From MIC to Mutant Selection Window

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-27

Quick Answer

• Measure mutant prevention concentration (MPC) by plating a high-density bacterial inoculum onto agar containing serial dilutions of the antimicrobial and identifying the lowest concentration that prevents all colony growth after extended incubation.
• Use an inoculum of approximately 10^10 CFU/mL to ensure that resistant subpopulations present at low frequency are captured in the assay.
• MPC interpretation requires careful distinction from minimum inhibitory concentration (MIC) results, and the mutant selection window is the concentration range between the MIC and the MPC where resistant mutants are selectively enriched.

At a Glance

Understanding the Mutant Selection Window

The mutant selection window is a conceptual range of antimicrobial concentrations that inhibit susceptible cells but allow growth of resistant mutants. The lower boundary is the MIC, the concentration that prevents visible growth of the standard inoculum. The upper boundary is the MPC, the concentration that prevents growth of the most resistant subpopulation present in a large bacterial population. When drug concentrations fall inside this window, susceptible cells are suppressed while resistant mutants can replicate and become enriched. Concentrations above the MPC suppress both susceptible and resistant cells, reducing the probability that resistance emerges during therapy.

The clinical relevance of the mutant selection window lies in its application to dosing strategies. If antimicrobial concentrations at the infection site remain above the MPC for the entire dosing interval, the selective advantage for resistant mutants is minimized. If concentrations fall into the window for prolonged periods, resistant subpopulations have an opportunity to amplify. This framework is particularly relevant for antimicrobials with concentration-dependent killing and for infections with high bacterial burdens, such as respiratory infections, where the total number of organisms increases the probability that a resistant mutant exists before therapy begins.

Core Principles of MPC Determination

The Biological Basis for MPC

The MPC concept rests on the observation that bacterial populations are not genetically uniform. Within a large population, spontaneous mutations conferring reduced susceptibility to an antimicrobial arise at frequencies that depend on the organism, the antimicrobial, and the genetic target. For most antimicrobials, the frequency of resistance mutations is approximately 10^-7 to 10^-9 per cell division. A standard MIC inoculum of 10^5 CFU/mL may not contain any resistant mutants. A larger inoculum of 10^10 CFU/mL is likely to contain at least one cell with a resistance mutation, allowing the assay to detect the concentration that suppresses the most resistant subpopulation present.

The MPC is therefore a population-level measurement. It reflects the antimicrobial concentration that prevents the survival of the least susceptible cells in a large population. The difference between the MIC and the MPC defines the mutant selection window. When the window is wide, there is a greater concentration range where resistant mutants can be selected. When the window is narrow or absent, the antimicrobial suppresses both susceptible and resistant cells at concentrations close to the MIC.

Relationship Between MIC and MPC

The MIC and MPC are related but distinct measurements. The MIC is determined with a standard inoculum of approximately 10^5 CFU/mL and is used to classify organisms as susceptible, intermediate, or resistant according to established breakpoints. The MPC is determined with a much larger inoculum and does not have standardized breakpoints. The MPC is always equal to or greater than the MIC. The ratio of MPC to MIC provides a measure of the width of the mutant selection window. A ratio close to 1 indicates that the antimicrobial suppresses resistant mutants at concentrations near the MIC. A ratio of 8 or higher indicates a wide window where resistant mutants can be selected at concentrations that are achievable in the patient.

The MPC is not a substitute for the MIC in clinical decision making. The MIC remains the standard measurement for susceptibility testing and is used to guide therapy selection. The MPC provides additional information about the potential for resistance emergence during therapy. Researchers use the MPC to evaluate whether a dosing regimen can maintain concentrations above the mutant selection window throughout the treatment period.

Laboratory Protocol for MPC Determination

Preparation of the Bacterial Inoculum

The MPC assay requires a high-density inoculum of approximately 10^10 CFU/mL. This inoculum size ensures that resistant subpopulations present at frequencies of 1e-7 to 1e-9 are represented in the culture. To prepare the inoculum, grow the bacterial strain in an appropriate broth medium to the logarithmic phase of growth. Concentrate the culture by centrifugation and resuspend the pellet in a small volume of fresh broth to achieve the target cell density. Confirm the cell density by plating serial dilutions onto agar and counting colonies after incubation.

