Kill-curve studies provide important data

Rapid eradication of surface flora is critical for effective antimicrobial endophthalmitis prophylaxis. That information is derived from kill-curve studies, not minimum inhibitory concentration data. Results from kill-curve studies for staphylococcal strains show gatifloxacin 0.3% reduces bacterial viability more rapidly and more completely than moxifloxacin 0.5%. The presence of benzalkonium chloride in the commercial preparation of gatifloxacin may contribute to its faster eradication rate.

Key Points

Dr. McDermott, professor of ophthalmology, Wayne State University School of Medicine, Detroit, presented results from kill-curve studies performed to compare the antimicrobial activity of the two fourth-generation fluoroquinolones.

Seven strains of Staphylococcus aureus and one strain each of S hominis and S epidermidis were incubated at physiological temperature (37° C) with an aliquot of the commercially available ophthalmic preparations of gatifloxacin and moxifloxacin to yield a final microbial concentration of 10⁵ to 10⁶ CFU/ml.

For all of the staphylococcal strains studied, colony counts were reduced to the no-recovery level (≤10 CFU/ml) within 30 minutes of incubation with gatifloxacin, whereas viability was detected for all strains even after 60 minutes of incubation with moxifloxacin.

Gram-positive bacteria, he said, are the most common pathogens causing postoperative endophthalmitis, and the flora of the patient's ocular surface is the primary etiologic source. Preoperative antimicrobial prophylaxis administered while the patient is in the holding area represents the most reliable way to reduce the surface microbial load. "In that setting, rapid eradication is essential," Dr. McDermott said.

Although minimum inhibitory concentration (MIC) data typically are considered to guide decisions on antimicrobial selection, they provide no information on the antimicrobial kill rate.

"Gatifloxacin and moxifloxacin have similar MIC values against gram-positive and gram-negative ocular pathogens, and because the concentrations applied using the commercially available preparations of those fluoroquinolones far exceed the MIC values, eventual eradication is predicted. However, the most relevant parameter for evaluating the potential utility of an antimicrobial for preoperative prophylaxis is its speed of kill," he said.

In the kill-curve study, all strains of S aureus were reduced by gatifloxacin to the no-recovery level within 30 minutes, and that outcome was achieved within 15 minutes for five of the strains. Moxifloxacin did not eradicate all of the S aureus isolates after 60 minutes of exposure.