How to Perform and Interpret Biofilm Susceptibility Testing Using the MBEC
By Dr. Zubair Khalid, DVM, MS, PhD ·
Quick Answer
- The MBEC assay uses the Calgary Biofilm Device to grow 96 equivalent biofilms on polystyrene pegs, then exposes them to antibiotic gradients to determine the minimum biofilm inhibitory concentration (MBIC).
- Set up the assay with a standardized inoculum, a 24-hour biofilm growth phase, a 24-hour antibiotic challenge, and sonication to recover surviving cells for viable plate counts.
- The MBEC assay measures biofilm susceptibility under static conditions, which may not fully represent in vivo biofilm behavior, so results require confirmation with clinical response and standard susceptibility methods.
At a Glance
The MBEC assay is a high-throughput method for testing antimicrobial agents against bacterial biofilms. It uses a 96-well plate with a lid bearing 96 polystyrene pegs. Biofilms form on the pegs, then the lid transfers to a challenge plate containing antibiotic dilutions. After exposure, surviving cells are recovered by sonication and quantified.
| Assay Step | Purpose | Key Decision Point |
|---|---|---|
| Inoculum preparation | Standardize bacterial density | Use authenticated strains from a culture collection and adjust to a defined optical density |
| Biofilm growth | Allow attachment and maturation on pegs | Incubate 24 to 48 hours with gentle agitation to promote uniform biofilm formation |
| Antibiotic challenge | Expose biofilms to drug dilutions | Prepare serial twofold dilutions in the challenge plate, include growth and sterility controls |
| Recovery and enumeration | Quantify surviving cells | Sonicate pegs to dislodge biofilm, then plate serial dilutions for viable counts |
| Data interpretation | Determine MBEC | Compare survivor counts to the initial inoculum and define the MBEC as the lowest concentration that reduces viability below a set threshold |
Background and Rationale
The Problem of Biofilm-Associated Infections
Bacterial biofilms are communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix. These communities resist antibiotics and host defenses far more effectively than planktonic cells. The World Health Organization identifies antimicrobial resistance as a global public health threat, and biofilms contribute to persistent infections that are difficult to treat with conventional susceptibility testing methods.
Standard antimicrobial susceptibility testing, such as broth microdilution, measures the minimum inhibitory concentration (MIC) for planktonic bacteria. These methods do not capture the reduced susceptibility of biofilm-grown cells. The Clinical and Laboratory Standards Institute publishes standards for antimicrobial susceptibility testing, but these standards primarily address planktonic organisms [CLSI]. The MBEC assay addresses this gap by providing a method to test antibiotics against biofilm-grown bacteria.
The Calgary Biofilm Device
The Calgary Biofilm Device, also known as the MBEC (Minimum Biofilm Eradication Concentration) assay, was developed to standardize biofilm susceptibility testing. The device consists of a 96-well plate lid with 96 pegs that fit into a standard 96-well plate. Each peg provides a surface for biofilm formation, and each well can contain a different antibiotic concentration or growth condition.
The MBEC assay allows researchers to grow 96 equivalent biofilms simultaneously, then expose them to a range of antibiotic concentrations. This design enables the determination of the minimum biofilm inhibitory concentration (MBIC), which is the lowest antibiotic concentration that prevents visible growth of biofilm-derived cells after recovery.
Core Principles of the MBEC Assay
Biofilm Formation on Pegs
The MBEC assay relies on the ability of bacteria to attach to and form biofilms on the polystyrene pegs. The pegs are designed to be hydrophobic, which promotes bacterial adhesion. The bacteria attach to the peg surface, produce extracellular polymeric substances, and form a mature biofilm over time.
The growth medium and incubation conditions must support biofilm formation. Common media include tryptic soy broth, Mueller-Hinton broth, or other media appropriate for the test organism. The incubation period typically ranges from 24 to 48 hours, depending on the organism and the desired biofilm maturity.
Antibiotic Challenge
After biofilm formation, the peg lid is transferred to a challenge plate containing serial dilutions of the antibiotic. The biofilms are exposed to the antibiotic for a defined period, usually 24 hours. The antibiotic concentrations should span a range that includes concentrations below and above the planktonic MIC.
