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

Cefazolin/tobramycin, cefuroxime/gentamicin, and moxifloxacin were compared using bacterial keratitis isolates to determine whether empiric therapy constituted optimal antibacterial treatment. 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. 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. 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.