week 11 quiz Flashcards

Why is antimicrobic sensitivity testing done?

• To identify the bacteria causing an infection
• To obtain all of this information
• To determine what antimicrobials the patient is susceptible to
• To determine what antimicrobials would be most effective for treating the infection

Which choice below briefly describes the Kirby-Bauer method of antimicrobic sensitivity testing?

• broth disk elution
• disk diffusion
• minimal inhibitory concentration testing
• broth dilution

Resistant and nonresistant E. coli bacteria samples were spread on the agar plates below and incubated for 24 hours. Select the true statements from the list below.

• The resistant E. coli strain grows on streptomycin negative plates.The resistant E. coli strain grows on streptomycin negative plates.
• The nonresistant E. coli strain grows on streptomycin negative plates.The nonresistant E. coli strain grows on streptomycin negative plates.
• The nonresistant E. coli strain grows the same on streptomycin positive or negative plates.The nonresistant E. coli strain grows the same on streptomycin positive or negative plates.
• The resistant E. coli strain grows on streptomycin positive plates.The resistant E. coli strain grows on streptomycin positive plates.
• Both resistant and nonresistant E. coli strain show no growth on streptomycin positive plates.Both resistant and nonresistant E. coli strain show no growth on streptomycin positive plates.
• The nonresistant E. coli strain shows more growth on the streptomycin positive than the resistant E. coli strain.

How will you select and grow a resistant strain of E. coli in this experiment?

• Samples of bacteria are taken from the culture and observed under a microscope for signs of susceptibility or resistance. Those that are resistant are separated and plated.
• A culture of E. coli will be mixed with streptomycin so that the antibiotic can alter the genetic composition of the bacteria
• Expose a sample of E. coli to streptomycin by inoculating it onto a streptomycin negative plate. Any colonies that grow will carry a mutation for resistance.
• Expose a sample of E. coli to streptomycin by inoculating it onto a streptomycin positive plate. Any colonies that grow will carry a mutation for resistance.

Bacteria from three different cultures were plated on agar plates containing ampicillin, an antibiotic. The number of colonies that grew on each plate can be seen in the graph below.

If you assume that 100 bacteria were plated on each plate, what percentage of the bacteria on plate B were resistant to ampicillin?

• 10%
• 3%
• 0%
• 100%

Mueller-Hinton agar plates are poured to a uniform thickness of 4mm. Zone size interpretive criteria are based on Mueller-Hinton plates of this thickness. If a plate had been poured that was 7 mm thick, how would this affect the size of the zones of inhibition?

• The zone diameters would be unaffected.
• The zones would be smaller.Correct
• The zones would be larger.

A chemical compound, regardless of its source (either natural or synthetic), that is capable of killing or inhibiting microorganisms is referred to as

• semisynthetic.
• a chemosynthetic agent.
• an antibiotic.
• an antimicrobial.

Natural selection occurs when

• only when nonrandom mating occurs.
• when a small number of individuals of a population break away and form a new population.
• a new genetic variation occurs that allows individuals to have a reproductive and survival advantage.
• a factor in the environment alters the genetic information in DNA of all members of a species.

How does an antibiotic act as a selective agent in promoting bacterial resistance?

• Bacteria with a random mutation that allows resistance to that antibiotic are able to survive exposure and reproduce, passing the resistance mutation on to subsequent generations of bacteria.
• Exposure to a specific antibiotic causes a mutation that allows the affected bacteria to survive and reproduce, passing the mutation to future generations.
• Antibiotics are designed to selectively kill some bacteria while promoting the survival of all others.
• A specific antibiotic selects favorable mutations that promote susceptibility to that antibiotic.

Two agar plates, one containing the antibiotic streptomycin and one without antibiotics, are inoculated with E. coli. After incubation, the streptomycin negative plate has many bacterial colonies, while the streptomycin positive plate has only a few colonies. Samples of the colonies from the positive plate are placed on another positive plate, and after incubation many colonies grow. In this experiment, what is the selective agent?

• the agar medium
• the antibiotic streptomycin
• the period of incubation
• the E. coli

Let's say you inoculate four agar plates, each containing a different antibiotic (A, B, C, and D), and you add a swab from a lesion taken from a patient. After incubating the plates, you observe that plate A has only a few colonies, while the others all have many colonies. How do you interpret the results?

• Bacteria in the patient's sample carry a mutation that makes them resistant to antibiotic A
• Bacteria in the patient's sample are resistant to antibiotic A, but not B, C, and D
• None of these antibiotics are an acceptable choice to use to treat the patient for this bacterial infection
• Bacteria in the patient's sample are susceptible to antibiotic A but not B, C, and D

Which of the following apply to the development of streptomycin resistance in E. coli?

• The E. coli population has genetic variation.
• The selective agent is streptomycin.
• The mutation resulting in streptomycin resistance is random.
• E. coli changes its gene structure to become antobiotic resistant in the presence of streptomycin.

Staphylococcus aureus with 11mm

Streptococcus pyogenes with 22mm

Staphylococcus aureus with 13mm

Pseudomonas aeruginosa with 17mm

Staphylococcus aureus with 16mm

Acinetobacter with 11mm

Place the steps of the Kirby Bauer method of antimicrobic sensitivity testing in the correct order.

• Tap down disks with sterile forceps.
• Dispense antibiotic disks onto the inoculated surface using sterile forceps or an automatic dispenser.
• Label a sterile Mueller-Hinton agar plate with organisms name and your name.
• Measure diameters of zones of inhibition and compare zone diameters to interpretive charts and determine if S, I, or R.
• Incubate for 18-24 hours at 35^oC.
• Obtain a dilute suspension of the organism in nutrient broth to use as the inoculum.
• Using a swab, inoculate the agar plate uniformly by swabbing in three directions.