32 terms

Lab Hw:Chemical control of growth

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What is the difference between a disinfectant and an antiseptic?
Disinfectants are used on inanimate objects, antiseptics are used on living tissue.
Why must you consider the pH of the location you plan to treat with a disinfectant?
All available answers are correct.
You have a spill in the lab, contaminating the benchtop with an organism that is very naturally resistant to disinfectants. How might you increase the potency of the disinfectant you use?
Increase the concentration of the disinfectant
Which of the following is most resistant to disinfectants?
Bacterial endospores
Which of the following is NOT an environmental condition that needs to be considered when selecting a disinfectant?
Length of exposure
Why are the tests performed in the lab not always a good indicator of how effective a disinfectant will be in the real world?
The lab tests often involve lots of bacteria growing on nutrient agar; nutrient agar is rarely disinfected in reality.
Which chemical is best to use for disinfection regardless of the environmental or microbiological conditions?
None of the available answers are correct.
Since longer exposures to disinfectants result in better disinfection, why don't we just disinfect everything we need to for a minimum of 5 days?
All available answers are correct.
You and your lab partner decide to test the effectiveness of two particular phenolic compounds in killing bacteria. You each add 10 µl of one of the compounds to your test and incubate it. Because of a miscommunication, you incubate at 37°C and your partner incubates at 25°C. Based on the results, it appears that your compound killed many more bacteria than did your partner's. What could explain this data?
You have to rerun the experiment because the higher temperature may have made your compound more effective.
Which of the following is indicative of an effective disinfectant?
None of the above is correct.
Some household disinfectants claim that they kill 99.99% of germs. Do you think this is likely?
It is impossible to evaluate this claim without knowing how they did the experiment.
Think about those household cleaners that claim to kill 99.99% of germs. Which of the following experiments would best test this claim?
Inoculate a broth with bacteria swabbed off of a desk. Determine the number of bacteria in the tube. Subject the contents to the disinfectant. Determine the number of bacteria still in the tube.
After performing this lab, your lab partner decides that she will disinfect her forceps with bleach rather than with alcohol. Why is this a bad idea?
All available answer choices are true.
As a control for an experiment, you add a disk impregnated with a relatively low amount of alcohol to a plate on which you had streaked a large quantity of bacteria. After incubating this at 37°C for 2 days, you are surprised by the small zone of inhibition. Which of the following could explain these data?
The small amount of alcohol evaporated and, therefore, could not kill the bacteria.
Why is it critical that disks impregnated with disinfectants be placed as far from one another as possible on the agar plate?
All of the above are correct.
Why is lab disinfectant often used as a control when testing a new substance's ability to kill microorganisms in broth?
It ensures that your testing system is working since you know lab disinfectant will kill certain microorganisms.
What is an antibiotic?
A chemical that inhibits microbial growth
What does the Kirby-Bauer test assess?
The susceptibility of an organism to a set of antibiotics
Which of the following best describes what a single Kirby-Bauer plate is testing?
Is a particular bacterium susceptible to any of a variety of antibiotics?
What characteristic of the Kirby-Bauer test helps scientists to determine if a microorganism is susceptible to an antibiotic?
The diameter of the zone of inhibition surrounding the antibiotic disks
What is a zone of inhibition?
The area around the antibiotic disk in which bacteria cannot grow
Which of the following characteristics does NOT need to be standardized in a Kirby-Bauer assay?
The method used to develop a bacterial lawn on the agar plate
Why is it critical to use aseptic technique when transferring the antibiotic disks to the agar?
All available answers are correct.
What are the three possible outcomes for each antibiotic: microorganism pair in a Kirby-Bauer assay?
Susceptible, resistant, or intermediate
You perform a Kirby-Bauer assay with two antibiotics. Antibiotic X has a zone of inhibition of 9 mm. Antibiotic Y has a zone of inhibition of 11 mm. Which antibiotic is better at killing this particular microorganism?
It is impossible to tell from the information given.
You perform a Kirby-Bauer assay, but you neglect to use aseptic technique. Your plate becomes contaminated with a bright red organism. You can easily tell this organism apart from the one you intended to inoculate (which has grown into a nice lawn all over the plate). In order to conserve time, you measure the zone of inhibition for your intended organism, ignoring the contaminants. Are you assured to get the correct answer using this method?
No, the contaminant adds too many variables to make this assay usable.
You measure the zone of inhibition around an erythromycin disk as 12 mm. How would you report the susceptibility of this organism to erythromycin?
Resistant
You measure the zone of inhibition around a tetracycline disk as 19 mm. How would you report the susceptibility of this organism to tetracycline?
Susceptible
You measure the zone of inhibition around a rifampin disk as 17 mm. How would you report the susceptibility of this organism to rifampin?
Intermediate
You perform a Kirby-Bauer test for a patient with a bladder infection caused by a Gram-positive organism. Considering the following data, which antibiotic would you prescribe?

Chloramphenicol: 12 mm zone of inhibition
Cefoxitin: 18 mm zone of inhibition
Streptomycin: 14 mm zone of inhibition
Ampicillin: 26 mm zone of inhibition
Cefoxitin
Compare the zone of inhibition chart for ampicillin (when testing Gram-positive organisms) and chloramphenicol. What might you predict based on these data?

Chloramphenicol: 12 mm zone of inhibition
Ampicillin: 26 mm zone of inhibition
Ampicillin diffuses much faster/farther into the agar than does chloramphenicol.
You perform a Kirby-Bauer test with a ciprofloxacin-resistant microorganism. You place two ciprofloxacin-containing disks on the agar: one with 5 µg and one with 10 µg. What would you expect the zones of inhibition to be for these two disks?
5 µg = 12 mm, 10 µg = impossible to predict