# Quality Control for Culture Media Used in Fastidious Organism Identification

## Quick Answer

- Use authenticated ATCC reference strains for growth promotion testing of each new lot of culture media before diagnostic use.
- Document incubation conditions, expected colony characteristics, and recovery results for each medium and organism pair.
- Growth promotion testing confirms media support fastidious organism growth but does not replace biochemical or molecular identification.

## Purpose of Quality Control for Fastidious Organism Media

Fastidious organisms require enriched media, specific atmospheric conditions, and precise incubation parameters to grow in the laboratory. Culture media quality control exists to verify that each batch of medium supports the growth of the target organism before it is used for diagnostic work. Laboratories that identify fastidious bacteria such as *Haemophilus influenzae*, *Neisseria gonorrhoeae*, *Streptococcus pneumoniae*, and *Mycobacterium* species depend on reliable media performance for accurate results.

Quality control for culture media is a systematic process that includes testing each new batch of prepared medium, documenting the results, and taking corrective action when a batch fails to perform as expected. The [American Type Culture Collection](https://www.atcc.org/) provides authenticated reference strains that laboratories use to verify that their media support the growth of known organisms. These strains carry documented characteristics and are maintained under controlled conditions to preserve their viability and biochemical properties.

The [Clinical and Laboratory Standards Institute](https://clsi.org/standards/products/microbiology) publishes standards for antimicrobial susceptibility testing and related microbiology procedures. These standards provide the framework that laboratories use to establish quality control protocols for culture media. While the standards address susceptibility testing directly, the same quality principles apply to media preparation and performance verification.

Quality control for fastidious organism media requires attention to several variables that do not affect routine media. Fastidious organisms often need specific pH ranges, reducing agents, growth factors, and atmospheric conditions. A medium that supports robust growth of *Escherichia coli* may fail completely for *Haemophilus* species if the required growth factors are absent or degraded. The quality control program must therefore include both the medium itself and the conditions under which it is used.

## Core Principles of Culture Media Quality Control

### Growth Promotion Testing

Growth promotion testing is the central procedure for verifying that a culture medium supports the growth of the target organism. The test involves inoculating the medium with a small number of viable organisms from a reference strain and observing whether growth occurs under the specified incubation conditions. The [American Type Culture Collection](https://www.atcc.org/) supplies reference strains that have been authenticated for their identity and characteristics.

For fastidious organisms, growth promotion testing requires careful attention to the inoculum size. A heavy inoculum can mask poor medium performance because even a suboptimal medium may support growth when large numbers of organisms are present. A light inoculum that approximates the number of organisms expected in a clinical specimen provides a more meaningful test of medium performance.

The inoculum preparation method affects the reliability of growth promotion testing. Organisms should be grown under conditions that produce healthy, viable cells. The suspension should be prepared in a diluent that maintains organism viability, and the number of organisms in the inoculum should be confirmed by colony count or turbidity measurement. Laboratories should document the method used to prepare the inoculum and the expected number of organisms in each test.

### Reference Strain Selection

Reference strains for quality control should be obtained from a recognized culture collection such as the [American Type Culture Collection](https://www.atcc.org/). These strains have documented characteristics, including their growth requirements, biochemical reactions, and genetic markers. Using authenticated strains ensures that the quality control test measures the performance of the medium instead of the viability of an uncharacterized laboratory strain.

The reference strain should be representative of the organisms that the medium is designed to support. For example, a medium intended for the isolation of *Campylobacter* species should be tested with a *Campylobacter* reference strain. The strain should be maintained according to the culture collection instructions to preserve its viability and characteristics. Subculturing beyond the recommended number of passages can alter the characteristics of the strain and reduce its usefulness for quality control.

### Documentation Requirements

Quality control documentation provides the evidence that a medium batch was tested and performed as expected. The documentation should include the medium name and lot number, the preparation date, the reference strain used, the inoculum concentration, the incubation conditions, and the observed growth result. The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) from the FDA describes the documentation practices used in food microbiology laboratories and provides a model for the level of detail expected in quality control records.

