In vitro fluoroquinolone resistance in staphylococcal endophthalmitis isolates

To evaluate the in vitro susceptibility and cross-resistance of gatifloxacin and moxifloxacin vs older fluoroquinolones among coagulase-negative staphylococci recovered from patients with clinical endophthalmitis.

A combination of E tests and disk diffusion methods was used to determine in vitro susceptibility and cross-resistance for 111 coagulase-negative staphylococci isolates recovered during a 15-year period (January 1, 1990, to December 31, 2004) against 5 fluoroquinolones.

In vitro susceptibilities (percentage sensitive) in descending order were as follows: gatifloxacin, 74.5%; moxifloxacin, 72.1%; levofloxacin, 69.3%; ciprofloxacin, 65.6%, and ofloxacin, 60.4%. More than 65% of the coagulase-negative staphylococci resistant to ciprofloxacin (n = 38) demonstrated in vitro cross-resistance to gatifloxacin (25 [65.8%] of 38) and moxifloxacin (27 [71.1%] of 38). During the initial 5 years (January 1, 1990, to December 31, 1994), 96.6% of the coagulase-negative staphylococci were sensitive to gatifloxacin and moxifloxacin, with minimal inhibitory concentration required to inhibit or kill 90% of the isolates of 0.19 microg/mL and 0.12 microg/mL, respectively. During the last 5-year period (January 1, 2000, to December 31, 2004), the percentage of sensitive coagulase-negative staphylococci declined to 65.4% for gatifloxacin and moxifloxacin (P=.02). Minimal inhibitory concentration required to inhibit or kill 90% of the isolates was 32 microg/mL or greater for both drugs.

Gatifloxacin and moxifloxacin demonstrated an in vitro efficacy of less than 80% for coagulase-negative staphylococci endophthalmitis in the present study. Ciprofloxacin resistance may serve as a surrogate for concurrent in vitro resistance for gatifloxacin and moxifloxacin. Resistance increased significantly during the last 5 years. Declining in vitro susceptibility to gatifloxacin and moxifloxacin may have important implications for the prevention and treatment of postoperative endophthalmitis.