The challenge plate should include a growth control (no antibiotic) and a sterility control (no bacteria). These controls are essential for validating the assay and interpreting the results.
Recovery and Quantification
After the antibiotic challenge, the peg lid is removed, and the biofilms are rinsed to remove loosely attached cells. The remaining biofilm cells are then dislodged from the pegs by sonication. The sonicated suspension is serially diluted and plated on agar to determine the number of viable cells.
The MBIC is defined as the lowest antibiotic concentration that reduces the viable cell count below a defined threshold, often a 99.9% reduction compared to the growth control. This threshold is analogous to the minimum bactericidal concentration (MBC) for planktonic bacteria.
Practical Workflow for the MBEC Assay
Step 1: Prepare the Inoculum
Use authenticated bacterial strains from a recognized culture collection such as the American Type Culture Collection (ATCC) to ensure reproducibility [ATCC]. Prepare the inoculum by growing the bacteria in an appropriate broth medium to the logarithmic phase. Adjust the cell density to a standardized concentration, typically 10^6 to 10^7 colony-forming units per milliliter (CFU/mL).
Measure the optical density at 600 nm (OD600) and adjust the culture to the desired density. Confirm the cell count by plating serial dilutions on agar and counting colonies after incubation.
Step 2: Inoculate the MBEC Device
Add the standardized inoculum to the wells of a 96-well plate. Place the peg lid onto the plate so that each peg is submerged in the inoculum. Cover the device and incubate at the appropriate temperature for the test organism, typically 35 to 37 degrees Celsius for most pathogens.
Incubate for 24 to 48 hours with gentle agitation to promote uniform biofilm formation on all pegs. The agitation ensures that the bacteria are exposed to the peg surface and that the biofilm develops consistently across the device.
Step 3: Prepare the Antibiotic Challenge Plate
Prepare serial twofold dilutions of the antibiotic in a fresh 96-well plate. The concentration range should include the planktonic MIC and extend above and below it. Include a growth control well with no antibiotic and a sterility control well with only the medium.
The antibiotic dilutions should be prepared in the same medium used for biofilm growth to maintain consistent conditions. The volume in each well should be sufficient to cover the peg completely.
Step 4: Transfer the Biofilm to the Challenge Plate
Remove the peg lid from the biofilm growth plate and rinse the pegs gently in a buffer solution to remove loosely attached cells. Place the peg lid onto the antibiotic challenge plate so that each peg is submerged in the antibiotic solution.
Incubate the challenge plate for 24 hours at the same temperature used for biofilm growth. This exposure period allows the antibiotic to act on the biofilm.
Step 5: Recover and Quantify Survivors
After the antibiotic challenge, remove the peg lid from the challenge plate. Rinse the pegs in a sterile buffer to remove residual antibiotic. Place the peg lid in a recovery plate containing a neutralizer or a fresh medium.
Sonicate the pegs to dislodge the biofilm cells. The sonication time and power should be optimized for the test organism to ensure complete recovery without killing the cells. After sonication, remove the peg lid and serially dilute the recovered cell suspension.
Plate the dilutions on agar and incubate for 24 to 48 hours. Count the colonies and calculate the number of viable cells per peg.
Step 6: Interpret the Results
Determine the MBIC as the lowest antibiotic concentration that reduces the viable cell count below the defined threshold. The threshold is often a 99.9% reduction compared to the growth control. This threshold is consistent with the definition of a bactericidal effect.
Compare the MBIC to the planktonic MIC to assess the difference in susceptibility between the biofilm and planktonic states. A large difference indicates that the biofilm is significantly less susceptible to the antibiotic.
Options and Tradeoffs
Choice of Growth Medium
The growth medium can influence biofilm formation and antibiotic activity. Some media promote more robust biofilm formation than others. For example, tryptic soy broth is commonly used for many organisms, while other media may be required for specific bacteria.
The medium should be selected based on the test organism and the purpose of the assay. A medium that supports good biofilm formation is essential for obtaining reliable results. The medium should also be compatible with the antibiotic being tested.
