Related Experiment Video
Updated: Jun 25, 2026
Development and Assessment of Intracellular Infection Models for Staphylococcus aureus
Published on: January 17, 2025
Intracellular activity of antibiotics against Staphylococcus aureus in a mouse peritonitis model
Anne Sandberg1, Jonas H R Hessler, Robert L Skov
1National Center for Antimicrobials & Infection Control, Statens Serum Institut, 5 Artillerivej, Building 47, Room 201, Copenhagen S DK-2300, Denmark. asa@ssi.dk
Abstract:
Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extra- and intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra- and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log(10) number of CFU/ml (Delta log(10) CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 Delta log(10) CFU/ml) > cefuroxime (3.56 Delta log(10) CFU/ml) > rifampin (1.86 Delta log(10) CFU/ml) > gentamicin (0.61 Delta log(10) CFU/ml) > azithromycin (0.21 Delta log(10) CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.
Insights
Treating Staphylococcus aureus infections is challenging due to recurrence. A new mouse model shows antibiotics have varying intracellular effects, crucial for effective treatment strategies against persistent bacteria.
Area of Science:
- Microbiology
- Pharmacology
- Infectious Diseases
Background:
- Antibiotic treatment of Staphylococcus aureus infections faces challenges like slow response and recurrence.
- Intracellular persistence of S. aureus is a key factor contributing to treatment difficulties.
- Understanding intracellular antibiotic activity is vital for improving infection outcomes.
Purpose of the Study:
- To develop and utilize a novel in vivo mouse model to assess the intracellular activity of antibiotics against Staphylococcus aureus.
- To differentiate between extracellular and intracellular effects of various antibiotics in a peritonitis/sepsis model.
- To evaluate the correlation between antibiotic accumulation and intracellular efficacy.
Main Methods:
- A mouse peritonitis/sepsis model was employed as the primary in vivo system.
- Quantitative ex vivo assays were used to differentiate extra- and intracellular bacterial counts.
- Electron microscopy confirmed the intracellular presence of S. aureus.
- Five antibiotics (dicloxacillin, cefuroxime, gentamicin, azithromycin, rifampin) were tested for their intracellular effects.
Main Results:
- The in vivo model successfully distinguished between extracellular and intracellular antibiotic effects.
- Antibiotics showed varying intracellular efficacy, ranked as follows (Delta log(10) CFU/ml): dicloxacillin > cefuroxime > rifampin > gentamicin > azithromycin.
- Factors such as dose, exposure time, and treatment timing significantly influenced intracellular activity.
- A weak correlation was observed between antibiotic intracellular accumulation and actual intracellular effect.
Conclusions:
- The developed mouse model is effective for evaluating in vivo intracellular antibiotic activity against S. aureus.
- Actual intracellular efficacy, not just accumulation or MICs, is critical for predicting treatment success.
- Optimizing dosing strategies and treatment timing is essential for targeting intracellular bacteria.
- This study highlights the need for in vivo experimental models to accurately assess antibiotic performance against intracellular pathogens.