The choice of bacterial strain is important for reproducibility. Use authenticated reference strains from a culture collection such as the American Type Culture Collection (ATCC) when available. Reference strains provide a consistent genetic background and reduce variability between experiments. Clinical isolates can be used for research purposes, but the results may be less reproducible due to genetic heterogeneity. Document the strain identifier, the source, and the passage history in the laboratory records.

Preparation of Agar Plates

The MPC is measured on agar plates containing serial dilutions of the antimicrobial. Prepare Mueller-Hinton agar according to the manufacturer instructions and autoclave to sterilize. Cool the agar to approximately 50 degrees Celsius before adding the antimicrobial. Prepare a stock solution of the antimicrobial at a concentration that allows the addition of a small volume to the molten agar. Mix the antimicrobial thoroughly into the agar to ensure a uniform distribution. Pour the plates and allow the agar to solidify.

The concentration range of the antimicrobial should span the expected MPC. Start with a concentration that is several dilutions above the MIC and extend to several dilutions below the MIC. A typical range is from 0.25 times the MIC to 32 times the MIC. Use at least 10 to 12 concentrations to define the MPC accurately. Include a growth control plate without antimicrobial to confirm that the inoculum is viable and that the plating technique is correct.

Inoculation and Incubation

Apply a fixed volume of the high-density inoculum to each agar plate. A common approach is to spot 100 microliters of the 10^10 CFU/mL suspension onto the agar surface and spread it evenly. The final cell density on the agar should be approximately 10^9 CFU per plate. Allow the inoculum to absorb into the agar before incubating the plates.

Incubate the plates at the appropriate temperature for the bacterial species. The incubation time for MPC determination is longer than for MIC determination. Standard MIC testing uses 24 hours of incubation. The MPC requires 48 to 72 hours of incubation to allow slow-growing resistant mutants to form visible colonies. Examine the plates daily and record the presence or absence of colony growth at each antimicrobial concentration.

Determination of the MPC Endpoint

The MPC is the lowest concentration of the antimicrobial that prevents the growth of any colony on the agar plate. After the incubation period, examine each plate for the presence of colonies. The MPC is the lowest concentration at which no colonies are visible. If colonies appear at a concentration but not at the next higher concentration, the MPC is the higher concentration. Record the result as the MPC value for the bacterial strain and antimicrobial combination.

The endpoint can be difficult to interpret when colonies appear at high concentrations. These colonies may represent resistant mutants that have emerged during the incubation period. Confirm that the colonies are the test organism and not contaminants. Subculture the colonies onto fresh agar and confirm the identification. The presence of colonies at concentrations above the MPC may indicate that the inoculum was too large or that the antimicrobial concentration was not uniform in the agar.

Data Analysis and Interpretation

Constructing the MPC Curve

The MPC curve is a plot of the number of colonies recovered at each antimicrobial concentration against the concentration. The curve typically shows a sharp decline in colony count as the concentration approaches the MPC. The MPC is the concentration at which the colony count reaches zero. The curve can be used to visualize the mutant selection window by comparing the MIC and the MPC on the same plot.

The shape of the MPC curve provides information about the resistance profile of the bacterial population. A steep curve indicates that the population is relatively homogeneous in its susceptibility. A shallow curve indicates that the population contains a range of susceptibility levels and that resistant subpopulations are present at higher frequencies. The curve can also reveal the presence of a resistant subpopulation that grows at concentrations above the MIC but below the MPC.

Calculating the Mutant Selection Window

The mutant selection window is the concentration range between the MIC and the MPC. The width of the window is calculated as the ratio of the MPC to the MIC. A ratio of 1 indicates that the window is absent and that the antimicrobial suppresses resistant mutants at concentrations close to the MIC. A ratio of 4 or higher indicates a wide window and a greater potential for resistant selection during therapy.

The clinical relevance of the window depends on the achievable antimicrobial concentrations at the infection site. If the maximum concentration at the infection site exceeds the MPC, the window is closed and resistant selection is minimized. If the maximum concentration falls within the window, resistant mutants can be selected during the dosing interval. The window concept is used to design dosing regimens that maintain concentrations above the MPC for the entire treatment period.

Limitations of the MPC Assay

The MPC assay has several limitations that affect its interpretation. The assay requires a high-density inoculum that is not representative of all clinical infections. The inoculum size can affect the MPC result, and the assay is not standardized across laboratories. The MPC is not a clinical breakpoint and cannot be used to classify an organism as susceptible or resistant. The MPC is a research tool that provides information about the potential for resistance selection.