Documentation should also include the expected result for each test. For example, a growth promotion test for chocolate agar with *Haemophilus influenzae* should specify the expected colony size, appearance, and the time required for visible growth. The documentation should also include the corrective action taken when a medium fails the quality control test.

## Media Preparation and Storage

### Preparation Variables That Affect Fastidious Organism Growth

The preparation of culture media for fastidious organisms requires attention to several variables that can affect the final product. The quality of the water used to prepare the medium is important because trace metals and organic contaminants can inhibit the growth of sensitive organisms. The pH of the medium must be checked after preparation because the autoclaving process can change the pH of the medium.

The addition of supplements to the medium must be performed under conditions that preserve the activity of the supplements. For example, antibiotics that are added to selective media must be prepared and stored according to the manufacturer instructions to maintain their activity. The temperature of the medium when supplements are added is also important. Some supplements are heat-labile and must be added after the medium has cooled to the appropriate temperature.

The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) describes the preparation of media for food microbiology testing and emphasizes the importance of following the manufacturer instructions for each medium. The same principle applies to media used for fastidious organisms in clinical microbiology. Each medium has specific preparation requirements that must be followed to produce a product that supports the growth of the target organism.

### Storage Conditions and Shelf Life

Culture media for fastidious organisms must be stored under conditions that preserve their performance. Most prepared media should be stored at refrigerated temperatures and protected from light and moisture. The storage conditions should be documented and monitored to ensure that the media remain within the acceptable temperature range.

The shelf life of prepared media depends on the medium type and the storage conditions. Media that contain labile components, such as antibiotics or growth factors, may have a shorter shelf life than media that contain only stable components. The manufacturer instructions or the laboratory validation data should specify the shelf life for each medium type. Laboratories should label each batch of medium with the preparation date and the expiration date.

The [American Type Culture Collection](https://www.atcc.org/) provides guidance on the storage and maintenance of reference strains, which includes the conditions needed to preserve viability. The same principles apply to the storage of prepared media. Media that are stored beyond their shelf life should be discarded and replaced with a fresh batch.

## Growth Promotion Testing Procedures

### Inoculum Preparation

The preparation of the inoculum for growth promotion testing begins with the reference strain. The strain should be grown on a medium that supports its growth under the recommended conditions. The growth should be examined for purity and the expected colony characteristics. A suspension of the organism should be prepared in a sterile diluent that maintains viability.

The concentration of the suspension should be adjusted to produce a test inoculum that contains a small number of organisms. The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) describes the use of a McFarland standard to adjust the concentration of bacterial suspensions. The suspension should be diluted so that the test inoculum contains the target number of organisms.

The number of organisms in the test inoculum should be confirmed by plating a sample of the suspension onto a nonselective medium and counting the colonies after incubation. This step provides the actual number of organisms used in the test and allows the laboratory to interpret the results accurately. If the inoculum contains too many organisms, the test may not detect a medium that is only partially deficient.

### Inoculation and Incubation

The test medium should be inoculated with the prepared suspension using a method that distributes the organisms evenly across the surface of the medium. The inoculum should be spread over the surface to allow the organisms to grow as individual colonies. The plates should be incubated under the conditions specified for the medium and the organism.

The incubation conditions include the temperature, the atmosphere, and the duration. Fastidious organisms often require an atmosphere that contains carbon dioxide or a reduced oxygen concentration. The incubation conditions should be documented for each test and should match the conditions used for clinical specimens.

The plates should be examined at the specified intervals to determine whether growth has occurred. The growth should be compared to the expected characteristics for the organism and the medium. The number of colonies should be counted and the appearance of the colonies should be recorded.

