In vitro comparison of combination and monotherapy for the empiric and optimal coverage of bacterial keratitis based on incidence of infection.

PURPOSE: Cefazolin/tobramycin, cefuroxime/gentamicin, and moxifloxacin were compared using bacterial keratitis isolates to determine whether empiric therapy constituted optimal antibacterial treatment.
METHODS: Based on percent incidence of corneal infection, 27 Staphylococcus aureus, 16 Pseudomonas aeruginosa, 10 Serratia marcescens, 4 Moraxella lacunata, 3 Haemophilus influenzae, 9 coagulase-negative staphylococci, 7 Streptococcus viridans, 6 Streptococcus pneumoniae, 7 assorted Gram-positive isolates, and 11 assorted Gram-negative isolates were tested for minimum inhibitory concentrations to cefazolin, tobramycin, cefuroxime, gentamicin, and moxifloxacin using E-tests to determine susceptibility and potency.
RESULTS: The in vitro coverage (susceptible to at least one antibiotic) of cefuroxime/gentamicin (97%) was statistically equal to cefazolin/tobramycin (93%) and moxifloxacin (92%) (P = 0.29). Double coverage (susceptible to both antibiotics) was equivalent (P = 0.77) for cefuroxime/gentamicin (42%) and cefazolin/tobramycin (40%). The susceptibilities of individual coverage were moxifloxacin (92%), gentamicin (89%), tobramycin (74%), cefazolin (58%), and cefuroxime (52%). Methicillin-resistant S. aureus was best covered by gentamicin 100% (9 of 9). Tobramycin was more potent (P = 0.00001) than gentamicin for P. aeruginosa, whereas cefazolin was more potent (P = 0.0004) than cefuroxime for S. aureus.
CONCLUSIONS: Although there seems to be no in vitro empiric coverage advantage between cefazolin/tobramycin, cefuroxime/gentamicin, and moxifloxacin monotherapy, potency differences may occur and optimal treatment can best be determined with laboratory studies.