The MPC assay is also limited by the choice of the antimicrobial concentration range and the incubation period. The concentration range must be wide enough to capture the MPC, and the incubation period must be long enough to allow slow-growing colonies to appear. The assay is time-consuming and labor-intensive, which limits its use in routine clinical laboratories. The MPC is best used in research settings where the goal is to evaluate the resistance potential of an antimicrobial.

Quality Controls and Reproducibility

Reference Strains and Quality Control Organisms

The use of reference strains is essential for the reproducibility of the MPC assay. Reference strains with known MIC values should be included in each experiment to verify the antimicrobial concentrations and the assay conditions. The American Type Culture Collection provides authenticated reference strains that are suitable for susceptibility testing ATCC. The reference strain should be tested alongside the clinical isolates to confirm that the antimicrobial concentrations are accurate and that the assay is performing correctly.

Quality control organisms should be tested with each batch of MPC plates. The MIC of the quality control organism should fall within the expected range for the antimicrobial. If the MIC is outside the expected range, the antimicrobial stock solution or the agar preparation may be incorrect. The quality control results should be recorded in the laboratory records and used to monitor the performance of the assay over time.

Reproducibility and Variability

The MPC assay is subject to variability from several sources. The inoculum size is a major source of variability, and the cell density must be confirmed by viable plate counts. The antimicrobial concentration in the agar can vary if the stock solution is not prepared accurately or if the antimicrobial is not mixed thoroughly into the agar. The incubation conditions, including temperature and duration, can also affect the result.

To reduce variability, use the same batch of agar and the same antimicrobial stock solution for all plates in the experiment. Prepare the inoculum fresh for each experiment and confirm the cell density by viable plate counting. Incubate all plates under the same conditions and examine the plates at the same time points. Repeat the MPC determination on separate days to confirm the result and to assess the reproducibility of the assay.

Recording and Reporting Results

The MPC results should be recorded in a standardized format that includes the bacterial strain, the antimicrobial, the inoculum size, the concentration range, and the incubation period. The results should be reported as the MPC concentration in micrograms per milliliter. The MIC should be reported alongside the MPC to allow the calculation of the mutant selection window.

The laboratory records should include the quality control results and any deviations from the standard protocol. The records should be reviewed by a supervisor or a qualified laboratory professional before the results are used for research or publication. The records should be stored in a secure location and retained for the period required by the laboratory quality management system.

Common Failure Patterns and Troubleshooting

No Growth on the Control Plate

If the control plate without antimicrobial shows no growth, the inoculum may not be viable or the agar may be defective. Check the viability of the inoculum by plating a dilution onto a fresh agar plate and incubating overnight. Confirm that the agar was prepared correctly and that the autoclave cycle was effective. The inoculum may have been exposed to a temperature or a chemical that reduced viability.

Growth at All Concentrations

If colonies appear at all concentrations of the antimicrobial, the inoculum may be too large or the antimicrobial concentration may be too low. Confirm the inoculum density by viable plate counting and reduce the inoculum if necessary. Check the antimicrobial stock solution and the agar preparation to confirm that the concentrations are correct. The antimicrobial may have degraded during the preparation or the storage.

No Growth at Any Concentration

If no growth appears at any concentration, including the control, the inoculum may be too dilute or the incubation conditions may be incorrect. Confirm the inoculum density and the viability of the culture. Check the incubation temperature and the duration of the incubation. The agar may be too dry or the plates may have been incubated for too long.

Colonies at Concentrations Above the MPC

If colonies appear at concentrations above the MPC, the resistant subpopulation may be present at a higher frequency than expected. The colonies should be subcultured and confirmed as the test organism. The presence of colonies at high concentrations may indicate that the antimicrobial concentration is not uniform in the agar or that the inoculum was too large. Repeat the experiment with a fresh inoculum and a wider concentration range.

Biosafety and Laboratory Practice

Risk Assessment and Containment

The MPC assay involves the manipulation of large volumes of bacterial cultures and the use of antimicrobial agents. The risk assessment should be based on the biosafety level of the organism and the procedures used in the assay. The CDC and NIH provide guidance on biosafety in microbiological and biomedical laboratories CDC BMBL. The risk assessment should consider the pathogenicity of the organism, the route of exposure, and the concentration of the inoculum.