### Interpretation of Results

The result of a growth promotion test is interpreted by comparing the observed growth to the acceptance criteria for the medium. The acceptance criteria should specify the minimum number of colonies that must be present and the expected colony characteristics. A medium that supports the growth of the target organism at the expected level passes the test.

A medium that fails to support the growth of the target organism, or that supports growth that is less than the expected level, fails the test. The failure should be documented and the medium should not be used for clinical specimens. The cause of the failure should be investigated and corrective action should be taken.

The [Clinical and Laboratory Standards Institute](https://clsi.org/standards/products/microbiology) provides the framework for quality control in microbiology laboratories and emphasizes the importance of documenting the results of quality control tests. The documentation should include the interpretation of the result and the action taken when a test fails.

## Selective Media and Inhibitory Components

### Selective Agent Verification

Selective media for fastidious organisms contain antibiotics or other components that inhibit the growth of competing organisms. The quality control program must verify that the selective agents are present at the correct concentration and that they inhibit the growth of the target organisms. The selective agents must also not inhibit the growth of the target organism.

The quality control test for a selective medium should include a test with the target organism to confirm that the medium supports its growth. The test should also include an organism that the selective agents are intended to inhibit. The inhibitory organism should not grow on the medium, or should grow only to a limited extent that is specified in the acceptance criteria.

The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) describes the use of selective media for the isolation of foodborne pathogens and the quality control procedures that are used to verify the performance of these media. The same principles apply to the selective media used for fastidious organisms in clinical laboratories.

### Inhibitory Organism Selection

The selection of the organism used to test the inhibitory properties of a selective medium should be based on the organisms that the medium is intended to exclude. For example, a selective medium for *Campylobacter* species should be tested with a strain of *Escherichia coli* or another organism that is expected to be inhibited by the selective agents.

The inhibitory organism should be tested at a concentration that is similar to the concentration of the target organism. The test should confirm that the inhibitory organism does not grow on the medium or that its growth is limited to the extent specified in the acceptance criteria. The result should be documented in the quality control records.

The [American Type Culture Collection](https://www.atcc.org/) provides reference strains for a wide range of organisms, including the organisms that are used to test the selectivity of culture media. The use of authenticated strains ensures that the test results are reliable and that the medium performs as expected.

## Documentation and Record Keeping

### Quality Control Records

The quality control records for culture media should include all the information needed to trace the performance of each batch of medium. The records should include the medium name and lot number, the preparation date, the expiration date, the reference strain used, the inoculum concentration, the incubation conditions, and the observed result. The records should also include the interpretation of the result and the corrective action taken when the test failed.

The records should be maintained in a format that allows the laboratory to review the performance of each medium over time. The review should identify any trends in the quality control results, such as a medium that is failing more frequently than expected. The review should also identify any changes in the preparation or storage procedures that may have affected the performance of the medium.

The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) describes the documentation requirements for food microbiology laboratories and emphasizes the importance of accurate and complete records. The same principles apply to the documentation of quality control for culture media in clinical laboratories.

### Corrective Action Documentation

When a medium fails the quality control test, the laboratory must document the corrective action taken. The corrective action should include the investigation of the cause of the failure, the action taken to correct the problem, and the verification that the corrective action was effective. The corrective action should be documented in the quality control records.

The investigation of a failed medium should include a review of the preparation records, the storage conditions, and the test procedure. The investigation should also include a review of the reference strain to confirm that the strain was viable and that the inoculum was prepared correctly. The corrective action may include the preparation of a new batch of medium, the adjustment of the preparation procedure, or the replacement of a component.

The [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html) provides guidance on the safe handling of microorganisms in the laboratory. The corrective action for a failed medium should be performed in accordance with the biosafety procedures for the laboratory.

## Common Failure Patterns and Troubleshooting

### Contamination of the Medium

Contamination of the culture medium is a common cause of quality control failure. The contamination can occur during the preparation of the medium, during the storage, or during the inoculation of the test. The contamination can be detected by the presence of colonies that do not match the expected characteristics of the target organism.

