Moxifloxacin resistance: intrinsic to antibiotic or related to mutation?

To evaluate and compare the pattern of resistance to topical moxifloxacin of ocular isolates recovered during comparable periods within 2 years.

Records of 611 consecutive cases of bacterial corneal ulcers that presented to our center between April and August 2007 were compared with records of 417 cases that presented in April and August 2006. Records of culture and sensitivity to cefazolin, tobramycin, gatifloxacin, and moxifloxacin at the time of presentation were analyzed. Parameters recorded included total number of cases in each period, culture-positive cases, organisms isolated, and sensitivity by Kirby-Bauer disk-diffusion test. The data from 2007 was compared with those from 2006 using χ test.

Culture-positive rate was 70.2% in 2007 compared with 65.0% in 2006 (p = 0.143). The percentage of culture-positive cases resistant to moxifloxacin was 5.46% in 2007 compared with 5.42% in 2006 (p = 0.977). There was no significant difference between resistance against cefazolin during the two periods (p = 0.895). Resistance to gatifloxacin and tobramycin was significantly higher in samples tested in 2007 as compared with those tested in 2006 (gatifloxacin, p = 0.037; tobramycin, p = 0.031).

Resistance to moxifloxacin did not change significantly during a period of 2 years in our study. The static resistance pattern to moxifloxacin may indicate that such resistance to moxifloxacin is intrinsic in the antibiotic rather than new resistance stemming from mutations.