The MPC assay should be performed in a biosafety cabinet when the organism is a potential respiratory pathogen or when the inoculum is aerosolized. The cabinet should be certified and maintained according to the laboratory safety program. The laboratory should have a written biosafety manual that describes the procedures for handling the cultures and the waste.

Decontamination and Waste Disposal

All cultures and contaminated materials should be decontaminated before disposal. The cultures should be autoclaved at the appropriate temperature and pressure for the required time. The agar plates and the pipette tips should be placed in a biohazard container and autoclaved. The antimicrobial stock solutions should be disposed of according to the laboratory waste management plan.

The work surfaces should be decontaminated after the experiment with an appropriate disinfectant. The disinfectant should be effective against the bacterial species used in the assay. The laboratory should have a spill response plan that covers the procedures for containing and decontaminating a spill.

Professional Training and Competency

The MPC assay should be performed by laboratory professionals who have the training and the competency to handle the bacterial cultures and the antimicrobial agents. The American Society for Microbiology provides professional education and resources for laboratory professionals ASM. The laboratory should provide training on the MPC protocol and the biosafety procedures before the staff performs the assay.

The laboratory should have a competency assessment program that verifies the ability of the staff to perform the assay correctly. The competency assessment should include the preparation of the inoculum, the preparation of the agar plates, and the interpretation of the results. The assessment should be documented in the personnel records.

Antimicrobial Resistance Context

Global Burden of Antimicrobial Resistance

Antimicrobial resistance is a global public health threat that requires coordinated action across human health, animal health, and the environment. The World Health Organization defines antimicrobial resistance as the ability of microorganisms to resist the effects of antimicrobials that were previously effective WHO AMR. The emergence of resistant bacteria reduces the effectiveness of the treatment and increases the risk of the spread of the infection.

The MPC concept is one of the tools used to understand the emergence of resistance. The mutant selection window provides a framework for evaluating the potential of an antimicrobial to select for resistant mutants during therapy. The MPC is used in research to evaluate the dosing strategies that minimize the selection of resistance.

Surveillance and Stewardship

The surveillance of antimicrobial resistance is essential for the detection of the emerging resistance and the guidance of the treatment. The WHO coordinates the global surveillance of antimicrobial resistance and provides the data to inform the stewardship WHO AMR. The surveillance data are used to identify the resistance trends and to guide the development of the new antimicrobials.

The antimicrobial stewardship programs promote the appropriate use of the antimicrobials to reduce the selection of resistance. The MPC concept can be used to inform the dosing strategies that minimize the resistance selection. The stewardship programs should consider the MPC data when the dosing regimens are designed for the treatment of the infections with a high bacterial burden.

The Role of the Clinical Laboratory

The clinical laboratory plays a central role in the detection and the surveillance of antimicrobial resistance. The laboratory performs the susceptibility testing and reports the MIC values to the clinicians. The laboratory also participates in the surveillance programs that track the resistance trends. The Clinical and Laboratory Standards Institute provides the standards for the antimicrobial susceptibility testing CLSI. The standards ensure that the testing is performed consistently and that the results are reliable.

The MPC assay is not a standard susceptibility test and is not performed in the routine clinical laboratory. The MPC is a research tool that is used to evaluate the resistance potential of the antimicrobial. The clinical laboratory may perform the MPC assay for the research purposes, but the results should be interpreted in the context of the research question and not used for the clinical decision-making.

Applications of the MPC in Research

Evaluating the New Antimicrobials

The MPC is used in the research and the development of the new antimicrobials. The MPC provides a measure of the potential of the antimicrobial to select the resistance. The MPC is compared to the MIC to calculate the mutant selection window. A narrow window indicates that the antimicrobial is less likely to select the resistance at the concentrations that are achievable in the patient.

The MPC is also used to evaluate the dosing regimens for the new antimicrobials. The goal is to design a dosing regimen that maintains the concentration of the antimicrobial above the MPC for the entire treatment period. The MPC data are used to the pharmacokinetic and the pharmacodynamic models that predict the concentration of the antimicrobial at the infection site.

The Combination Therapy

The MPC concept can be used to evaluate the combination of the antimicrobials. The combination of the antimicrobials can reduce the mutant selection window by the different mechanisms of the action. The MPC of the combination is measured by the plating the inoculum onto the agar containing the combination of the antimicrobials. The MPC of the combination is compared to the MPC of the individual antimicrobials to determine the effect of the combination.

