Injectable vs. Research Antibiotics

The answer is YES. However, research antibiotics will give accurate results to the best of my knowledge. Talk to your supervisor and follow his /her advice. Make sure you are following guidelines from the team working elsewhere.

While it is possible to use injectable antibiotics for broth microdilution, it is generally not recommended for determining MIC values. Here's why:

Reasons for Caution:-

Formulation Differences: Injectable antibiotics often contain additives and preservatives that may interfere with the MIC determination1. These additional components could affect bacterial growth or interact with the antibiotic, leading to inaccurate results.

Concentration Uncertainty: Research-grade antibiotics come with precise concentration information, while injectable formulations may have some variability. This could lead to inconsistencies in preparing the correct dilutions for the MIC assay.

Standardization Issues: Standard protocols for MIC determination, such as those from CLSI and EUCAST, typically specify pure, research-grade antibiotics. Using injectable formulations may deviate from these standards, making comparing results across laboratories or with published data difficult.

Quality Control: Research-grade antibiotics undergo rigorous quality control to ensure purity and potency, which is crucial for accurate MIC determination. Injectable formulations may not meet the same standards for this specific purpose.

Practical Considerations

If you must use injectable antibiotics due to unavailability of research-grade options, consider the following:

Validation: Perform parallel tests with both injectable and research-grade antibiotics (if possible) to validate your results.

Documentation: Document the use of injectable antibiotics in your methodology and results, as this is a deviation from standard protocols.

Concentration Adjustment: When calculating concentrations for your dilution series, account for any additives or diluents in the injectable formulation.

Quality Control Strains: Use appropriate quality control strains to ensure your results' reliability, as CLSI and EUCAST recommended.

Yes, While injectable antibiotics for MIC determination are not ideal, it may be a temporary solution if research-grade antibiotics are unavailable. However, it's crucial to interpret the results cautiously and consider repeating the tests with research-grade antibiotics to ensure accuracy and compliance with standard protocols.

Using injectable antibiotics for broth microdilution and minimum inhibitory concentration (MIC) determination presents unique challenges compared to research-grade antibiotics, potentially affecting accuracy and standardisation in antimicrobial susceptibility testing. These differences, including formulation variations and quality control issues, can impact MIC results and the observed inoculum effect, highlighting the importance of careful consideration when selecting antibiotic formulations for laboratory testing.

Challenges with Injectable Antibiotics

Using injectable antibiotics for broth microdilution presents several challenges that can affect the accuracy of MIC determination. These formulations often contain additives and preservatives that may interfere with bacterial growth or interact with the antibiotic, potentially leading to inaccurate results. Additionally, injectable antibiotics may have concentration uncertainties, making preparing precise dilutions for MIC assays difficult.

The use of injectable antibiotics also deviates from standard protocols specified by organizations like CLSI and EUCAST, which typically require pure, research-grade antibiotics. This deviation can make comparing results across laboratories or with published data challenging. Furthermore, injectable formulations may not undergo the same rigorous quality control as research-grade antibiotics, potentially compromising the reliability and reproducibility of MIC determinations.

Differences in MIC Results

MIC results can vary significantly when using injectable antibiotics instead of research-grade formulations. The presence of additives and preservatives in injectable antibiotics may alter the dose-response curve, potentially leading to different MIC values. This can result in:

• Inconsistent concentration information affecting the accuracy of dilution series preparation
• Altered pharmacokinetic/pharmacodynamic relationships, complicating the interpretation of in vivo efficacy based on MIC values.
• Reduced comparability with standardised methods and published data
• Potential exaggeration or masking of the actual inoculum effect, particularly for high-inoculum infections or bacteria with higher MIC values.

To mitigate these issues, researchers should consider validating results obtained with injectable antibiotics against those from research-grade antibiotics when possible and clearly document the use of injectable formulations in their methodology and results reporting.

Inoculum Effect Variability

The inoculum effect, characterised by reduced antimicrobial activity at high bacterial counts, can vary between injectable and research-grade antibiotics. This variability stems from formulation differences, with injectable antibiotics potentially exaggerating or masking the actual inoculum effect due to additives and preservatives. High-inoculum infections more rapidly reach the stationary phase, diminishing the effect of antibiotics targeting penicillin-binding proteins. Researchers have observed this phenomenon with various antibiotics, including carbapenems and fluoroquinolones, regardless of formulation. To accurately assess the inoculum effect:

• Use appropriate quality control strains as recommended by CLSI and EUCAST
• Consider performing parallel tests with both injectable and research-grade antibiotics when possible
• Account for potential concentration variability in injectable formulations when preparing dilution series
• Document any deviations from standard protocols in methodology and results reporting

Practical Considerations for Researchers

Researchers should implement several precautionary measures when faced with the necessity of injectable antibiotics for MIC determination. Performing parallel tests with injectable and research-grade antibiotics (if available) can help validate results and identify discrepancies. It's crucial to meticulously document the use of injectable antibiotics in the methodology and results, as this represents a deviation from standard protocols. Researchers must also carefully adjust concentrations to account for any additives or diluents in the injectable formulation when preparing a dilution series. To ensure result reliability, employing appropriate quality control strains recommended by CLSI and EUCAST is essential. While not ideal, these steps can help mitigate some challenges associated with injectable antibiotics for MIC determination when research-grade options are unavailable.

Yes you can.

But you should set the concentration.

Formally, it is incorrect, for two reasons. First, injectables are not 100% pure substance, so the quantities would have to be readjusted.

Secondly, antibiotics for clinical use contain other substances (excipients, etc.) which, eventually, at least in theory, could alter the results of sensitivity tests, so that the results, especially at the quantitative level, may be erroneous.

Yes, injectable antibiotics can be used for broth microdilution to determine MIC, as long as they are properly reconstituted to a known concentration. However, research-grade antibiotics are typically preferred because they are standardized for laboratory use and may have fewer impurities or variations. It’s practical, but you may encounter minor variability with injectable forms, so it's important to verify concentrations accurately.