Effect of long-term treatment with eyedrops for glaucoma on conjunctival bacterial flora Rio Honda1, Hiroshi Toshida1, Chikako Suto2, Takuro Fujimaki2, Tairo Kimura2, Toshihiko Ohta1, Akira Murakami2 1Department of Ophthalmology, Juntendo University Shizuoka Hospital, Izunokuni, Shizuoka, 2Department of Ophthalmology, Juntendo University School of Medicine, Tokyo, Japan Objective: The effect of eyedrops for glaucoma on conjunctival bacterial flora was investigated by comparing a group of patients treated with such eyedrops for at least 1 year to a control group that did not use eyedrops. Methods: In both groups, bacterial culture came from scrapings of the conjunctival sac, and the bacterial infection rate and pattern of drug resistance were determined. Findings were analyzed in various subgroups stratified by age, frequency of instillation, and concentration of antiseptic benzalkonium chloride in the eyedrops. Results: The culture-positive rate was significantly lower in the glaucoma eyedrop group (43/119 eyes, 40.3%) than in the control group (19/28 eyes, 67.8%) (P < 0.05). No differences in infection rate were found among the different age groups. The most frequent bacteria in both groups was coagulase-negative staphylococci. Gram-negative bacteria were only detected in the glaucoma eyedrop group. Retrospective evaluation was possible for 86 eyes of patients from the glaucoma eyedrop group, among which 45 eyes (52.3%) showed some corneal epithelium damage. There was no difference in the culture-positive rate of bacteria between patients who used eyedrops containing 0.01% or higher dose of benzalkonium chloride and those containing less than 0.01%. Strains that showed resistance to levofloxacin were significantly less frequent in the glaucoma eyedrop group (six strains, 15.0%) than in the control group (11 strains, 39.3%) (P < 0.05). Conclusion: Patients using eyedrops for glaucoma had a lower culture-positive rate of bacteria in the conjunctival sac, probably due to being washed out by the eyedrops. However, Gram-negative bacteria were detected in the eyedrop group. Bacteria isolated from the eyedrop group had lower resistance to levofloxacin, a finding that may have clinical relevance. Keywords: glaucoma, eyedrop, benzalkonium chloride, conjunctival flora, levofloxacin