BACTE - MT111 - Lecture 5 - Antimicrobial Susceptibility Testing

Antimicrobial Susceptibility Testing

Procedures used to produce antimicrobial susceptibility profiles and detect resistance to therapeutic agents.

Resistance

The ability of bacteria to acquire and express resistance to antimicrobial agents commonly used to treat infections.

Therapeutic agents

Antimicrobial agents selected for treatment of bacterial infections.

Bacterial isolate

An organism that has been isolated in the laboratory for characterization and testing.

Antimicrobial resistance

The ability of bacteria to resist the effects of antimicrobial agents.

Clinical specimens

Samples taken from patients for laboratory testing.

Infections

Diseases caused by bacteria.

Microbial flora

The bacteria that naturally reside in a specific area of the body.

Susceptibility testing

Testing performed on bacteria isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections caused by the bacteria.

Disk diffusion

A method of susceptibility testing where paper disks containing different antimicrobial agents are placed on a culture plate to determine the bacteria's susceptibility.

Minimal inhibitory concentration (MIC)

A method of susceptibility testing that determines the lowest concentration of an antimicrobial agent that inhibits the growth of bacteria.

Clinical and Laboratory Standards Institute (CLSI)

An organization that publishes standards for antimicrobial susceptibility testing methods.

Commercial manual or automated antimicrobial susceptibility test systems

Testing systems available for clinical laboratories to perform antimicrobial susceptibility testing.

Laboratory reports

Documents that communicate the results of laboratory tests, including antimicrobial susceptibility testing.

Probable cause

The likelihood that a bacterial isolate is responsible for a patient's infection.

Predictably susceptible

Bacteria that are known to be susceptible to commonly used antimicrobial agents.

Group A β-hemolytic Streptococcus

A type of bacteria that is universally susceptible to penicillin.

Staphylococcus aureus

A type of bacteria that may or may not be susceptible to oxacillin, the recommended agent for treating infections caused by this bacterium.

Body site

The location in the body from which the bacteria was isolated.

Normal inhabitants

Bacteria that are normally present in a specific body site.

Quality of the specimen

The condition and suitability of the sample taken for testing.

Host's status

The overall health and immune status of the patient.

Traditional Antimicrobial Susceptibility Test Methods

Methods used for susceptibility testing, such as disk diffusion and dilution methods.

Dilution Susceptibility Testing Methods

Methods that determine the minimal inhibitory concentration (MIC) of an antimicrobial agent.

Broth Dilution

A dilution method where the antimicrobial agent is added to liquid broth containing the bacteria to determine the MIC.

Commercial Susceptibility Testing

Commercially available systems for antimicrobial susceptibility testing.

Phenotypic Methods

Methods that detect antimicrobial resistance based on observable characteristics of the bacteria.

Genotypic Methods

Methods that detect antimicrobial resistance based on the genetic makeup of the bacteria.

Bactericidal Tests

Tests that determine the ability of an antimicrobial agent to kill bacteria.

Tests for Activity of Antimicrobial Combinations

Tests that determine the effectiveness of combinations of antimicrobial agents.

Antimicrobial Susceptibility Testing

A laboratory test used to determine the effectiveness of antimicrobial drugs against specific microorganisms.

Contamination

The presence of unwanted microorganisms in a sample, which can affect the accuracy of antimicrobial susceptibility testing.

Pure culture

A culture containing only a single species of microorganism, which is less likely to be contaminated.

Colony-forming units (CFU)

A measure of viable microorganisms in a sample.

True infection

An actual infection caused by a specific microorganism.

Normal biota

Microorganisms that are normally present in or on the human body without causing harm.

Gram stain

A staining technique used to classify bacteria into Gram-positive or Gram-negative based on their cell wall structure.

Host status

The condition or characteristics of the patient that can influence susceptibility testing decisions.

Immunocompromised

Having a weakened immune system, making a person more susceptible to infections.

Penicillin-allergic

Having an allergic reaction to penicillin, a commonly used antibiotic.

Erythromycin

An antibiotic used to treat various bacterial infections.

Standardization

The process of ensuring consistency and reliability in antimicrobial susceptibility testing.

Bacterial inoculum size

The amount of bacteria used in the testing, which needs to be standardized for accurate results.

Growth medium

The substance or environment in which bacteria are grown in the laboratory.

pH

The measure of acidity or alkalinity of a solution.

Cation concentration

The concentration of positively charged ions in a solution.

Blood and serum supplements

Additional components added to the growth medium to support the growth of certain bacteria.

Thymidine content

The presence of thymidine, a nucleotide, in the growth medium.

Incubation atmosphere

The specific conditions (such as oxygen levels) in which bacteria are incubated for growth.

Incubation temperature

The temperature at which bacteria are incubated for growth.

Incubation duration

The length of time bacteria are incubated for growth.

Traditional Antimicrobial Susceptibility Test Methods

Conventional methods used for antimicrobial susceptibility testing.

Inoculum Preparation

The process of preparing a sample containing a specific number of bacteria for testing.

McFarland Turbidity Standards

A standardized method for determining the turbidity (cloudiness) of a bacterial suspension, used for inoculum standardization.

McFarland 0.5 standard

A commonly used standard in antimicrobial susceptibility testing that provides turbidity comparable to a bacterial suspension containing approximately 1.5 x 108 CFU/mL.

