We read with interest the article by Hsu and associates on ocular flora and antibiotic resistance in patients undergoing cataract surgery. As the authors indicated, the most common prophylactic practice in many countries outside Europe is perioperative topical antibiotics, despite the fact that the ESCRS endophthalmitis study showed that intracameral cefuroxime (with or without topical levofloxacin) was more effective than either no antibiotics or topical levofloxacin. Furthermore, in that large study, no differences were found when comparing perioperative topical antibiotic plus antisepsis prophylaxis vs placebo plus antisepsis prophylaxis. Another clinical trial showed that antibiotic use conferred no additional benefit beyond that provided by povidone-iodine alone.
So, on the one hand the evidence tends to favor the intraocular route as more effective, and on the other hand there is no plausible clinical evidence that topical prophylactic antibiotics are more effective than local povidone-iodine antisepsis. However, as the authors correctly pointed out, the source of infection in postoperative endophthalmitis is almost always the conjunctival flora, and reducing it to a minimum before surgery seems reasonable.
In the study by Hsu and associates, 95% of gram-positive bacteria were susceptible to gentamicin, while 75% and 78% were susceptible to moxifloxacin and gatifloxacin, respectively. Susceptibility testing of gram-positive bacteria to tobramycin was not performed. These findings are consistent with those reported by other researchers in the United States but contrast with those from other countries. Chung and associates in Korea found that 41.3% of methicillin-susceptible coagulase-negative Staphylococcus (CNS) conjunctival isolates showed intermediate resistance to tobramycin and 59.3% of methicillin-resistant CNS showed resistance to tobramycin, while 0% methicillin-susceptible CNS showed resistance to moxifloxacin and gatifloxacin, and 11.1% and 3.7% of methicillin-resistant CNS showed intermediate resistance to gatifloxacin and moxifloxacin, respectively.
Recently Blanco and associates in Italy, studying staphylococci from respiratory tract and ocular infections, found that 100% of methicillin-resistant Staphylococcus aureus isolates and 58% of methicillin-resistant Staphylococcus epidermidis were resistant to tobramycin, while 75% and 25% of them, respectively, were resistant to moxifloxacin.
We performed a study on isolates from keratitis and endophthalmitis cases from June 2011 to January 2012 (92 samples) at Fundación Oftalmológica de Santander FOSCAL (Floridablanca, Colombia) and found that 0% of gram-positive bacteria were resistant to moxifloxacin, 1.1% to gatifloxacin, and 85% to tobramycin. A total of 8.3% of gram-negative bacteria were resistant to moxifloxacin, 0% to gatifloxacin, and 27.3% to tobramycin. With these results we strongly discourage the use of tobramycin, either for prophylaxis or for treatment.
All these data confirm significant geographical differences in antimicrobial resistance patterns. Many studies conducted over the last decade have identified 1 main factor influencing prevalence of antibiotic resistance: antibiotic consumption and abuse, within both human medicine and veterinary medicine. Undoubtedly, regional variations in these practices will be reflected in differences in local resistance rates to antimicrobials. We consider that the suggestion of the authors, that aminoglycosides could be a good alternative to fluoroquinolones in prophylaxis of endophthalmitis, should be taken with caution, as these findings may not be applicable to other regions of the world.