Related Experiment Video
Updated: Jul 11, 2026
06:36
Improved Enzyme Protection Assay to Study Staphylococcus aureus Internalization and Intracellular Efficacy of Antimicrobial Compounds
Published on: September 8, 2021
Daptomycin inoculum effects and mutant prevention concentration with Staphylococcus aureus.
Brian Quinn1, Syed Hussain, Muhammad Malik
1Public Health Research Institute and Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, 225 Warren Street, Newark, NJ 07103, USA.
The Journal of Antimicrobial Chemotherapy
|October 2, 2007
Summary
Determining antimicrobial mutant prevention concentration (MPC) is affected by bacterial inoculum size. Using liquid cultures with calcium reduces inoculum effects and helps identify daptomycin resistance mutations in Staphylococcus aureus.
Area of Science:
- Microbiology
- Pharmacology
- Antimicrobial Resistance
Background:
- Antimutant activity of antimicrobials is assessed by comparing drug pharmacokinetics with the mutant prevention concentration (MPC).
- The impact of large bacterial inocula, which can reduce susceptibility, on MPC determination has not been extensively studied.
- Staphylococcus aureus is a significant human pathogen where understanding antimicrobial activity is crucial.
Purpose of the Study:
- To investigate the effect of inoculum size on the determination of daptomycin's mutant prevention concentration (MPC) in Staphylococcus aureus.
- To evaluate the influence of calcium (Ca2+) on MIC and MPC values.
- To assess the genetic stability of recovered mutants.
Main Methods:
- Staphylococcus aureus inoculum size was varied in solid (agar) and liquid media containing daptomycin and varying concentrations of Ca2+.
- Minimum Inhibitory Concentration (MIC) and Mutant Prevention Concentration (MPC) were determined.
- Population analysis profiles were used to extrapolate MPC values.
- Recovered bacterial colonies were re-tested for susceptibility and screened for genetic mutations.
Main Results:
- Increasing Ca2+ concentration from 1 to 50 mM reduced daptomycin's MIC by 2-fold and MPC from 20 to 3 mg/L.
- In liquid medium with 1 mM Ca2+ and an inoculum density of 10(7) CFU/mL, the MPC was between 2.5 and 5 mg/L.
- Bacteria recovered from liquid assays showed a 4- to 8-fold increase in MIC and possessed point mutations in the mprF gene, unlike those from agar.
Conclusions:
- Inoculum effects on MPC can be mitigated by using low-density liquid bacterial cultures.
- Retesting putative mutants is essential, as stable mutants with genetic variations (mprF gene) were recovered from liquid but not agar media.
- Daptomycin's MPC in S. aureus was found to be below the minimal plasma drug concentration at approved doses, suggesting resistance rarely emerges.
Related Concept Videos
01:25Clinical Significance of Antibiotic Resistance
Methicillin-resistant Staphylococcus aureus (MRSA) presents a critical public health threat, arising from its capacity to resist β-lactam antibiotics due to acquisition of the mecA gene within the staphylococcal cassette chromosome mec (SCCmec). This gene encodes penicillin-binding protein 2a (PBP2a), which impairs binding efficacy of methicillin and other β-lactams. MRSA has evolved into distinct clonal lineages impacting humans and animals alike, reinforcing its significance within the One...
01:20Estimation of k and VD of Aminoglycosides
Aminoglycosides are a class of antibiotics used to treat various bacterial infections. Clinicians must determine the elimination rate constant (k) and volume of distribution (VD) to optimize therapeutic efficacy and minimize toxicity. The k value represents the rate at which the drug is removed from the body, and the VD reflects the degree to which the drug distributes into body tissues. Accurately estimating these parameters allows healthcare professionals to tailor drug dosing to individual...
01:28Antimicrobial Effectiveness
The effectiveness of antimicrobial agents depends on various factors influencing their ability to eliminate microbial populations. Larger microbial populations require more time for complete eradication, emphasizing the importance of population size analysis when evaluating antimicrobial efficacy.Microbial resistance to antimicrobial agents varies significantly. Highly resilient microorganisms include endospores, gram-negative bacteria, and non-enveloped viruses, while prions are exceptionally...