Control of Microbial Growth - Antimicrobic Sensitivity Testing (Kirby-Bauer Method) 1 PDF

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CONTROL OF MICROBIAL GROWTH • ANTIMICROBIC SENSITIVITY TESTING (KIRBY-BAUER METHOD) INTRODUCTIO N

L ABO RATORY S IMUL ATIO N

Key Concepts

Overview

Before you begin

Several different approaches can be used to remove, destroy, or inhibit the growth of microbes. The choice of approach usually depends on whether body tissues or inanimate objects are involved and the extent to which control is

There are many reasons to control the growth of microbes both in the lab and in real life. Without adequate control methods, lab results may show contamination, food would spoil more quickly, water and beverages would be

After the Kirby-Bauer test is complete and the zones of inhibition are measured, an assessment of all the drugs tested is made.

necessary.

undrinkable and we would have less ability to prevent and treat infectious disease.

The largest zone of inhibition is not automatically assumed to be the best. This

Physical control methods include the use of heat, cold, irradiation, and mechanical removal. An example is the use of heat to sterilize an inoculating loop.

The discovery of antimicrobial drugs has been one of the most significant developments in the history of microbiology. These drugs have had an incredible impact on public health over the past half century.

Chemical control methods involve the use of antimicrobial chemicals like alcohol or bleach. Some are appropriate for skin, like alcohol, while bleach would not be safe on human skin but could be used on inanimate objects. All control methods target a specific part of a microbe. Targets include the cell wall, cell membrane, nucleic acid and protein synthesis, proteins, and specific metabolic pathways. Antibiotics are natural compounds produced by microorganisms that can be used to inhibit the growth of other microorganisms. Some antibiotics are synthetic or semi-synthetic, meaning they are completely or partially synthesized to have similar effects on microbial growth. Antibiotics can have multiple effects on the growth of microorganisms: Sensitive: Cells are affected by the drug and growth is inhibited Intermediate: Drug has a moderate effect on the cells. Resistant: Cells can tolerate the drug and grow in its presence. Antibiotics vary across the types of microbes they inhibit or kill. Some work only against a small range of microorganisms and are called narrow-spectrum. An

Recently, resistance to antimicrobial drugs has become a major concern. Because of this drug resistance issue, it is very important to choose antimicrobial treatments based on the effectiveness of these agents against specific pathogens. The Kirby-Bauer test is a commonly used method to determine the effectiveness of several different antibiotics against a particular bacterial agent. In this lab simulation, you will learn about antimicrobial drugs and the common laboratory test that is used to determine their effectiveness. An organism suspected to be the cause of an infection is placed onto a plate of Mueller-Hinton agar. This particular type of culture media allows for the growth of any microorganism, it does not promote or inhibit the growth of any specific organism. It also has an agar concentration that promotes the diffusion of drugs through the medium leading to a larger zone of inhibition. Once a lawn of bacteria is created, a dispenser places antimicrobial discs containing different concentrations of specific antibiotics onto the surface of the plate.

is a standardized test in that many factors are taken into account to determine which drugs are not only effective but most effective. For example, diffusion of the drugs from each disc will vary which would impact the results. Charts are available to compare the measured zone of inhibition with its effectiveness. The tested organism on the plate can be susceptible or resistant to a given drug. Sometimes the effect falls in between these two and is described as having an intermediate effect. Evaluating the Results of a Kirby-Bauer Antimicrobial Test Drug

Zone of Inhibition Resistant

Zone of Inhibition Susceptible

Bacitracin

> 13

18

< 12

Erythromycin

> 18

< 13

Gentamicin

> 13

< 12

Staphylococcus aureus Gentamicin Zone of Inhibition: 21 millimeters Susceptible

In this example, gentamicin measures a 21 mm zone of inhibition. For gentamicin, anything greater than 13 mm is considered a susceptible result. CONT IN UE TO:

Laboratory Simulation...