA together, permanently denature proteins and halt metabolism, and interfere with the gradient of hydrogen ions across the cell membrane that enables ATP synthase to produce ATP.
13. How do the Lactobacillus species of the normal vaginal flora inhibit the growth of yeast?
- a) They produce products that maintain a pH below 5.5, which is too low for organisms requiring a higher pH
- b) They produce a chemical agent that is virucidal
- c) They raise the pH above 8, which yeast cannot tolerate
- d) They consume the available oxygen, creating anaerobic conditions
Lactobacillus species require a pH less than 5.5 and produce products that help maintain that pH within the vagina. In doing so they inhibit the growth of microorganisms that require a higher pH, such as yeast.
14. Why are naked viruses considered more resistant to chemical agents than enveloped viruses?
- a) Enveloped viruses have a lipid membrane that can be disrupted by any chemical damaging to lipids, and naked viruses lack that envelope
- b) Naked viruses form endospores when they are exposed to chemicals
- c) Enveloped viruses are larger and therefore easier for a chemical to reach
- d) Naked viruses reproduce faster than the chemical agent can act
Enveloped viruses have a lipid membrane derived from their hosts that could be disrupted by any chemical damaging to lipids. Naked viruses, lacking the lipid envelope, would be unaffected by such agents.
15. Why can some chemical agents be used as disinfectants but not as antiseptics?
- a) Some agents used solely as disinfectants are too harsh to be applied to living tissues
- b) Disinfectants are microbiostatic while antiseptics are microbicidal
- c) Antiseptics must be capable of destroying endospores
- d) Disinfectants cannot be applied to inanimate objects
Not all chemical agents can classify as both, because some of the agents used solely as disinfectants are too harsh to be applied to living tissues.
16. In the antiseptic and disinfectant activity, a filter disk is soaked in sterile water and placed on the agar alongside the others. What is the purpose of this disk?
- a) It serves as the control
- b) It provides the moisture the other disks need in order to diffuse
- c) It marks the center of the plate for measuring
- d) It rinses away any chemical agent that spreads too far
A filter disk is dipped into the microcentrifuge tube containing sterile water, which serves as your control.
17. How is a lawn of growth created on the surface of an agar plate?
- a) The surface is swabbed multiple times, rotating the plate between swabs, so it is completely covered in bacteria
- b) A single streak is made down the center of the plate
- c) The plate is flooded with broth culture and the excess is poured off
- d) Four quadrants are streaked, flaming the loop between each one
Use the cotton swab applicator to introduce the bacterial sample to the surface of the agar. Swab the surface multiple times, rotating the plate in between swabs. The surface of the agar should be completely covered in bacteria in order to create a lawn of growth.
18. Why must the antimicrobial sensitivity test be performed using a standardized method?
- a) So the results obtained are consistent across the healthcare industry
- b) So that only one drug needs to be tested on each plate
- c) So the plates can be incubated at room temperature
- d) So the zones can be judged by eye without measurement
The antimicrobial sensitivity test must be performed using a standardized method so the results obtained are consistent across the healthcare industry.
19. Which factors may affect the size of the zone of inhibition? Select all that apply.
- a) The molecular weight of the drug
- b) The type of media used for the test
- c) The concentration of the bacterial culture
- d) The color of the filter disk
Several factors may affect the size of the zone of inhibition, including the molecular weight of the drug, the type of media used for the test, and the concentration of the bacterial culture.
20. What is the clinical purpose of performing an antimicrobial susceptibility test?
- a) To determine which drugs would be potentially effective in treating an infection
- b) To determine the temperature group to which the organism belongs
- c) To determine whether the organism is capable of forming endospores
- d) To measure the rate at which the organism reproduces
The antimicrobial susceptibility test allows for the determination of which drugs would be potentially effective in treating an infection.
21. When using a spectrophotometer to analyze bacterial growth in the broth cultures, which mode and wavelength are used?
- a) Absorbance mode at 600 nm
- b) Transmittance mode at 600 nm
- c) Absorbance mode at 450 nm
- d) Transmittance mode at 450 nm
Set the mode to absorbance and the wavelength to 600 nm.
22. What is used to calibrate the spectrophotometer before samples are measured?
- a) An uninoculated medium
- b) A sample of the most turbid culture
- c) Sterile water in an open tube
- d) A cuvette left empty
Calibrate the spectrophotometer using an uninoculated medium.
23. Why are the sides of the cuvette wiped with a Kim wipe before it is placed in the spectrophotometer?
- a) To ensure no skin oils or fingerprints interfere with the measurement
- b) To sterilize the outside of the cuvette
- c) To warm the sample to the temperature of the instrument
- d) To remove condensation that would dilute the sample
Wipe the sides of the cuvette with a Kim wipe to ensure no skin oils or fingerprints interfere with the measurement.
24. Using the visual analysis scale for assessing growth in the broth tubes, what does a recording of +++ indicate?
- a) Maximal growth, seen as maximum turbidity
- b) Moderate growth, seen as moderate turbidity
- c) Minimal growth, seen as minimal turbidity
- d) No growth
On the visual analysis scale, - is no growth, + is minimal growth (minimal turbidity), ++ is moderate growth (moderate turbidity), and +++ is maximal growth (maximum turbidity).