Incubation Time
The incubation time for biofilm formation affects the maturity of the biofilm. A 24-hour incubation may produce a young biofilm, while a 48-hour incubation may produce a more mature biofilm with a thicker matrix. The choice depends on the research question and the organism.
A longer incubation time may result in a more robust biofilm that is more resistant to antibiotics. However, it also increases the risk of contamination and may require more resources. The incubation time should be standardized for the assay to ensure reproducibility.
Antibiotic Concentration Range
The antibiotic concentration range should be selected to cover the expected MBIC. The range should include concentrations below the planktonic MIC and extend several dilutions above it. This ensures that the MBIC can be determined even if the biofilm is significantly less susceptible.
The concentration range should be based on the known susceptibility of the test organism and the pharmacokinetic properties of the antibiotic. The range should be broad enough to capture the full dose-response curve.
Recovery Method
The recovery method is critical for obtaining accurate viable counts. Sonication is the standard method for dislodging biofilm cells from the pegs. The sonication time and power must be optimized to remove all cells without causing cell death.
Alternative recovery methods include vortexing or scraping, but these may be less effective or less reproducible. The recovery method should be validated for the test organism to ensure that the viable counts are representative of the biofilm population.
Observations and Measurements
Viable Cell Counts
The primary measurement in the MBEC assay is the viable cell count recovered from each peg after the antibiotic challenge. This count is expressed as CFU per peg. The counts are used to calculate the reduction in viability compared to the growth control.
The viable cell counts should be recorded for each antibiotic concentration. The data should be plotted as a dose-response curve to visualize the relationship between antibiotic concentration and cell survival.
Minimum Biofilm Inhibitory Concentration
The MBIC is the key result of the MBEC assay. It is defined as the lowest antibiotic concentration that reduces the viable cell count below the threshold. The threshold is typically a 99.9% reduction, which corresponds to a 3-log reduction in viable cells.
The MBIC is a measure of the antibiotic concentration needed to inhibit the growth of biofilm-derived cells. It is distinct from the planktonic MIC and provides a more relevant measure for biofilm-associated infections.
Comparison with Planktonic MIC
The MBIC is often compared to the planktonic MIC to assess the impact of the biofilm on antibiotic susceptibility. A higher MBIC indicates that the biofilm is less susceptible to the antibiotic. This comparison can help guide the selection of antibiotics for biofilm-associated infections.
The ratio of MBIC to MIC can be used to quantify the increase in resistance. A ratio of 1 indicates no difference, while a ratio of 10 or higher indicates a significant increase in resistance.
Records and Documentation
Assay Records
Maintain detailed records of the MBEC assay, including the test organism, the strain, the growth medium, the incubation time, the antibiotic concentrations, and the recovery method. These records are essential for reproducibility and for troubleshooting.
The records should include the optical density of the inoculum, the viable cell count of the inoculum, and the viable cell counts from each antibiotic concentration. The records should also include the MBIC and the planktonic MIC for comparison.
Quality Control
Quality control is essential for the MBEC assay. Use reference strains with known susceptibility profiles to validate the assay. The reference strains should be tested in parallel with the test organisms to ensure that the assay is performing correctly.
The quality control should include the growth control and the sterility control. The growth control should show a high viable cell count, and the sterility control should show no growth. Any deviation from these expectations indicates a problem with the assay.
Common Failure Patterns
Poor Biofilm Formation
Poor biofilm formation can result in low viable cell counts and unreliable MBIC values. This can be caused by an inadequate inoculum, a short incubation time, or a medium that does not support biofilm formation.
To address this, increase the inoculum density, extend the incubation time, or change the growth medium. The biofilm formation should be verified by staining or by viable cell counts before proceeding with the antibiotic challenge.
Incomplete Recovery
Incomplete recovery of biofilm cells can lead to underestimation of the viable cell count and an artificially low MBIC. This can be caused by insufficient sonication time or power.
To address this, optimize the sonication conditions for the test organism. The recovery efficiency should be verified by comparing the viable cell count from the sonicated pegs to the total cell count determined by other methods.
