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In vitro activity of ciprofloxacin against methicillin-susceptible and methicillin-resistant staphylococci
B Gahrn-Hansen1, P Søgaard, M Arpi
1Department of Antibiotics, Statens Seruminstitut, Copenhagen S, Denmark.
Abstract:
Ciprofloxacin was found to be active against selected strains of methicillin-resistant as well as methicillin-susceptible Staphylococcus aureus and Staphylococcus epidermidis, having minimal inhibitory concentrations less than 0.8 micrograms/ml with inocula of approximately 10(4) CFU/ml. When killing kinetic studies were done with inocula of approximately 10(6) CFU/ml and various ciprofloxacin concentrations, the killing effect was low (less than 3 log10 in 6 h) and regrowth of the culture was seen after 24 h in at least one Staphylococcus aureus strain. After 48 h incubation, mutants with MICs eightfold that of the parental strain had grown to a number of approximately 10(10) CFU/ml.
Insights
Ciprofloxacin shows activity against Staphylococcus aureus and Staphylococcus epidermidis strains. However, high bacterial loads may lead to reduced killing efficacy and the emergence of resistant mutants.
Area of Science:
- Microbiology
- Pharmacology
- Infectious Diseases
Background:
- Staphylococcus aureus and Staphylococcus epidermidis are significant human pathogens.
- Methicillin-resistance is a growing clinical concern.
- Fluoroquinolones, including ciprofloxacin, are broad-spectrum antibiotics.
Purpose of the Study:
- To evaluate the in vitro activity of ciprofloxacin against methicillin-resistant and methicillin-susceptible Staphylococcus strains.
- To assess the killing kinetics and potential for resistance development.
Main Methods:
- Minimal Inhibitory Concentration (MIC) determination.
- Killing kinetic studies with varying ciprofloxacin concentrations.
- Mutant selection and growth analysis.
Main Results:
- Ciprofloxacin exhibited low MICs (<0.8 µg/ml) against both susceptible and resistant strains at low inocula (10^4 CFU/ml).
- Killing kinetics at higher inocula (10^6 CFU/ml) showed limited killing (<3 log10 in 6h) and regrowth by 24h for S. aureus.
- Mutants with eightfold increased MICs grew to high densities (approx. 10^10 CFU/ml) after 48h incubation.
Conclusions:
- Ciprofloxacin demonstrates potent in vitro activity against Staphylococcus strains at standard testing concentrations.
- High bacterial burdens may compromise ciprofloxacin's bactericidal effect, potentially promoting the selection of resistant mutants.
- Clinical efficacy may be strain-dependent and influenced by bacterial load.