Understanding Antimicrobial Testing: A Guide to MIC, MBC, and More
Antimicrobial resistance is a growing concern in healthcare and pharmaceutical research, making robust antimicrobial testing crucial for developing effective therapies. At Emery Pharma, we specialize in antimicrobial testing, including Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC), and Minimum Fungicidal Concentration (MFC) assays. This blog answers common questions about these assays and provides insight into how they contribute to drug discovery and development.
What is a Minimum Inhibitory Concentration (MIC) Assay?
A Minimum Inhibitory Concentration (MIC) assay determines the lowest concentration of an antimicrobial agent needed to prevent visible microbial growth. This assay is widely used to evaluate the effectiveness of antibiotics, antifungals, and other antimicrobial compounds.
What Are Minimum Bactericidal Concentration (MBC) and Minimum Fungicidal Concentration (MFC) Assays?
The Minimum Bactericidal Concentration (MBC) assay identifies the lowest concentration of an antimicrobial agent required to kill 99.9% of bacterial cells. Similarly, the Minimum Fungicidal Concentration (MFC) assay determines the lowest concentration needed to kill fungal cells. These assays provide crucial data for understanding an antimicrobial’s potency beyond growth inhibition.
Antibiotic Susceptibility Testing at Emery Pharma
Emery Pharma follows Clinical and Laboratory Standards Institute (CLSI) guidelines and offers multiple antibiotic susceptibility testing methods, including:
- Broth microdilution and macrodilution – Quantitative methods to determine MIC values in liquid media.
- Agar disk diffusion – A qualitative test measuring the zone of inhibition around an antimicrobial disc.
- Agar well diffusion – A qualitative test measuring the zone of inhibition around a well containing the antimicrobial material.
- Agar dilution – A quantitative method for testing bacterial resistance at different antibiotic concentrations in agar.
Which Susceptibility Testing Method is Right for My Compound?
Choosing the right method depends on the physical and chemical properties of the test material:
- Broth microdilution: Ideal for screening multiple compounds efficiently. Requires less test material than other methods.
- Macro-dilution: Suitable for viscous materials or compounds difficult to assess in microplate formats.
- Agar dilution: Preferred quantitative method for many anaerobic microorganisms and suitable for test materials that are strongly colored or form precipitate.
- Agar well diffusion: Great for quick, qualitative assessment of highly viscous material or compounds that are otherwise difficult to assess in liquid media.
- Agar disk diffusion: Provides a quick, qualitative assessment, best for water-soluble compounds.
What Types of Test Articles Can Be Used in MIC and MBC Assays?
At Emery Pharma, we can test a wide range of antimicrobial compounds, including:
- Natural or synthetic compounds
- Purified substances or complex mixtures
Which Microorganisms Can Emery Pharma Test Against?
We test against hundreds of bacterial and fungal strains, including multi-drug resistant (MDR) clinical isolates and ESKAPE pathogens.
What Are ESKAPE Pathogens and Why Are They Important?
ESKAPE pathogens are a group of antibiotic-resistant bacteria that cause serious hospital-acquired infections. They include:
- E. faecium
- S. aureus
- K. pneumoniae
- A. baumannii
- P. aeruginosa
- E. cloacae
These pathogens are particularly concerning due to their ability to “escape” the effects of antibiotics, making new antimicrobial development a critical priority.
How is an MIC/MBC Assay Performed at Emery Pharma?
The MIC/MBC assay process includes:
- Serial dilution of the test compound into various concentrations.
- Inoculation of microorganisms into the test system.
- Incubation to allow microbial growth or inhibition.
- MIC determination by identifying the lowest concentration that prevents visible growth.
- MBC determination by plating samples on agar and checking for bacterial survival following an MIC assay.
Schematic of a MIC/MBC assay. (1) first prepare 2-fold serial dilutions of the test compounds (up to 7) and one quality control (QC) antibiotic in a microdilution plate. (2) Create the inoculum by taking a few colonies from an agar plate with a sterile swab, preparing a McFarland standard, and diluting the McFarland standard into media. (3) Dispense the inoculum into the microdilution plate with the serial diluted test compounds and incubate the microdilution plate. (4) Read the microdilution plate to determine the MIC value. (5) Plate a portion of each well on an appropriate agar media, incubate the agar, and check for colonies to determine the MBC.
What is a Time-Kill Assay?
A time-kill assay examines how quickly an antimicrobial agent reduces live bacteria or fungi over time. This data helps determine the speed of action of a drug, which is critical for therapeutic applications.
How Does Emery Pharma Conduct Time-Kill Assays?
- The test compound is prepared at a relevant concentration (e.g., 4x or 8x the MIC value).
- Microorganisms are added, and the mixture is incubated.
- Samples are taken at different time points to measure colony counts.
- Fast-acting agents like antiseptics show results within minutes, while antibiotics targeting cellular functions may take hours.
A sample time-kill kinetics assay is depicted. Three different test compounds, Compound 1, Compound 2, and Compound 3 are added to media containing a starting culture of bacteria. A vehicle only control and a growth control are included as negative controls. The log CFU/mL for all groups is determined at time 0 and at subsequent time points up to 24 hours, depending on the bacteria strain and the media used. In this example, Compound 1 exhibited bactericidal effect, reducing the starting log CFU/mL by greater than 3 logs. Compound 2 exhibited bacteriostatic effect, as the log CFU/mL over time remained roughly the same as the starting log CFU/mL concentration. Compound 3 exhibited little antimicrobial effect, as the bacteria in the presence of this compound grew over time to a level similar to the vehicle only control.
Why Choose Emery Pharma for Antimicrobial Testing?
Emery Pharma offers cutting-edge microbiology and analytical services to support drug development and ensure compliance with regulatory standards. We provide customized solutions for antimicrobial testing, ensuring high-quality, reproducible results for your research needs.
Contact Us
To learn more about our antimicrobial testing services or to discuss your project, please contact us:
Email: info@emerypharma.com
Visit our website and click on the Contact Us page: emerypharma.com
Please see our other videos here or contact us today to see how we can support your antimicrobial research!
