Combination Effect of Antibiotics Against Bacteria Isolated From Keratitis Using Fractional Inhibitory Concentration Index

Purpose: To study the in vitro interaction of fluoroquinolones such as levofloxacin (LVFX), gatifloxacin (GFLX), or moxifloxacin (MFLX) in combination with tobramycin (TOB) or cefmenoxime (CMX) against clinical isolates of bacteria from keratitis. Methods: The activity of each drug alone was determined by an agar dilution method. Checkerboard synergy testing was then performed against 47 isolates, including Staphylococcus species, Streptococcus species, and Pseudomonas aeruginosa. Antimicrobial combinations were classified as synergistic, additive, indifferent, or antagonistic, according to their fractional inhibitory concentration. Results: The average fractional inhibitory concentration indexes of combined use of LVFX/CMX or GFLX/CMX in Staphylococcus species and Streptococcus species, and LVFX/CMX, GFLX/CMX, MFLX/CMX in gram-negative rods were low. The additive activity of the following drug combinations were seen in more than 70% of isolates: LVFX/CMX and GFLX/CMX against gram-positive cocci and LVFX/CMX, GFLX/CMX, MFLX/CMX against gram-negative rods. No consistent synergistic or antagonistic effect was observed with the combinations used. Conclusion: The combination of LVFX/CMX or GFLX/CMX was predominantly additive for all tested isolates.

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