Antibiotic Carryover
Antibiotic carryover can occur when the antibiotic is not completely removed from the pegs before recovery. This can lead to a false reduction in viable cell count and an artificially low MBIC.
To address this, rinse the pegs thoroughly in a sterile buffer after the antibiotic challenge. The rinse should be sufficient to remove all residual antibiotic. The recovery medium should also contain a neutralizing agent if the antibiotic is known to carry over.
Contamination
Contamination can occur during the assay, leading to unreliable results. This can be caused by improper handling of the device or the media. The sterility control should be used to detect contamination.
To address this, follow strict aseptic technique throughout the assay. The sterility control should show no growth, and any contamination should be investigated and corrected.
Safety and Regulatory Context
Laboratory Biosafety
The MBEC assay involves the handling of bacterial cultures, which requires appropriate biosafety precautions. The Biosafety in Microbiological and Biomedical Laboratories (BMBL) provides guidelines for the safe handling of microorganisms [CDC BMBL]. The risk assessment should be based on the biosafety level of the test organism.
The assay should be performed in a biosafety cabinet to prevent the release of aerosols. The laboratory should be equipped with appropriate personal protective equipment, including gloves, lab coats, and eye protection. The waste should be decontaminated before disposal.
Antimicrobial Resistance Surveillance
The MBEC assay can be used to assess the susceptibility of biofilms to antibiotics, which is relevant to antimicrobial resistance surveillance. The World Health Organization emphasizes the importance of surveillance in addressing antimicrobial resistance [WHO AMR]. The data from the MBEC assay can contribute to understanding the resistance patterns of biofilm-associated infections.
The assay results should be interpreted in the context of the clinical situation and the pharmacokinetics of the antibiotic. The MBIC should not be used as a direct guide for clinical dosing without considering the drug concentrations achieved at the site of infection.
Limitations and Evidence Gaps
In Vitro Nature of the Assay
The MBEC assay is an in vitro model that does not fully reproduce the in vivo biofilm environment. The biofilm formed on the pegs may differ from the biofilm in a host, which includes host factors and the immune response. The results should be interpreted with caution.
The MBIC may not predict the clinical outcome of antibiotic treatment. The clinical efficacy of an antibiotic depends on many factors, including the pharmacokinetics and pharmacodynamics of the drug, the host immune response, and the site of infection.
Standardization
The MBEC assay is not fully standardized across laboratories. The variations in the protocol, such as the growth medium, the incubation time, and the recovery method, can affect the results. The Clinical and Laboratory Standards Institute provides standards for antimicrobial susceptibility testing, but these standards do not cover the MBEC assay [CLSI].
The lack of standardization makes it difficult to compare results across studies. The assay should be validated in each laboratory, and the results should be interpreted in the context of the specific protocol used.
The Need for Clinical Validation
The MBEC assay has not been extensively validated in clinical studies. The relationship between the MBIC and the clinical outcome is not well established. The assay should be used in conjunction with clinical data and standard susceptibility testing.
The results of the MBEC assay should be confirmed by clinical trials and by the response to treatment. The assay should not be used as a sole basis for clinical decisions.
Professional Escalation Criteria
When to Seek Expert Advice
If the MBEC assay results are inconsistent or if the assay fails to produce reliable results, consult with a laboratory professional or a clinical microbiologist. The expert can help troubleshoot the assay and interpret the results.
If the MBIC is significantly higher than the planktonic MIC, the results should be discussed with a clinician. The clinician can consider the implications for treatment and the need for alternative antibiotics.
When to Use Alternative Methods
If the MBEC assay is not suitable for the test organism or the antibiotic, consider alternative methods for biofilm susceptibility testing. These methods may include the use of a different device or a different protocol.
The alternative methods should be validated and should provide reliable results. The choice of method should be based on the research question and the available resources.