The combination of the antimicrobials can also reduce the frequency of the resistant mutants. The combination of the antimicrobials with the different targets reduces the probability that a single mutation confers the resistance to both the antimicrobials. The MPC data are used to evaluate the potential of the combination to prevent the resistance selection.

The Clinical Application

The MPC concept has been applied to the clinical setting to evaluate the dosing of the antimicrobials. The MPC is used to identify the dosing regimens that achieve the concentration above the MPC at the infection site. The MPC is also used to evaluate the potential of the antimicrobial to select the resistance during the therapy.

The clinical application of the MPC is limited by the lack of the standardized breakpoints and the lack of the clinical data. The MPC is not used to guide the treatment of the individual patient. The MPC is used to inform the development of the dosing strategies that minimize the resistance selection.

The Mutant Selection Window in the Context of the Susceptibility Testing

The Standard Susceptibility Testing

The standard susceptibility testing is performed according to the standards of the Clinical and Laboratory Standards Institute CLSI. The standard testing measures the MIC of the antimicrobial against the bacterial isolate. The MIC is used to classify the isolate as susceptible, susceptible, or resistant to the antimicrobial. The classification is based on the breakpoints that are established by the CLSI and the other standards organizations.

The standard susceptibility testing uses a standard inoculum of approximately 10^5 CFU/mL and an incubation period of 24 hours. The standard testing does not detect the resistant subpopulations that are present at a low frequency. The MIC is a measure of the susceptibility of the majority of the population and does not reflect the potential for the resistance selection.

The MPC and the MIC

The MPC is a measure of the concentration that prevents the growth of the most resistant subpopulation in a large population. The MPC is always greater than or equal to the MIC. The difference between the MPC and the MIC is the mutant selection window. The window is the concentration range where the resistant mutants can be selected.

The MPC and the MIC are used together to evaluate the potential of the antimicrobial to select the resistance. The ratio of the MPC to the MIC provides a measure of the width of the window. A ratio of 1 indicates that the window is absent. A ratio of 4 or higher indicates a wide window and a greater potential for the resistance selection.

The Interpretation of the MPC

The MPC is interpreted in the context of the antimicrobial concentration at the infection site. The MPC is compared to the maximum concentration of the antimicrobial at the infection site. If the maximum concentration exceeds the MPC, the window is closed and the resistance selection is minimized. If the maximum concentration falls within the window, the resistant mutants can be selected.

The MPC is not a clinical breakpoint and cannot be used to classify the isolate as susceptible or resistant. The MPC is a research tool that provides information about the potential of the antimicrobial to select the resistance. The MPC is used to inform the dosing strategies and the development of the new antimicrobials.

The MPC and the Bacterial Population

The Heterogeneity of the Bacterial Population

The bacterial population is not uniform in the susceptibility to the antimicrobial. The population contains the cells with the different levels of the susceptibility. The majority of the cells are susceptible to the antimicrobial at the concentration of the MIC. A small subpopulation of the cells may be resistant to the antimicrobial at the concentration of the MIC.

The resistant subpopulation arises from the mutations that occur during the growth of the bacterial population. The frequency of the resistant mutants depends on the mutation rate and the size of the population. The larger the population, the greater the probability that a resistant mutant is present.

The Selection of the Resistant Mutants

The resistant mutants are selected when the antimicrobial concentration falls within the mutant selection window. The susceptible cells are inhibited by the antimicrobial, while the resistant mutants can survive and grow. The resistant mutants become the majority of the population over time.

The selection of the resistant mutants is a major concern in the treatment of the infections with a high bacterial burden. The high bacterial burden increases the probability that a resistant mutant is present before the treatment. The MPC is used to identify the concentration that prevents the growth of the resistant mutants.

The Prevention of the Resistance Selection

The resistance selection can be prevented by maintaining the antimicrobial concentration above the MPC at the infection site. The concentration above the MPC kills the susceptible cells and the resistant mutants. The dosing regimen should be designed to achieve the concentration above the MPC for the entire treatment period.

The MPC is used to inform the dosing of the antimicrobials. The dosing regimen should be designed to achieve the concentration above the MPC at the infection site. The MPC is also used to evaluate the potential of the antimicrobial to select the resistance.