The investigation of a contaminated medium should include a review of the preparation procedure and the storage conditions. The preparation should be performed in a clean environment and the equipment should be sterilized before use. The storage conditions should be checked to confirm that the medium was stored at the correct temperature and that the container was sealed.

### Loss of Viability of the Reference Strain

The reference strain can lose viability if it is not maintained under the correct conditions. The strain should be stored according to the instructions from the culture collection. The strain should be subcultured only the recommended number of times to prevent the loss of viability or the change in characteristics.

The investigation of a failed growth promotion test should include a check of the viability of the reference strain. The strain should be subcultured on a medium that is known to support its growth and the growth should be observed. If the strain does not grow on the control medium, the strain should be replaced with a fresh culture from the culture collection.

### Incorrect Incubation Conditions

The incubation conditions are critical for the growth of fastidious organisms. The temperature, the atmosphere, and the duration of the incubation must be correct for the organism and the medium. The incubation conditions should be checked and documented for each test.

The investigation of a failed growth promotion test should include a review of the incubation conditions. The temperature of the incubator should be checked and the atmosphere should be verified. The duration of the incubation should be reviewed to confirm that the plates were incubated for the correct time.

## Biosafety and Containment Considerations

### Risk Assessment for Reference Strains

The reference strains used for quality control testing are biological materials that require handling in accordance with the biosafety principles. The [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html) provides the framework for the risk assessment and the containment of microorganisms in the laboratory. The risk assessment should consider the characteristics of the organism, the procedures that are performed, and the equipment that is used.

The reference strains for fastidious organisms may include organisms that are classified at a biosafety level that requires specific containment. The laboratory should follow the biosafety procedures for the level of the organism. The procedures should include the use of personal protective equipment, the use of a biological safety cabinet for procedures that generate aerosols, and the decontamination of the waste.

### Decontamination and Waste Disposal

The waste from the quality control testing should be decontaminated before disposal. The decontamination should be performed in accordance with the laboratory biosafety procedures. The decontamination method should be appropriate for the organism and the waste type.

The [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html) provides guidance on the decontamination of the laboratory waste. The waste should be placed in the appropriate containers and the decontamination should be performed in a manner that is verified to be effective.

## Antimicrobial Resistance and Quality Control Context

The quality control of culture media is connected to the broader context of antimicrobial resistance. The [World Health Organization](https://www.who.int/health-topics/antimicrobial-resistance) describes antimicrobial resistance as a global health threat that requires surveillance and stewardship. The accurate identification of the organisms is the foundation for the antimicrobial susceptibility testing that informs the treatment decisions.

The quality control of the culture media ensures that the organisms are correctly identified and that the susceptibility testing is performed on the organisms that are present in the specimen. The use of the reference strains for quality control supports the accuracy of the identification and the susceptibility testing.

The [Clinical and Laboratory Standards Institute](https://clsi.org/standards/products/microbiology) provides the standards for the antimicrobial susceptibility testing. The quality control of the culture media is a component of the quality assurance program that supports the accuracy of the susceptibility testing results.

## Professional Escalation Criteria

The laboratory professional should escalate the quality control issue when the failure cannot be resolved by the routine investigation. The escalation should include the documentation of the investigation and the corrective action that was performed. The escalation should be directed to the laboratory supervisor or the quality assurance officer.

The escalation criteria include the repeated failure of a medium that was prepared according to the standard procedure, the failure of a medium that cannot be traced to a specific cause, and the failure of a medium that affects the clinical results. The escalation should also include the failure of a reference strain that cannot be resolved by the replacement of the strain.

The [American Society for Microbiology](https://asm.org/) provides professional resources for the laboratory professionals and emphasizes the importance of the quality assurance in the microbiology laboratory. The escalation of the quality control issues is a component of the professional practice that ensures the accuracy of the laboratory results.

