Comparison of fluoroquinolone kinetics of kill in susceptible and resistant gram-positive conjunctival pathogens

The purpose of this study was to compare moxifloxacin's rate of kill of susceptible and resistant Gram-positive organisms with that of ciprofloxacin and ofloxacin, using concentrations found in human conjunctiva after instillation of one drop.

Staphylococcus aureus (S. aureus) and Streptococcus pneumoniae (S. pneumoniae) isolates were exposed to moxifloxacin, ciprofloxacin, or ofloxacin diluted to human conjunctival concentrations achieved after instillation of one drop. These treated isolates were cultured on blood agar plates at 0, 15, 30, and 60 minutes after exposure, and incubated to observe the number of surviving colony-forming units/mL.

In susceptible S. pneumoniae, moxifloxacin showed the most rapid reduction of colonies at 15 and 30 minutes, with the fewest colonies at 60 minutes compared with ciprofloxacin and ofloxacin. In S. pneumoniae resistant to ciprofloxacin and ofloxacin, moxifloxacin had rapid reduction in colonies at each time point and near-eradication at 60 minutes, while ciprofloxacin and ofloxacin had an increase in colonies at 60 minutes. In susceptible S. aureus, moxifloxacin had a rapid decrease in colonies at 15 and 30 minutes, compared with a slight reduction in colonies at these intervals for the other antibiotics. In methicillin-resistant S. aureus with cross-resistance to fluoroquinolones and other antibiotics, moxifloxacin had a decrease in colonies at each time point compared with an increase at each time point for ciprofloxacin and ofloxacin.

Moxifloxacin showed an increased speed of kill against both of the common susceptible Gram-positive conjunctival pathogens, compared with the inconsistency of killing activity of two other fluoroquinolones tested. In addition, at the concentration level achieved in the conjunctiva after the instillation of one drop, moxifloxacin effectively and rapidly killed resistant Gram-positive conjunctival pathogens, while ciprofloxacin and ofloxacin had no effect against these organisms.