In vitro selection of resistance by in vivo concentrations of fluoroquinolones in Pseudomonas aeruginosa and Staphylococcus aureus

Abstract number: P1046

Drago L., Nicola L., Zanini S., De Vecchi E.

Objective: To compare the ability to select for resistance in Staphylococcus aureus and Pseudomonas aeruginosa of levofloxacin (LVX), ciprofloxacin (CIP), and prulifloxacin (PRU).

Method: Twenty strains of S. aureus and of P. aeruginosa susceptible to fluoroquinolones, isolated from lower respiratory tract, mainly of nosocomial origin, were considered. Frequency of spontaneous single-step mutation allowing for bacterial growth in presence of plasma and epithelial lining fluid (ELF) peak and trough concentrations were calculated. Multistep selection of resistance was evaluated by performing 10 serial subcultures on agar plates containing a linear gradient ranging from the peak to the trough concentration of each drug, followed by 10 subcultures on antibiotic-free agar. LVX was tested by considering concentrations obtained after 500 mg and 750 mg administration. MICs were determined after 1,5,10 passages on antibiotic-gradient plates and after 10 subcultures on antibiotic free agar.

Resistant strains selected were characterised for DNA mutations by sequencing gyrA, gyrB, grlA, parC and parE genes.

Results: Frequencies of mutations in P. aeruginosa were similar at plasma and ELF peak concentrations for LVX (median: 8×10-10 and <10-11 at plasma, 4×10-11 and <10-11 at ELF, LVX 500 mg and LVX 750 mg, respectively) and CIP (median: <10-11 and 8.67×10-10, at plasma and ELF, respectively) but higher for PRU (median: 1.25×10-8 and 1.01×10-7 at plasma and ELF, respectively). In S. aureus higher frequencies of mutations were obtained both with CIP (median: 1.11×10-7) and PRU (median: 2.76×10-10) than with LVX (< 10-11) at plasma peak concentration.

P. aeruginosa was more prone than S. aureus in becoming resistant to fluoroquinolones after multi step selection (58 resistant strains vs 47, P. aeruginosa vs S. aureus). The lowest number of resistant strains was selected by LVX 750 mg both in P. aeruginosa (n = 33) and S. aureus (n = 3).

Mutations in gyrA, parC, grlA and gyrB were mostly associated with resistance.

Conclusions: Fluoroquinolones are characterised by a different ability to select for resistance, depending on bacterial species and in vivo drug concentrations. Generally, ELF concentrations selected for less resistant strains than plasma concentrations. Globally, LVX selected for less resistance than comparators both in P. aeruginosa and S. aureus.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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