This prospective study found that repeated exposure to azithromycin, and to a lesser degree fluoroquinolone antibiotics, significantly increases the presence of conjunctival Staphylococcus epidermidis at the expense of other commensal flora.
The study included 48 eyes of 24 patients undergoing serial unilateral intravitreal injection for choroidal neovascularization followed for a year. They received four consecutive monthly unilateral intravitreal injections and were then treated as needed.
Each patient was randomized to one of four antibiotics (azithromycin 1%, gatifloxacin 0.3%, moxifloxacin 0.5% or ofloxacin 0.3%) and used only the assigned antibiotic for four days after each injection.
Cojunctival cultures revealed that in azithromycin-treated eyes, Staphylococcus epidermidis and Staphylococcus aureus accounted for 54.5 percent and 18.2 percent of cultured isolates, respectively, at baseline and 90.9 percent (P < 0.01) and 4.5 percent (P < 0.01), respectively, after azithromycin exposure. In fluoroquinolone-treated eyes, 45.7 percent and 6.5 percent of cultured isolates at baseline were S epidermidis and S aureus, respectively, but this increased to 63.4 percent (P < 0.03) and 13 percent (P = 0.24), respectively, after fluoroquinolone exposure.
The percentage of gram-negative species decreased from 8.7 percent at baseline to 1.6 percent (P < 0.05) in fluoroquinolone-treated eyes, while the percentage of S epidermidis isolated from azithromycin-treated eyes was significantly greater when compared with fellow control eyes (P < 0.01) or fluoroquinolone-treated eyes (P < 0.01).
The authors say the high percentage (75 percent) of baseline resistance to azithromycin may have allowed resistant S epidermidis strains to outcompete other commensal flora. They note that 100 percent of S epidermidis strains cultured from visits one to four after antibiotic exposure were resistant to azithromycin. However, S aureus also demonstrated similar levels of baseline resistance to azithromycin (around 80 percent) but instead declined after exposure.
They say the results demonstrate that repeated application of macrolide or fluoroquinolone antibiotics increases the percentage of S epidermidis on ocular flora. S epidermidis is one of the most commonly identified opportunistic pathogens. They believe this to be the first controlled, longitudinal study to demonstrate these changes, which may have considerable clinical implications because S epidermidis is a major cause of ocular infection.