## Records and Measurements

### Standardized Test Records

The quality control records should be standardized to allow the comparison of the results over time. The records should include the fields that are needed for the review of the results. The records should be reviewed on a regular basis to identify the trends and to ensure that the quality control program is effective.

The records should include the following information for each test:

| Record Field | Description | Example |
| --- | --- | --- |
| Medium name and lot number | The identity of the medium and the batch that was tested | Chocolate agar, lot 2024-001 |
| Preparation date | The date the medium was prepared | 2024-01-15 |
| Reference strain | The strain used for the test | ATCC 49247 |
| Inoculum concentration | The number of organisms used in the test | 100 CFU per plate |
| Incubation conditions | The temperature, atmosphere, and duration | 35 degrees Celsius, 5 percent carbon dioxide, 24 hours |
| Observed result | The growth observed on the medium | Growth present, 50 colonies |
| Interpretation | The pass or fail result | Pass |
| Corrective action | The action taken when the test failed | The medium was discarded and a new batch was prepared |

### Decision Table for Quality Control Actions

The decision table below summarizes the actions that should be taken based on the quality control test results:

| Test Result | Interpretation | Action |
| --- | --- | --- |
| The target organism grows at the expected level | The medium passes the quality control test | The medium can be used for clinical specimens |
| The target organism does not grow | The medium fails the quality control test | The medium should be discarded and the cause of the failure should be investigated |
| The target organism grows but the colony characteristics are not typical | The medium may be deficient | The medium should be discarded and the preparation procedure should be reviewed |
| The inhibitory organism grows on the selective medium | The selective agents are not working | The medium should be discarded and the selective agent concentration should be checked |

## Practical Implementation Steps

### Step 1: Establish the Quality Control Program

The laboratory should establish a quality control program that includes the procedures for the testing of each medium that is used for the fastidious organisms. The program should specify the reference strains that are used, the test procedures, the acceptance criteria, and the documentation requirements. The program should be reviewed and approved by the laboratory supervisor.

### Step 2: Obtain the Reference Strains

The reference strains should be obtained from the [American Type Culture Collection](https://www.atcc.org/) or another recognized culture collection. The strains should be stored according to the instructions from the culture collection. The strains should be maintained and the viability should be checked on a regular basis.

### Step 3: Prepare the Test Inoculum

The test inoculum should be prepared from the reference strain. The organism should be grown on a medium that supports its growth. The suspension should be prepared in a sterile diluent and the concentration should be adjusted to the target number of organisms. The actual number of organisms should be confirmed by plating the suspension on a nonselective medium.

### Step 4: Inoculate and Incubate the Test Medium

The test medium should be inoculated with the prepared suspension. The plates should be incubated under the conditions specified for the medium and the organism. The plates should be inspected at the incubation time and the growth should be recorded.

### Step 5: Interpret the Result and Document the Outcome

The result should be interpreted against the acceptance criteria. The result should be documented in the quality control records. If the test fails, the corrective action should be performed and documented.

## Common Failure Patterns

### The Medium Does Not Support the Growth of the Target Organism

The most common failure pattern is the medium that does not support the growth of the target organism. The cause of the failure can be the incorrect preparation of the medium, the degradation of the components, or the incorrect incubation conditions. The investigation should include a review of the preparation records and the incubation conditions.

### The Medium Supports the Growth of the Target Organism but the Colony Characteristics Are Not Typical

The medium may support the growth of the target organism but the colony characteristics may not be typical. This can indicate that the medium is deficient in a component that affects the colony morphology. The medium should be discarded and the preparation procedure should be reviewed.

### The Selective Medium Does Not Inhibit the Growth of the Competing Organisms

The selective medium may fail to inhibit the growth of the competing organisms. This can indicate that the selective agents are not present at the correct concentration or that the selective agents have lost their activity. The medium should be discarded and the selective agent concentration should be checked.

