Kinetics of kill of bacterial conjunctivitis isolates with moxifloxacin, a fluoroquinolone, compared with the aminoglycosides tobramycin and gentamicin

To compare the kinetics and speed of kill of Streptococcus pneumoniae and Haemophilus influenzae on exposure to three topical ophthalmic antibiotic solutions. Bacterial conjunctivitis isolates of S. pneumoniae and H. influenzae were exposed to 1:1000 dilutions of moxifloxacin 0.5%, tobramycin 0.3%, gentamicin 0.3%, and water (control). At 15, 30, 60, 120, and 180 minutes after exposure, aliquots were collected, cells were cultured, and viable cell counts were determined using standard microbiological methods. Moxifloxacin achieved 99.9% kill (3-log reduction) at approximately 2 hours for S. pneumoniae and at 15 minutes for H. influenzae. Tobramycin and gentamicin did not achieve 3-log reduction of S. pneumoniae during the 180-minute study period. An increase in bacterial growth was noted for these isolates. Gentamicin took more than 120 minutes to achieve the 3-log reduction of H. influenzae and tobramycin did not reach the 3-log reduction of this pathogen during the 180-minute study period. Moxifloxacin killed S. pneumoniae and H. influenzae in vitro faster than tobramycin and gentamicin, suggesting its potential clinical benefit as a first-line treatment for bacterial conjunctivitis to minimize patient symptoms and to limit the contagiousness of the disease.