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2001 Ocular Microbiology and Immunology Group, Abstract 15

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Postantibiotic Effect of Ciprofloxacin and Ofloxacin on Staphylococcus aureus
R.M. Gander, D.C. Cavuoti, P.V. Berkley, K. Alford and J.P. McCulley
UT Southwestern Medical Center, Dallas, TX

Purpose: The postantibiotic effect (PAE) describes the delayed growth of bacteria after a short exposure to antimicrobial agents. Determination of PAEs were used to compare the activity of ciprofloxacin and ofloxacin against ocular strains of S.aureus.

Methods: Using commercial ophthalmic solutions of ciprofloxacin and ofloxacin, three human ocular isolates of methicillin-ciprofloxacin - and ofloxacin-susceptible S.aureus were exposed to 3.0 µg/ml and 30.0 µg/ml of the two fluoroquinolones for 15, 30, 60 and 120 min. time periods. Suspensions of bacteria in logarithmic growth (approx 108 org/ml) were incubated in a shaking water bath at 37oC in the presence of antibiotics for specified time periods. Control suspensions without antibiotics were treated in a similar manner. After incubation, bacteria were removed from broth by filtration and resuspended in media. Again, cultures were incubated in a shaking water bath at 37oC. Colony counts were performed immediately after resuspension (time 0) and hourly thereafter. The PAE was calculated as the difference in time (hours) required for the test culture concentration to increase one log above that observed immediately after drug removal, compared to the time required for the control culture to increase one log from time 0.

Results: PAEs increased with increasing antimicrobial concentrations and exposure time for the S.aureus strains. Bacterial strain variations in PAE results were also observed. The S.aureus results for ciprofloxacin and ofloxacin were similar, as was reported previously for S.epidermidis strains.

This study was supported in part by Alcon Laboratories, INC.

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