Trends in antibiotic susceptibility and resistance in ocular infections

ByPenny A. Asbell, MD, FACS, MBA

Surveillance data of a longitudinal, prospective approach continues to gain support because of the opportunity to provide clinicians with a larger, nationwide picture of resistance and susceptibility patterns in infections. The TRUST (Tracking resistance in the United States today) study^1,2 began in 1996 to test in vitro susceptibility of isolates collected yearly from approximately 200 clinical laboratories throughout the United States. Although ocular isolates were included in the study, they were submitted periodically and were not the primary focus of the analysis. For this reason, the TRUST study was expanded to incorporate the Ocular TRUST study,³ an annual survey of ocular isolates collected from diverse populations throughout the United States.

Exploring the Ocular TRUST study

“Surveillance data of a longitudinal, prospective approach continues to gain support because of the opportunity to provide clinicians with a larger, nationwide picture of resistance and susceptibility patterns in infections.”
— Penny A. Asbell, MD, FACS, MBA

The Ocular TRUST study is the world’s largest collection of surveillance data on eye infections. Each ocular isolate is sent to an independent, centralized laboratory and tested for in vitro susceptibility using standardized, validated duplicate testing methods set forth by the Clinical Laboratory and Standards Institute. Broth microdilution techniques were used to obtain the minimal inhibitory concentration (MIC) of the ocular isolates. The MIC required to inhibit the growth of 90% of organisms (MIC₉₀) was determined to be susceptible, intermediate or resistant based on standardized systemic breakpoints. Data were prospectively collected from more than 100 sites across the country and tested against a comprehensive set of antibiotics often used to treat eye infections.

The pathogen panel evaluated in the Ocular TRUST 1 study included 197 Staphylococcus aureus isolates—both methicillin-susceptible (MSSA) and methicillin-resistant (MRSA)—49 Streptococcus pneumoniae isolates and 32 Haemophilus influenzae isolates, all of which were prospectively collected. The researchers archived 760 S pneumoniae isolates and 356 H influenzae isolates. The antimicrobial panel used in the Ocular TRUST 1 study included fluoroquinolones currently available for topical use (ciprofloxacin, gatifloxacin, levofloxacin, moxifloxacin), macrolides (azithromycin), beta-lactams (penicillin), lipopeptides (polymyxin B), aminoglycosides (tobramycin) and folate pathway inhibitors (tri-methoprim), all of which were analyzed for each isolate.

Examining Ocular TRUST study results

The Ocular TRUST data show that MSSA and MRSA have identical susceptibility patterns when tested against fluoroquinolones—gatifloxacin, levofloxacin and moxifloxacin. MSSA susceptibility to fluoroquinolones ranged from 79.9% to 81.1%, and MRSA susceptibility to fluoroquinolones was 15.2%. The data show that MRSA is resistant to azithromycin, penicillin, polymyxin B and tobramycin, whereas tri-methoprim is shown to be highly effective against MRSA. MSSA was susceptible to tobramycin, tri-methoprim and fluoroquinolones, which were more effective against MSSA than azithromycin. The results show that MRSA is also resistant to many antibiotics, not only fluoroquinolones, which is typical of organisms exhibiting multiple resistance.

All of the antimicrobials were effective against S pneumoniae, an organism commonly seen in eye infections, whereas ciprofloxacin was effective against 89.9% of isolates. All H influenzae isolates were susceptible to the antimicrobials, except tri-methoprim.

Conclusion

The Ocular TRUST study is a valuable source of data that can help ophthalmologists understand susceptibility patterns for eye infections, not only on a local level but also on a national level. The Ocular TRUST 1 study results show that fluoroquinolones continue to be an effective treatment option for ocular infections. In addition, susceptibility patterns are shown to be similar among all the fluoroquinolones, with the exception of S pneumoniae, which was reported to have lower susceptibility to ciprofloxacin compared with other fluoroquinolones. However, when bacteria with multimarker resistance such as MRSA are present, other treatment options should be considered. Understanding antibiotic surveillance data can help ophthalmologists make informed choices of antibiotics for their patients with ocular infections.

Editor’s note: Iquix solution is indicated for the treatment of corneal ulcer.

Penny A. Asbell, MD, FACS, MBA, is a professor of ophthalmology at the Mount Sinai School of Medicine, director of Cornea and Refractive Services at the Mount Sinai Medical Center in New York, New York, and the Contact Lenses Section editor of the Ocular Surgery News Editorial Board. Dr. Asbell can be reached at 212-241-7977; fax: 212-289-5945; e-mail: penny.asbell@mssm.edu.

References:

Thornsberry C, Jones ME, Hickey ML, Mauritz Y, Kahn J, Sahm D. Resistance surveillance of Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis isolated in the United States, 1997-1998. J Antimicrob Chemother. 1999;44:749-759.
Thornsberry C, Sahm DF, Kelly LJ, et al. Regional trends in antimicrobial resistance among clinical isolates of Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the United States: results from the TRUST surveillance program, 1999-2000. Clin Infect Dis. 2002;34:S4-S16.
Asbell PA, Colby KA, Deng S, et al. Ocular TRUST: nationwide antimicrobial susceptibility patterns in ocular isolates. Am J Ophthalmol. 2008;145:951-958.