Decision Framework for Selecting MBEC Assay Conditions
Organism-Specific Assay Design Decisions
The MBEC assay requires deliberate choices at each step, and these choices depend on the bacterial species under investigation. A laboratory scientist must decide whether the assay will use a reference strain from a culture collection or a clinical isolate. The American Type Culture Collection provides authenticated strains with documented growth characteristics, which supports reproducibility across experiments and laboratories [ATCC]. Clinical isolates introduce variability because their biofilm-forming capacity and antibiotic susceptibility profiles differ from reference strains. For initial assay validation, use authenticated reference strains to establish baseline performance, then apply the same protocol to clinical isolates with documented passage history.
The growth medium selection should follow the nutritional requirements of the test organism. Mueller-Hinton broth supports the growth of many nonfastidious pathogens and aligns with standard susceptibility testing practice [CLSI]. Tryptic soy broth often produces more robust biofilm formation for staphylococci and Pseudomonas species. For fastidious organisms, supplement the medium with the required nutrients, but verify that the supplement does not interfere with antibiotic activity. The medium chosen for biofilm growth should also be used for the antibiotic challenge plate to maintain consistent ionic strength and pH across the assay.
Static Versus Dynamic Exposure Decisions
The MBEC assay as described in the existing protocol uses static conditions, where the biofilm remains in contact with a fixed antibiotic concentration for the entire challenge period. This design measures the response to a constant drug level. In vivo, antibiotic concentrations fluctuate with dosing intervals, clearance, and tissue penetration. A laboratory can modify the MBEC protocol to simulate dynamic exposure by transferring the peg lid to fresh antibiotic dilutions at defined intervals. This modification requires additional plates and careful timing but provides a more realistic assessment of how a biofilm responds to the concentration-time profile of a drug.
The choice between static and dynamic exposure depends on the research question. For a screening assay to compare multiple antibiotics, static exposure is practical and reproducible. For a pharmacokinetic-pharmacodynamic study, dynamic exposure provides information about the effect of drug concentration decay on biofilm viability. Document the exposure mode in the assay records because the MBIC from a static assay will not match the MBIC from a dynamic assay for the same organism and antibiotic.
Threshold Selection for MBIC Definition
The MBIC is defined as the lowest antibiotic concentration that reduces the viable cell count below a set threshold. The most common threshold is a 99.9% reduction, which corresponds to a 3-log reduction in viable cells compared to the growth control. This threshold is analogous to the minimum bactericidal concentration for planktonic bacteria. However, the 99.9% threshold is not the only possible definition. Some laboratories use a 99% reduction, which is a 2-log reduction, to define a more sensitive endpoint. Others use a 99.99% reduction, which is a 4-log reduction, to define a more stringent endpoint.
The choice of threshold affects the reported MBEC value. A 99% threshold will produce a lower MBEC than a 99.9% threshold for the same data set. The threshold should be selected before the assay and documented in the protocol. The threshold should also be aligned with the clinical context. A more stringent threshold may be appropriate for a device-associated infection where the goal is eradication, while a less stringent threshold may be appropriate for a screening study where the goal is to identify candidate antibiotics.
Control Design Decisions
The MBEC assay requires a growth control and a sterility control. The growth control is a well with no antibiotic, and it provides the baseline viable cell count for calculating the reduction. The sterility control is a well with only the medium, and it detects contamination. These controls are essential, but they are not sufficient for a well-designed assay.
A positive control with a known antibiotic and a known resistant organism should be included to validate the assay performance. A negative control with a known susceptible organism and a known effective antibiotic should also be included. These controls provide a reference for the expected MBEC range and help identify systematic errors in the assay. The positive and negative controls should be tested in parallel with the test organisms on the same plate to account for plate-to-plate variation.
Replication and Statistical Power
The MBEC device provides 96 pegs, which allows for multiple replicates of each antibiotic concentration. A common design uses four replicates per concentration, which provides 24 concentrations per plate. The number of replicates should be based on the expected variability of the assay and the desired statistical power. A pilot study with the test organism and the antibiotic can estimate the variability and guide the choice of replicate number.
The data from the replicates should be reported as the mean and standard deviation of the viable cell counts. The MBEC should be reported as the lowest concentration that meets the threshold in all replicates. If the replicates show a high degree of variability, the assay should be repeated, and the source of the variability should be investigated. The variability can be caused by inconsistent biofilm formation, incomplete recovery, or antibiotic carryover.