The MPC and the Antimicrobial Resistance

The Mechanism of the Resistance

The antimicrobial resistance can arise from the mutations in the bacterial genome or the acquisition of the resistance genes. The mutations can alter the target of the antimicrobial, the permeability of the cell, or the efflux of the antimicrobial. The resistance genes can encode the enzymes that inactivate the antimicrobial or the proteins that protect the target.

The MPC is a measure of the concentration that prevents the growth of the most resistant subpopulation. The MPC is affected by the mechanism of the resistance and the frequency of the resistant mutants. The MPC is higher for the antimicrobials that have a high frequency of the resistance.

The MPC and the Resistance

The MPC is used to evaluate the potential of the antimicrobial to select the resistance. The MPC is compared to the MIC to calculate the mutant selection window. The window is the concentration range where the resistant mutants can be selected.

The MPC is also used to evaluate the potential of the antimicrobial to select the resistance during the therapy. The MPC is compared to the concentration of the antimicrobial at the infection site. If the concentration falls within the window, the resistant mutants can be selected.

The MPC and the Clinical Outcome

The MPC is not used to predict the clinical outcome of the treatment. The MPC is a research tool that provides the information about the potential of the antimicrobial to select the resistance. The clinical outcome is affected by the many factors, including the immune response of the patient and the severity of the infection.

The MPC is used to inform the development of the dosing strategies that minimize the resistance selection. The MPC is also used to evaluate the potential of the new antimicrobials to select the resistance.

The MPC and the Laboratory

The Laboratory Protocol

The MPC is measured in the laboratory using the agar dilution method. The agar dilution method is used to measure the MIC of the antimicrobial. The MPC is measured using the same method but with a larger inoculum and a longer incubation period.

The MPC protocol is not standardized across the laboratories. The protocol should be described in the laboratory records and the results should be interpreted in the context of the protocol. The MPC is a research tool and the results should be interpreted with the caution.

The Quality Control

The quality control is essential for the reproducibility of the MPC assay. The quality control should include the reference strains and the quality control organisms. The reference strains should be used to verify the antimicrobial concentrations and the test conditions.

The quality control results should be recorded in the laboratory records. The quality control results should be monitored to identify the problems with the assay. The quality control results should be used to the corrective action if the assay is not performing correctly.

The Data Analysis

The data analysis of the MPC assay involves the plotting of the colony count against the concentration. The MPC is the concentration at which the colony count is zero. The data analysis should be performed using the appropriate software and the results should be recorded.

The data analysis should include the calculation of the mutant selection window. The window is the difference between the MPC and the MIC. The window should be reported in the research report.

The MPC and the Clinical

The Clinical Application

The MPC has been applied to the clinical setting to evaluate the dosing of the antimicrobials. The MPC is used to identify the target that achieves the concentration above the MPC at the infection site. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The clinical application of the MPC is limited by the lack of the standardized data. The MPC is not used to predict the clinical outcome of the individual patient. The MPC is used to inform the development of the dosing strategies.

The Clinical Dosing

The MPC is used to inform the dosing of the antimicrobials. The dosing regimen should be designed to achieve the concentration above the MPC at the infection site. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The MPC is used to evaluate the dosing of the antimicrobials for the infections with a high bacterial burden. The high bacterial burden increases the probability of the resistant mutants. The MPC is used to identify the concentration that prevents the growth of the resistant mutants.

The Clinical Outcome

The MPC is not used to predict the clinical outcome of the individual patient. The clinical outcome is affected by the many factors, including the immune response of the patient and the severity of the infection. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The MPC is used to inform the development of the antimicrobial dosing strategies. The MPC is used to evaluate the potential of the antimicrobial to select the resistance during the treatment.

The MPC and the Research

The Research Applications

The MPC is used in the research to evaluate the potential of the antimicrobial to select the resistance. The MPC is used to evaluate the new antimicrobials and the combination of the antimicrobials. The MPC is used to evaluate the dosing of the antimicrobials.

The MPC is used in the research to understand the mechanism of the resistance. The MPC is used to evaluate the frequency of the resistant mutants. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The Research Limitations

The MPC is a research tool that has the limitations. The MPC is not standardized across the laboratories. The MPC is not used to predict the clinical outcome. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The MPC is limited by the inoculum size and the incubation period. The MPC is also limited by the concentration range of the antimicrobial. The MPC is used to evaluate the potential of the antimicrobial to select the resistance.

