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Ch. 10 - Controlling Microbial Growth in the Body: Antimicrobial Drugs
Bauman - Microbiology with Diseases by Taxonomy 6th Edition
Bauman6th EditionMicrobiology with Diseases by TaxonomyISBN: 9780134832302Not the one you use?Change textbook
All textbooksBauman 6th EditionCh. 10 - Controlling Microbial Growth in the Body: Antimicrobial DrugsProblem 2
Chapter 10, Problem 2

In a Kirby-Bauer susceptibility test, the presence of a zone of inhibition around disks containing antimicrobial agents indicates __________ .
a. That the microbe does not grow in the presence of the agents
b. That the microbe grows well in the presence of the agents
c. The smallest amount of the agent that will inhibit the growth of the microbe
d. The minimum amount of an agent that kills the microbe in question

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1
Understand the purpose of the Kirby-Bauer susceptibility test: it is used to determine the sensitivity of bacteria to various antimicrobial agents by observing bacterial growth around antibiotic disks on an agar plate.
Recognize that the 'zone of inhibition' is the clear area surrounding an antimicrobial disk where bacterial growth has been prevented or reduced.
Interpret the presence of a zone of inhibition as an indication that the antimicrobial agent is effective at inhibiting the growth of the microbe in that area.
Differentiate between inhibition of growth and killing of bacteria: the Kirby-Bauer test shows inhibition (bacteriostatic effect), not necessarily killing (bactericidal effect).
Conclude that the zone of inhibition means the microbe does not grow in the presence of the antimicrobial agent, corresponding to option (a).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Kirby-Bauer Susceptibility Test

The Kirby-Bauer test is a standardized method used to evaluate the effectiveness of antibiotics against bacteria. It involves placing antibiotic-impregnated disks on an agar plate inoculated with the test microbe. After incubation, zones of inhibition around disks indicate areas where bacterial growth is prevented.
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Zone of Inhibition

A zone of inhibition is a clear area surrounding an antibiotic disk where bacterial growth has been halted. Its size reflects the susceptibility of the microbe to the antimicrobial agent; a larger zone generally means greater sensitivity, indicating the antibiotic effectively inhibits growth.
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Antimicrobial Susceptibility vs. Resistance

Susceptibility means the microbe's growth is inhibited by the antimicrobial agent, while resistance means the microbe can grow despite the agent's presence. The Kirby-Bauer test helps distinguish between these by showing whether zones of inhibition form around antibiotic disks.
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Related Practice
Textbook Question

What specific test for antimicrobial efficacy is shown? What does this test measure? Draw an oval to predict the size and shape of the zone of inhibition if the drug concentration on the strip were increased twofold.

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Textbook Question

Label each of the accompanying figures to indicate the class of drug that is stopping polypeptide translation.


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a. ________________ b. _________________

block initiation. change 30S subunit.



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c. ________________ d. _________________

block ribosome attachment inhibits peptide bonding


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e. ________________ g. _________________

f. ________________ h. _________________

block ribosome movement block tRNA docking

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Textbook Question

Diffusion and dilution tests that expose pathogens to antimicrobials are designed to determine __________ .

a. The spectrum of action of a drug

b. Which drug is most effective against a particular pathogen

c. The amount of a drug to use against a particular pathogen

d. Both b and c

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Textbook Question

Why is the fact that drug Z destroys the NAM portions of a cell’s wall structure an important factor in considering the drug for chemotherapy?

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Textbook Question

Contrast narrow-spectrum and broad-spectrum drugs. Which are more effective?

Textbook Question

What characteristics would an ideal chemotherapeutic agent have? Which drug has these qualities?