Turbidity

The cloudiness or haziness of a liquid caused by the presence of suspended particles.

Inoculum standardization

The process of standardizing the inoculum (bacterial suspension) used in antimicrobial susceptibility testing to ensure consistency and accuracy of results.

Nephelometric or spectrophotometric device

Instruments used for objective standardization of bacterial inoculum in the clinical laboratory, providing a more precise alternative to visual adjustment.

Antimicrobial battery or panel

The selection of antimicrobial agents chosen for testing against a particular bacterial isolate in antimicrobial susceptibility testing.

CLSI

Clinical and Laboratory Standards Institute, an organization that publishes up-to-date tables listing potential antimicrobial agents recommended for inclusion in batteries for testing against specific organisms or organism groups.

Intrinsic resistance

Resistance to antimicrobial agents that is naturally present in an organism.

Acquired resistance patterns

Resistance to antimicrobial agents that is acquired by an organism over time.

Antimicrobial susceptibility testing method

The specific method used to determine the susceptibility of a bacterial isolate to antimicrobial agents.

MIC (Minimum Inhibitory Concentration)

The lowest concentration of an antimicrobial agent that inhibits the growth of a bacterial isolate.

Dilution susceptibility testing methods

Methods used to determine the MIC by testing varying concentrations of an antimicrobial agent added to broth or agar media.

Antimicrobial Susceptibility Testing

A test used to determine the effectiveness of antimicrobial agents against specific organisms.

Interpretive criteria

Guidelines used to interpret the results of antimicrobial susceptibility testing for different organisms or organism groups.

MIC breakpoint

The minimum inhibitory concentration (MIC) breakpoint is the concentration of an antimicrobial agent that separates susceptible from resistant results.

Susceptible

Organisms with MICs at or below the breakpoint are considered susceptible to the antimicrobial agent.

Resistant

Organisms with MICs above the breakpoint are considered resistant to the antimicrobial agent.

CLSI documents

Documents published by the Clinical and Laboratory Standards Institute that provide details on performing MIC tests using different methods.

Antimicrobial stock solutions

Solutions used in MIC tests that are prepared from reference standard antimicrobial powders, not from pharmaceutical preparations.

Broth macrodilution

A method of MIC testing that uses a liquid medium in large test tubes.

Broth microdilution

A method of MIC testing that uses a liquid medium in a single microtiter tray with small volumes.

Broth-macrodilution MIC tests

Broth dilution MIC tests performed in test tubes.

Broth-microdilution MIC tests

Broth dilution MIC tests performed in multiwall microdilution trays.

Mueller-Hinton broth

The recommended medium for broth dilution MIC tests of non-fastidious bacteria.

Growth-control tube

A tube used in broth dilution MIC tests that contains broth plus inoculum.

Uninoculated control tube

A tube used in broth dilution MIC tests that contains only broth.

Minimum inhibitory concentration (MIC)

The lowest concentration of an antimicrobial agent that inhibits the growth of an organism.

Broth macrodilution impractical

Broth macrodilution is not practical for routine testing of multiple antimicrobial agents or isolates.

Broth-microdilution tray

A tray containing multiple wells filled with small volumes of twofold dilution concentrations of antimicrobial agents in broth.

Inoculum suspension

A standardized suspension of test bacteria used in broth-microdilution MIC testing.

Tray-reading device

A device used to visually examine the wells of a broth-microdilution tray after incubation.

Growth-control well

A well in a broth-microdilution tray that serves as a positive control for growth.

Quality control organisms

Organisms used to ensure the accuracy and reliability of antimicrobial susceptibility testing.

Dispensing devices

Devices used to prepare broth microdilution panels.

Frozen panels

Commercially prepared panels stored at low temperatures until used.

Freeze-dried panels

Commercially prepared panels with dried or lyophilized drugs that are reconstituted before use.

Breakpoint (Cutoff) MIC panels

A variation of the standard broth microdilution MIC panel that tests only one or a few concentrations of each antimicrobial agent.

Breakpoint (Cutoff)

The concentration of an antimicrobial agent that coincides with a susceptible or intermediate MIC breakpoint for a particular drug.

Antimicrobial Susceptibility Testing

A method used to determine the effectiveness of antimicrobial agents against bacterial isolates.

Intermediate susceptibility

When there is growth in the low concentration but no growth in the high concentration, indicating that the isolate has intermediate susceptibility to the antimicrobial agent.

Resistant isolate

An isolate that grows in both low and high concentrations of the antimicrobial agent, indicating resistance to the agent.

Qualitative interpretation

The classification of isolates as non-susceptible, susceptible, intermediate, or resistant, rather than reporting the minimum inhibitory concentration (MIC).

Breakpoint panels

Panels used in antimicrobial susceptibility testing that allow testing of multiple drugs on a single panel.

Precise MIC

The exact minimum inhibitory concentration of an antimicrobial agent, which is not obtained with breakpoint panels.

Trailing growth

Heavy growth at lower concentrations followed by reduced growth in subsequent wells, commonly observed with certain antimicrobial agents.

Skipped wells

Growth at higher concentrations and no growth at one or more lower concentrations, which may occur due to various factors such as contamination or unusual resistance.

Penicillin susceptible

An isolate that shows a penicillin MIC of 0.03 µg/mL or less, indicating susceptibility to penicillin.