The Research Future

The MPC is a research tool that is used to evaluate the potential of the antimicrobial to select the resistance. The MPC is used to evaluate the new antimicrobials and the combination of the antimicrobials. The MPC is used to evaluate the dosing of the antimicrobials.

The MPC is a research tool that is used to evaluate the potential of the antimicrobial to select the resistance. The MPC is used to evaluate the antimicrobial resistance and the development of the new antimicrobials.

Frequently Asked Questions

What is the difference between the MIC and the MPC?

The MIC is the lowest concentration of an antimicrobial that prevents visible growth of a standard inoculum of approximately 10^5 CFU/mL after 24 hours of incubation. The MPC is the lowest concentration that prevents the growth of the most resistant subpopulation in a large inoculum of approximately 10^10 CFU/mL after 48 to 72 hours of incubation. The MPC is always equal to or greater than the MIC.

Why is the inoculum size important for the MPC?

The inoculum size determines the probability that a resistant mutant is present in the culture. A large inoculum of approximately 10^10 CFU/mL ensures that resistant subpopulations present at frequencies of 1e-7 to 1e-9 are represented. A smaller inoculum may not contain any resistant mutants and the MPC would be underestimated.

How is the mutant selection window calculated?

The mutant selection window is the concentration range between the MIC and the MPC. The width of the window is the difference between the MPC and the MIC. A wide window indicates a greater potential for the selection of resistant mutants during therapy.

Can the MPC be used to classify an organism as susceptible or resistant?

No. The MPC is a research tool and does not have established breakpoints. The classification of an organism as susceptible or resistant is based on the MIC and the breakpoints established by the standards organizations such as the CLSI.

What is the clinical relevance of the MPC?

The MPC is used to evaluate the potential of an antimicrobial to select resistant mutants during therapy. The MPC is compared to the concentration of the antimicrobial at the infection site. If the concentration exceeds the MPC, the selection of resistant mutants is minimized.

What are the limitations of the MPC assay?

The MPC assay is not standardized across laboratories. The results are affected by the inoculum size, the antimicrobial concentration range, and the incubation period. The MPC is not used to predict the clinical outcome of an individual patient.

How is the MPC measured in the laboratory?

The MPC is measured by plating a high-density inoculum of approximately 10^10 CFU/mL onto agar plates containing serial dilutions of the antimicrobial. The plates are incubated for 48 to 72 hours and the MPC is the lowest concentration that prevents all colony growth.

What is the role of the reference strains in the MPC assay?

Reference strains from a culture collection such as the ATCC ATCC provide a consistent baseline for the MPC assay. The reference strains are used to verify the antimicrobial concentrations and the test conditions. The quality control organisms are used to monitor the performance of the assay.

Using the Evidence

Related Bacteriology Guides

• Furunculosis in Salmonids: Detection and Prevention
• Leptospirosis in Dogs: Diagnosis, Treatment, and Prevention
• Bacterial Contamination in Poultry Meat: Risks, Detection, and Prevention
• Salmonella and E. coli in Poultry: Food Safety Risks and Prevention
• Salmonella in Poultry: Public Health Risks, Prevention, and Control

References and Further Reading

• Biosafety in Microbiological and Biomedical Laboratories. CDC and NIH.
• Microbiology Resources. American Society for Microbiology.
• Antimicrobial Resistance. World Health Organization.
• Bacteriological Analytical Manual. U.S. Food and Drug Administration.
• Antimicrobial Susceptibility Testing. Clinical and Laboratory Standards Institute.
• Culture Collections. American Type Culture Collection.
• NCBI Pathogen Detection. National Center for Biotechnology Information.
• Mitochondrial transfer from bone-marrow-derived stromal cells to pulmonary alveoli protects against acute lung injury.. Nature medicine, 2012.
• Changes in gut microbiota control metabolic endotoxemia-induced inflammation in high-fat diet-induced obesity and diabetes in mice.. Diabetes, 2008.
• Mutant prevention concentrations, in vitro resistance evolution dynamics, and mechanisms of resistance to imipenem and imipenem/relebactam in carbapenem-susceptible Klebsiella pneumoniae isolates showing ceftazidime/avibactam resistance.. Antimicrobial agents and chemotherapy, 2024.
• Revisiting the mutant prevention concentration to guide dosing in childhood tuberculosis.. The Journal of antimicrobial chemotherapy, 2017.

This article is educational and does not replace validated laboratory procedures, clinical diagnosis, institutional biosafety review, or public-health guidance.