## Limitations of the Quality Control Testing

The quality control testing confirms that the medium supports the growth of the reference strain under the test conditions. The test does not confirm that the medium will support the growth of all strains of the target organism. The clinical specimens may contain organisms that have different growth requirements than the reference strain.

The quality control testing does not confirm that the medium will support the growth of the organism from a clinical specimen. The clinical specimen may contain the organism in a low number or the organism may be stressed by the conditions in the specimen. The quality control testing provides a baseline for the performance of the medium but the clinical results should be interpreted in the context of the specimen.

The quality control testing does not replace the need for the identification of the organism. The growth of the organism on the medium is the first step in the identification process. The identification of the organism requires the additional testing that is performed after the organism is isolated.

## Risk-Based Media QC Prioritization and Workload Management

### The Problem of Unbounded QC Demands

Laboratories that test every medium batch against every relevant reference strain can create a quality control workload that competes with diagnostic work for time, incubator space, and staff attention. The [American Society for Microbiology](https://asm.org/) emphasizes that laboratory practice must balance quality assurance with operational efficiency. When a laboratory prepares 15 different media types and maintains 20 reference strains, the number of possible medium-strain combinations can exceed 300. Testing every combination for every batch is rarely feasible and often unnecessary.

A risk-based prioritization framework allows the laboratory to direct QC effort toward the media and organisms that present the greatest risk to patient care. The framework assigns each medium-organism pair to a testing frequency tier based on the consequences of a silent failure, the historical performance of the medium, and the difficulty of detecting a problem through other means.

### Risk Tier Assignment Criteria

The first tier includes media that are used for primary isolation of fastidious organisms from clinical specimens where no backup medium is available. A failure in this tier directly causes a missed or delayed diagnosis. Examples include chocolate agar for *Haemophilus influenzae* and *Neisseria gonorrhoeae*, buffered charcoal yeast extract agar for *Legionella* species, and selective media for *Campylobacter* species. These media should be tested with every new batch before any clinical use.

The second tier includes media that are used for confirmation, subculture, or antimicrobial susceptibility testing. A failure in this tier delays the result but does not necessarily cause a missed organism because the organism has already been isolated on the primary medium. Examples include Mueller-Hinton agar for susceptibility testing and the media used to maintain reference strains. These media should be tested with every new batch, but the testing can be scheduled to occur before the batch is released for the specific procedure.

The third tier includes media that are used infrequently, that have a long documented history of consistent performance in the laboratory, or that are used only for noncritical procedures. These media can be tested on a rotating schedule, such as every third batch or monthly, provided that the laboratory has documented evidence that the medium formulation and preparation procedure have not changed.

The [Clinical and Laboratory Standards Institute](https://clsi.org/standards/products/microbiology) standards for microbiology quality control describe the expectation that laboratories establish procedures that are appropriate for their testing volume and risk profile. The risk tier approach is a documented procedure that the laboratory can defend during an inspection because it is based on a written risk assessment instead of an arbitrary reduction in testing.

### Workload Calculation and Scheduling

The laboratory should calculate the total QC workload for a typical month before establishing the schedule. The calculation should include the number of medium batches prepared, the number of reference strains that require testing, the time required for each test, and the incubator capacity. The calculation should also include the time required for the documentation and the review of the results.

A practical scheduling method is to assign each medium-organism pair to a specific day of the week. For example, the laboratory can test the chocolate agar and the *Haemophilus influenzae* strain on Monday, the *Neisseria gonorrhoeae* strain on Tuesday, and the selective media on Wednesday. This approach distributes the workload evenly and ensures that the incubator is not overloaded on a single day.

The schedule should be documented and posted in the media preparation area. The schedule should include the medium name, the reference strain, the test date, and the expected completion date. The schedule should be reviewed monthly to confirm that the workload is manageable and that the testing is being completed on time.