Decision Matrix for Assay Troubleshooting
Low Viable Cell Count in the Growth Control
A low viable cell count in the growth control indicates that the biofilm did not form adequately on the pegs. The possible causes include an inoculum that is too dilute, an incubation time that is too short, or a medium that does not support biofilm formation. The first step is to verify the inoculum density by plating serial dilutions of the inoculum and counting the colonies. The inoculum should be adjusted to the target density, typically 10^6 to 10^7 CFU per milliliter.
If the inoculum density is correct, the next step is to extend the incubation time. A 24-hour incubation may not be sufficient for some organisms to form a mature biofilm. A 48-hour incubation may produce a more robust biofilm. The incubation temperature should also be verified, as some organisms require a specific temperature for optimal growth.
If the incubation time and temperature are correct, the medium should be evaluated. Some organisms require a specific medium for biofilm formation. The medium should be tested with the organism in a preliminary experiment to confirm that it supports biofilm formation. The biofilm formation can be verified by staining the pegs with crystal violet and measuring the optical density of the stained biofilm.
High Variability Between Replicate Pegs
High variability between replicate pegs indicates that the biofilm formation is not uniform across the device. The possible causes include inconsistent agitation during the growth phase, variations in the peg surface, or variations in the inoculum. The agitation should be gentle and consistent across the entire incubation period. The device should be placed on a platform shaker with a consistent speed and orbit.
The peg surface should be inspected for defects or contamination. The pegs should be handled with care to avoid scratching the surface. The inoculum should be mixed thoroughly before adding to the wells to ensure that each well receives the same number of cells.
If the variability persists, the recovery method should be investigated. The sonication time and power should be optimized to ensure that all cells are dislodged from the pegs. The sonication should be performed in a consistent manner, and the recovery plate should be kept on ice to prevent cell death.
MBEC Values That Are Too Low or Too High
An MBEC that is too low, meaning the antibiotic appears more effective than expected, can be caused by antibiotic carryover. The antibiotic may remain on the pegs after the challenge and inhibit the growth of the recovered cells. The pegs should be rinsed thoroughly in a sterile buffer after the challenge. The rinse should be performed in a consistent manner, and the buffer should be changed for each rinse.
An MBEC that is too high, meaning the antibiotic appears less effective than expected, can be caused by incomplete recovery of the biofilm cells. The sonication may not dislodge all the cells from the pegs, leading to an underestimation of the viable cell count. The sonication time and power should be optimized for the test organism. The recovery efficiency should be verified by comparing the viable cell count from the sonicated pegs to the total cell count determined by other methods.
Contamination in the Sterility Control
Contamination in the sterility control indicates that the assay was contaminated during the setup or the incubation. The possible causes include contaminated media, contaminated equipment, or improper aseptic technique. The sterility control should be checked at each step of the assay. The media should be prepared fresh and checked for contamination before use. The equipment should be sterilized properly, and the aseptic technique should be followed strictly.
If the contamination persists, the source should be investigated. The biosafety cabinet should be checked for proper airflow and the work surface should be cleaned. The personal protective equipment should be changed, and the hands should be washed thoroughly. The contamination should be documented in the assay records, and the assay should be repeated.
Decision Matrix for the MBEC Assay
The following decision matrix provides a structured approach to troubleshooting the MBEC assay. The matrix lists the common failure patterns, the possible causes, and the corrective actions.
| Failure Pattern | Possible Cause | Corrective Action |
|---|---|---|
| Low viable cell count in growth control | Inoculum too dilute | Verify inoculum density and adjust to target range |
| Low viable cell count in growth control | Incubation time too short | Extend incubation to 48 hours |
| Low viable cell count in growth control | Medium does not support biofilm | Test alternative media and select the one that supports biofilm |
| High variability between replicate pegs | Inconsistent agitation | Use a platform shaker with a consistent speed |
| High variability between replicate pegs | Uneven peg surface | Inspect pegs for defects and handle cleanly |
| High variability between replicate pegs | Incomplete recovery | Optimize sonication time and power |
| MBEC too low | Antibiotic carryover | Rinse pegs thoroughly in sterile buffer |
| MBEC too high | Incomplete recovery | Optimize sonication time and power |
| Contamination in sterility control | Contaminated media | Prepare fresh media and check for contamination |
| Contamination in sterility control | Contaminated equipment | Sterilize equipment and follow aseptic technique |
Validation and Verification Protocol
Initial Validation of the Assay
Before using the MBEC assay for a new organism or a new antibiotic, the assay should be validated in the laboratory. The validation should include a reference strain with a known susceptibility profile. The reference strain should be tested in parallel with the test organism to ensure that the assay is performing correctly.
