Francis S. Mah, MD, assistant professor of ophthalmology and co-medical director of the Charles T. Campbell Eye Microbiology Laboratory at the University of Pittsburgh Medical Center.
"...He added, however, that resistance based on in vitro data may be over-reported in the ophthalmology literature because that determination is based on serum susceptibility standards. In fact, results of a study evaluating the activity of a variety of antibiotics against methicillin-resistant Staphylococcus aureus (MRSA) showed that in vitro antibiotic resistance can be overcome in vivo with intensive therapy.
"In this study evaluating gatifloxacin, vancomycin [Vancocin, ViroPharma], levofloxacin, ciprofloxacin [Ciloxan, Alcon], and cefazolin [Apothecon], only vancomycin should have been effective for MRSA eradication, but gatifloxacin also significantly reduced the bacterial counts and was equivalent to cefazolin and vancomycin," Dr. Mah said.
Another issue to consider when evaluating in vitro susceptibility data is that those studies are performed using pure antibiotic rather than commercially available formulations. The relevance of that fact with respect to ophthalmic antibiotic preparations is that the preservative benzalkonium chloride (BAK) can increase the speed of microbial kill.
"In a rabbit model of MRSA infection, we showed the combination of gatifloxacin plus BAK hastened the speed of recovery compared with gatifloxacin alone," Dr. Mah said ..."