### Batch Size and Testing Frequency

The batch size for media preparation affects the QC workload. A laboratory that prepares small batches of media more frequently will perform more QC tests than a laboratory that prepares larger batches less frequently. The laboratory should consider the shelf life of the medium and the expected usage when deciding the batch size.

For media with a short shelf life, such as those that contain labile antibiotics or growth factors, the laboratory should prepare smaller batches to reduce the waste of expired media. The QC testing for these batches should be performed on every batch because the risk of a failed batch is higher when the components are labile.

For media with a long shelf life, the laboratory can prepare larger batches and test every batch. The larger batch size reduces the total number of QC tests per month while maintaining the testing of every batch.

### Documentation of the Risk Assessment

The risk assessment that supports the QC schedule should be documented. The documentation should include the list of media, the risk tier assigned to each medium, the rationale for the tier assignment, and the schedule for the testing. The documentation should be reviewed annually or when the laboratory changes the medium formulation, the preparation procedure, or the reference strains.

The [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam) describes the documentation practices for food microbiology laboratories and emphasizes the importance of the written procedures. The same principle applies to the documentation of the QC schedule in the clinical laboratory. The written risk assessment provides the evidence that the laboratory has considered the risks and has established a schedule that is appropriate for the laboratory.

### Monitoring the Effectiveness of the Schedule

The laboratory should monitor the effectiveness of the QC schedule by reviewing the failure rates and the turnaround times. The review should identify any medium that is failing more frequently than expected and any medium that is not being tested according to the schedule. The review should also identify any medium that is not being tested because the schedule is not realistic for the workload.

The review should be performed monthly and the results should be documented. The review should include the number of tests performed, the number of tests that passed, the number of tests that failed, and the corrective actions taken. The review should also include the number of clinical specimens that were processed during the month and the number of the specimens that had a delayed result because of the media failure.

The [World Health Organization](https://www.who.int/health-topics/antimicrobial-resistance) describes the importance of the surveillance and the monitoring for the antimicrobial resistance. The same principle applies to the monitoring of the QC program. The monitoring provides the data that the laboratory uses to improve the QC process and to reduce the risk of the media failure.

### Common Scheduling Errors

A common error is the testing of the reference strain on the same day that the medium is prepared. The medium may not have equilibrated to the correct pH or the supplements may not have been fully dissolved. The test should be performed after the medium has been stored under the correct conditions for the recommended time.

Another common error is the use of the same reference strain for multiple media without confirming that the strain is appropriate for each medium. The strain should be selected based on the organism that the medium is intended to support. The strain should be tested on a control medium that is known to support its growth to confirm the viability of the strain.

A third common error is the failure to document the schedule and the results. The schedule should be posted in the media preparation area and the results should be recorded in the QC log. The log should be reviewed by the laboratory supervisor on a regular basis.

### Escalation for Schedule Failures

The laboratory professional should escalate the schedule failure when the QC testing is not being completed according to the schedule. The escalation should include the reason for the missed test, the impact of the missed test on the clinical results, and the plan to complete the test. The escalation should be directed to the laboratory supervisor.

The escalation should also include the situation where the laboratory is unable to complete the QC testing because of the staffing or the workload. The supervisor should review the workload and the schedule and should make the adjustments that are needed to ensure that the QC testing is completed.

The [American Society for Microbiology](https://asm.org/) provides the professional resources for the laboratory professionals and emphasizes the importance of the quality assurance in the microbiology laboratory. The escalation of the QC schedule failures is a component of the professional practice that ensures the accuracy of the laboratory results.

## Frequently Asked Questions

### What is the purpose of growth promotion testing for culture media?

Growth promotion testing verifies that a new batch of culture medium supports the growth of the target organism before the medium is used for clinical specimens. The test uses a small number of organisms from a reference strain and checks that the growth occurs under the specified incubation conditions. The test provides the evidence that the medium is suitable for its intended use.