The validation should include a determination of the optimal inoculum density, the optimal incubation time, and the optimal recovery method. The validation should also include a determination of the reproducibility of the assay. The assay should be repeated on multiple days to assess the day-to-day variability.
The validation data should be documented in the assay records. The records should include the reference strain, the growth conditions, the antibiotic concentrations, and the MBEC values. The records should also include the viable cell counts from the growth control and the sterility control.
Ongoing Quality Control
The quality control should be performed on a regular basis to ensure that the assay is performing correctly. The quality control should include a reference strain with a known susceptibility profile. The reference strain should be tested in parallel with the test organisms.
The quality control should include the growth control and the sterility control. The growth control should show a high viable cell count, and the sterility control should show no growth. Any deviation from these expectations indicates a problem with the assay.
The quality control data should be documented in the assay records. The data should be reviewed regularly to identify trends and to detect problems early. The quality control should be performed at least once per week or more frequently if the assay is performed regularly.
Documentation of the Assay Conditions
The assay conditions should be documented in detail in the assay records. The records should include the test organism, the strain, the growth medium, the incubation time, the antibiotic concentrations, and the recovery method. The records should also include the optical density of the inoculum, the viable cell count of the inoculum, and the viable cell counts from each antibiotic concentration.
The records should include the MBIC and the planktonic MIC for comparison. The records should also include the quality control data and the validation data. The records should be stored in a secure location and should be available for review.
Comparison of the MBEC with Alternative Biofilm Susceptibility Methods
The MBEC versus the Colony Biofilm Method
The colony biofilm method is an alternative to the MBEC for testing biofilm susceptibility. In the colony biofilm method, the bacteria are grown on a membrane filter placed on an agar plate. The biofilm forms on the membrane, and the membrane is transferred to an antibiotic-containing agar plate. After the challenge, the membrane is transferred to a recovery plate, and the viable cells are counted.
The colony biofilm method produces a biofilm that is more mature and more robust than the MBEC biofilm. The colony biofilm method also allows for the testing of a wider range of antibiotic concentrations. However, the colony biofilm method is more labor-intensive and requires more time than the MBEC. The colony biofilm method is also more difficult to standardize across laboratories.
The MBEC is more suitable for high-throughput screening, while the colony biofilm method is more suitable for detailed studies of the biofilm. The choice between the two methods depends on the research question and the available resources.
The MBEC versus the Flow Cell System
The flow cell system is another alternative to the MBEC for testing biofilm susceptibility. In the flow cell system, the bacteria are grown in a flow cell, and the biofilm forms on a glass surface. The antibiotic is then introduced into the flow cell, and the biofilm is monitored in real time.
The flow cell system provides a more realistic model of the in vivo biofilm environment, as the flow of the medium and the antibiotic mimics the flow of the blood and the tissue fluid. The flow cell system also allows for the real-time monitoring of the biofilm response to the antibiotic.
However, the flow cell system is more complex and more expensive than the MBEC. The flow cell system also requires more time and more expertise to operate. The flow cell system is not suitable for high-throughput screening.
The MBEC is more suitable for high-throughput screening, while the flow cell system is more suitable for detailed studies of the biofilm response. The choice between the two methods depends on the research question and the available resources.
The MBEC versus the Standard Broth Microdilution
The standard broth microdilution method measures the susceptibility of planktonic bacteria to an antibiotic. The MBEC measures the susceptibility of biofilm-grown bacteria. The MBEC is typically higher than the MIC because biofilms are more resistant to antibiotics.