25. Each temperature classification group demonstrates a range. Which temperatures define that range? Select all that apply.
- a) A minimum temperature
- b) A maximum temperature
- c) An optimal temperature
- d) A sterilizing temperature
Each of these groups demonstrates a range that has a minimum temperature, a maximum temperature, and an optimal temperature for the organism's growth.
26. Serratia marcescens is inoculated onto two nutrient agar slants, one incubated at 25°C and one at 37°C. What does this activity demonstrate?
- a) That temperature can affect the phenotypic expression of an organism
- b) That Serratia marcescens is a thermophile
- c) That nutrient agar slants are selective media
- d) That temperature has no effect on observable characteristics
This activity assesses the effect of temperature on phenotypic expression. The same organism is grown at two temperatures and the growth on the surface of each slant is compared.
27. A neutrophile is inoculated into a broth adjusted to pH 5.5 and incubated. What is the most likely outcome?
- a) Growth is poor or absent, because the pH is outside the range the organism prefers
- b) Growth is maximal, because acidic conditions favor all bacteria
- c) The organism is converted into an acidophile
- d) Growth is unaffected, because pH does not influence bacterial growth
Neutrophiles prefer a pH range near pH 7. Changes in pH can detrimentally affect cellular function, disrupting hydrogen bonding in DNA and proteins and halting metabolism, so a neutrophile would grow poorly at pH 5.5.
28. A proton-pump inhibiting drug reduces the acid produced in the stomach, raising the pH of the stomach contents. Would a patient taking such a drug be more or less likely to acquire an infection with Vibrio cholerae?
- a) More likely, because Vibrio cholerae grows best at a pH slightly above 8
- b) Less likely, because Vibrio cholerae requires a pH below 5.5
- c) More likely, because Vibrio cholerae is a thermophile
- d) Less likely, because raising the pH destroys all bacteria
Vibrio cholerae is an alkaliphile that grows best at a pH slightly above pH 8. Raising the pH of the stomach contents creates conditions closer to what the organism prefers.
29. When bacteria in a culture medium carry out fermentation, they produce organic acids. How could this affect further growth in that medium?
- a) The pH of the medium falls, which may inhibit or kill organisms that require a higher pH
- b) The pH of the medium rises, which favors acidophiles
- c) The pH of the medium remains unchanged, since acids are neutral
- d) The organic acids serve as a final electron acceptor, increasing growth
Organic acids increase the hydrogen ion concentration, moving the medium toward 0 on the pH scale. Because changes in pH can disrupt hydrogen bonding and denature proteins, organisms preferring a higher pH may be inhibited or killed.
30. Some bacteria produce urease, which converts urea to ammonium. How could this affect growth in the medium?
- a) The pH of the medium rises, which may inhibit organisms that require a lower pH
- b) The pH of the medium falls, which may inhibit organisms that require a higher pH
- c) The medium becomes anaerobic, favoring anaerobes
- d) The medium is sterilized, preventing all further growth
Ammonium raises the pH of the medium, making it more alkaline. Organisms whose optimal pH is lower may then be inhibited, because changes in pH can disrupt hydrogen bonding and denature proteins.
31. Which of the following are given as examples of antiseptics? Select all that apply.
- a) Isopropyl alcohol
- b) Iodine
- c) Hydrogen peroxide
- d) Lysol
Antiseptics include substances such as isopropyl alcohol, iodine, and hydrogen peroxide. Bleach and Lysol are given as examples of disinfectants.
32. You are working in a clinical environment, helping to clean a room that had been occupied by a Mycobacterium tuberculosis patient. Can any cleaner available in the cleaning closet be used?
- a) No, because chemical agents do not work equally against all microorganisms; an agent effective against this organism must be selected
- b) Yes, because all disinfectants destroy all forms of microorganisms
- c) Yes, provided the cleaner is also approved as an antiseptic
- d) No, because only antiseptics may be used on inanimate surfaces
Chemical agents do not work equally against all forms of microorganisms, because some organisms are more resistant due to their structural make up. An agent known to be effective against the organism in question must be chosen.
33. Following incubation you notice a very large zone of inhibition around one of the antimicrobial disks. Can you assume the drug is effectively inhibiting the bacteria?
- a) No, because effectiveness cannot be determined visually; the measurement must be compared to a set of standardized values
- b) Yes, because the largest zone always indicates the most effective drug
- c) Yes, provided the zone is larger than the disk itself
- d) No, because zones of inhibition indicate resistance rather than susceptibility
It is important to note that effectiveness cannot be determined visually. The measurements must be compared to a set of standardized values, which indicate whether the organism is susceptible, intermediate, or resistant.
34. A Kirby-Bauer result of resistant (R) indicates that the bacteria are resistant:
- a) In vitro in the lab, and there is a likelihood the drug would be ineffective in treating the infection
- b) Only at higher than normal drug concentrations
- c) In the patient, but not under laboratory conditions
- d) To all drugs in the same class
A resistant result indicates that the bacteria are resistant in vitro in the lab, and if it is resistant in the lab there is a likelihood that it would be ineffective in treating the infection in the patient.
This page titled 5.5.4: Multiple Choice / Multiple Select was last modified on Sat, 25 Jul 2026 03:43:14 GMT and is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sherry Smith.
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