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

Abstract

Purpose: To compare the kinetics and speed of kill of Streptococcus pneumoniae and Haemophilus influenzae on exposure to three topical ophthalmic antibiotic solutions.

Materials and methods: 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.

Results: 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.

Conclusion: 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.

Keywords: Haemophilus influenzae; Streptococcus pneumoniae; aminoglycosides; bacterial conjunctivitis; fluoroquinolones; in vitro; kinetics of kill.

Figure 1

Percent survivors of Streptococcus pneumoniae (MCC 40211) as a function of time.

Figure 2

Photographs of Streptococcus pneumoniae (MCC 40211) grown on blood agar plates after exposure to 1:1000 dilutions of moxifloxacin (5 μg/mL), tobramycin (3 μg/mL), gentamicin (3 μg/mL), and water control.

Figure 3

Percent survivors of Haemophilus influenzae (MCC 95018) as a function of time.

Figure 4

Photographs of Haemophilus influenzae (MCC 95018) grown on chocolate agar plates after exposure to 1:1000 dilutions of moxifloxacin (5 μg/mL), tobramycin (3 μg/mL), gentamicin (3 μg/mL), and water control.