The standard broth microdilution is a well-established method that is standardized by the Clinical and Laboratory Standards Institute [CLSI]. The MBEC is not fully standardized, and the results may vary across laboratories. The standard broth microdilution is the first-line method for susceptibility testing, while the MBEC is a specialized method for biofilm susceptibility.
The MBEC should be used in conjunction with the standard broth microdilution to provide a complete picture of the antibiotic susceptibility of the organism. The MBEC should not be used as a replacement for the standard broth microdilution.
Professional Escalation Criteria for the MBEC Assay
When to Consult a Laboratory Professional
If the MBEC assay results are inconsistent or if the assay fails to produce reliable results, consult with a laboratory professional or a clinical microbiologist. The expert can help troubleshoot the assay and interpret the results. The expert can also help with the validation of the assay and the interpretation of the data.
If the MBIC is significantly higher than the planktonic MIC, the results should be discussed with a clinician. The clinician can consider the implications for treatment and the need for alternative antibiotics. The clinician can also help with the interpretation of the results in the context of the clinical situation.
When to Use Alternative Methods
If the MBEC assay is not suitable for the test organism or the antibiotic, consider alternative methods for biofilm susceptibility testing. These methods may include the colony biofilm method or the flow cell system. The alternative methods should be validated and should provide reliable results.
The choice of method should be based on the research question and the available resources. The MBEC is a suitable method for high-throughput screening, while the colony biofilm method and the flow cell system are more suitable for detailed studies of the biofilm.
When to Seek Clinical Guidance
The MBEC results should be interpreted in the context of the clinical situation. The MBIC should not be used as a direct guide for clinical dosing without considering the drug concentrations achieved at the site of infection. The clinician should be consulted to interpret the results and to make treatment decisions.
The MBEC results should be confirmed by clinical trials and by the response to treatment. The MBEC should not be used as a sole basis for clinical decisions. The clinician should be involved in the interpretation of the results and the decision-making process.
Frequently Asked Questions
What is the difference between the MBEC and the standard MIC?
The standard MIC measures the susceptibility of planktonic bacteria to an antibiotic. The MBEC measures the susceptibility of biofilm-grown bacteria. The MBEC is typically higher than the MIC because biofilms are more resistant to antibiotics.
How long does the MBEC assay take?
The MBEC assay typically takes 72 to 96 hours. This includes 24 to 48 hours for biofilm formation, 24 hours for the antibiotic challenge, and 24 to 48 hours for the recovery and enumeration of viable cells.
What is the minimum biofilm inhibitory concentration (MBIC)?
The MBIC is the lowest antibiotic concentration that reduces the viable cell count of the biofilm below a defined threshold, typically a 99.9% reduction. It is a measure of the antibiotic concentration needed to inhibit the biofilm.
Can the MBEC assay be used for all bacteria?
The MBEC assay can be used for many bacteria that form biofilms. However, the assay may not be suitable for all organisms. The growth conditions and the recovery method must be optimized for the test organism.
How do I choose the antibiotic concentration range for the MBEC assay?
The antibiotic concentration range should be based on the planktonic MIC and the expected MBIC. The range should cover concentrations below and above the planktonic MIC. The range should be broad enough to capture the dose-response curve.
What are the common sources of error in the MBEC assay?
Common sources of error include poor biofilm formation, incomplete recovery of cells, antibiotic carryover, and contamination. These errors can be minimized by optimizing the assay conditions and using appropriate controls.
How do I interpret the MBEC results?
The MBEC results should be interpreted in the context of the planktonic MIC and the clinical situation. A higher MBIC indicates that the biofilm is less susceptible to the antibiotic. The results should be confirmed by clinical trials.
What are the limitations of the MBEC assay?
The MBEC assay is an in vitro model that does not fully reproduce the in vivo biofilm environment. The results may not predict the clinical outcome. The assay is not fully standardized, and the results should be interpreted with caution.
Related Bacteriology Guides
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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.
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This article is educational and does not replace validated laboratory procedures, clinical diagnosis, institutional biosafety review, or public-health guidance.