In Vitro Antibiotic Susceptibility of Rapidly Growing Nontuberculous Mycobacteria Isolated from Patients with Microbial Keratitis

Abstract

Purpose: This study was undertaken to determine the antibiotic susceptibility and minimum inhibitory concentrations (MIC) of amikacin, tobramycin, ciprofloxacin, gatifloxacin, azithromycin, and clarithromycin against rapidly growing nontuberculous mycobacteria isolated from patients with keratitis.

Methods: A total of 15 rapidly growing nontuberculous mycobacteria isolated from corneal scrapings of keratitis patients from January 1999 through December 2007 were subjected to antimicrobial susceptibility testing by the E-Test to amikacin, tobramycin, ciprofloxacin, gatifloxacin, azithromycin, and clarithromycin.

Results: Out of 15 isolates, 13 were identified as Mycobacterium chelonae complex and 2 as Mycobacterium fortuitum complex. Based on minimum inhibitory concentration (MIC) cut off, all 15 (100%) isolates were sensitive to amikacin, azithromycin, and clarithromycin, 13 (86%) were sensitive to tobramycin, nine (60%) to gatifloxacin, and only 6 (40%) to ciprofloxacin. The MIC range was 0.25–4 µg/ml for amikacin, 0.5–1 µg/ml for azithromycin, 0.125–1 µg/ml for clarithromycin, 0.5–16 µg/ml for ciprofloxacin, and 0.25–16 µg/ml for tobramycin. MIC₉₀ for amikacin was 2 µg/ml, azithromycin 1 µg/ml, clarithromycin 0.75 µg/ml, ciprofloxacin 8 µg/ml, gatifloxacin 8 µg/ml, and for tobramycin it was 4 µg/ml.

Conclusions: All the isolates were sensitive to amikacin, azithromycin, and clarithromycin, but the MIC values of clarithromycin and azithromycin were lower than amikacin. Based on in vitro susceptibility results it appears that the topical amikacin in combination with oral clarithromycin or azithromycin is the best treatment option for rapidly growing nontuberculous mycobacterial keratitis.

KEYWORDS::

• Amikacin
• Azithromycin
• Clarithromycin
• E-test
• Keratitis
• Nontuberculous mycobacteria

ACKNOWLEDGMENT

The authors would like to thank Hyderabad Eye Research Foundation for funding this project.

Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and writing of the paper.