### Which reference strains should be used for quality control testing?

The reference strains should be obtained from a recognized culture collection such as the [American Type Culture Collection](https://www.atcc.org/). The strains should be authenticated for their identity and characteristics. The strain should be selected based on the organism that the medium is intended to support.

### How often should culture media be tested for quality control?

Each new batch of prepared medium should be tested before it is used for clinical specimens. The testing should be performed for each batch of medium that is prepared in the laboratory. The testing should also be performed when the preparation procedure is changed or when a component of the medium is changed.

### What should be done when a medium fails the quality control test?

The medium should be discarded and the cause of the failure should be investigated. The investigation should include a review of the preparation records, the storage conditions, and the test procedure. The corrective action should be performed and the result should be documented.

### What is the difference between a selective medium and a nonselective medium?

A selective medium contains components that inhibit the growth of the competing organisms while allowing the growth of the target organism. A nonselective medium does not contain the inhibitory components and supports the growth of a wider range of organisms. The quality control testing for a selective medium includes the verification of the inhibitory effect.

### How should the reference strains be stored?

The reference strains should be stored according to the instructions from the culture collection. The storage conditions should be documented and the strains should be checked for viability on a regular basis. The strains should be subcultured only the recommended number of times to preserve their characteristics.

### What is the role of the quality control in the antimicrobial susceptibility testing?

The quality control of the culture media ensures that the organism is isolated and identified correctly. The accurate identification is the foundation for the antimicrobial susceptibility testing. The [Clinical and Laboratory Standards Institute](https://clsi.org/standards/products/microbiology) provides the standards for the susceptibility testing and the quality control is a component of the quality assurance.

### What biosafety precautions should be followed during the quality control testing?

The quality control testing should be performed in accordance with the biosafety procedures for the organism. The risk assessment should be performed and the containment should be appropriate for the risk level. The waste should be decontaminated according to the laboratory procedures.

## Related Diagnostic Guides

- [Microbiology Quality Control: Essential Practices for Reliable Culture Results](/knowledge/diagnostics/microbiology/microbiology-quality-control-essential-practices-for-reliable-culture-results)
- [Cell Culture Media: A Guide to Selection and Optimization](/knowledge/diagnostics/microbiology/cell-culture-media-a-guide-to-selection-and-optimization)
- [Microbiology in Water Testing: Methods and Significance](/knowledge/diagnostics/microbiology/microbiology-in-water-testing-methods-and-significance)

## References and Further Reading

- [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html). CDC and NIH.
- [Microbiology Resources](https://asm.org/). American Society for Microbiology.
- [Antimicrobial Resistance](https://www.who.int/health-topics/antimicrobial-resistance). World Health Organization.
- [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam). U.S. Food and Drug Administration.
- [Antimicrobial Susceptibility Testing](https://clsi.org/standards/products/microbiology). Clinical and Laboratory Standards Institute.
- [Culture Collections](https://www.atcc.org/). American Type Culture Collection.
- [Laboratory Biosafety Manual](https://www.who.int/publications/i/item/9789240011311). World Health Organization.
- [NCBI Literature Resources](https://www.ncbi.nlm.nih.gov/). National Center for Biotechnology Information.
- [Antimicrobial susceptibility testing for Helicobacter pylori: sensitivity test results and their clinical relevance.](https://pubmed.ncbi.nlm.nih.gov/10974151). Current pharmaceutical design, 2000.
- [Antimicrobial Susceptibility Testing of Leptospira spp. in the Lao People's Democratic Republic Using Disk Diffusion.](https://pubmed.ncbi.nlm.nih.gov/30887950). The American journal of tropical medicine and hygiene, 2019.
- [Elucidation of auxotrophic deficiencies of Bacillus pumilus DSM 18097 to develop a defined minimal medium.](https://pubmed.ncbi.nlm.nih.gov/29986716). Microbial cell factories, 2018.

> This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.