Susceptibility of S pneumoniae to levofloxacin remains high

Fort Lauderdale, FL-Results of a nationwide antibiotic susceptibility tracking study demonstrate that despite more than 9 years of widespread systemic and ocular use, levo-floxacin ophthalmic solution 0.5% (Quixin, Santen) has continued to maintain potent activity against Streptococcus pneumoniae, including against multidrug-resistant strains, said Christopher N. Ta, MD.

Dr. Ta, assistant professor of ophthalmology, Stanford University, Stanford, CA, presented data from the Tracking Resistance in the United States Today (TRUST) Program at the annual meeting of the Association for Research in Vision and Ophthalmology.

The TRUST Program is a longitudinal, continuous-surveillance project that has been monitoring susceptibility of bacterial isolates collected from more than 200 institutions nationwide since 1996. Considering all 22,794 S pneumoniae isolates collected during the respiratory seasons from 2000-2001 through 2003-2004, 99% were found to be susceptible to levofloxacin, while 98% of 4,704 isolates of multidrug-resistant S pneumoniae (MDRSP) were also susceptible to levofloxacin.

"The emergence of bacterial resistance to commonly used antibiotics is a growing concern, and the prevalence of MDRSP has been increasing in the United States over the past decade," Dr. Ta said. "However, these data illustrate that prolonged and intensive use of an antibiotic does not necessarily lead to increased antimicrobial resistance over time.

"Among newer-generation fluoroquinolones, levofloxacin has been the most widely used for systemic infections," Dr. Ta added. "Nevertheless, it continues to provide excellent coverage against S pneumoniae, which is an important ocular pathogen that can cause severe keratitis and endophthalmitis. The favorable physicochemical and biological characteristics of levofloxacin appear to have allowed it to maintain a high degree of broad-spectrum activity."

Since it was launched, the TRUST study has analyzed susceptibility of more than 70,000 clinical isolates. The pathogens analyzed in TRUST are collected during the winter respiratory season, which is the peak time for systemic antibiotic exposure. TRUST 1 included data from the 1996-1997 respiratory season; TRUST 8 analyzed isolates collected in the 2003-2004 respiratory season. The bacterial isolates derived from patients with respiratory, sinus, and ocular infections.

"The susceptibility pattern in respiratory tract flora is likely reflective of that in bacteria colonizing eyelids and conjunctiva," Dr. Ta noted.

Antibiotic susceptibility is determined by results of minimum inhibitory concentration (MIC) testing using the broth microdilution technique. For levofloxacin, the MIC susceptible breakpoint is defined as ≤2.0 μg/ml.

For TRUST 5 through TRUST 8, the rate of S pneumoniae resistance to levofloxacin remained low at between 0.8% and 1.1%. Cephalosporins also continued to be associated with low rates of resistance (≤1.7%). In contrast, about 18% of S pneumoniae isolates were penicillin-resistant, and rates of macrolide and TMP/SMX resistance were even higher at about 25%.

During TRUST 5, 13.5% of S pneumoniae isolates were MDRSP, and that rate increased to 26.3% during TRUST 8. For all four collection periods analyzed, susceptibility rates to levofloxacin remained high (97.5% to 98.2%) with <1% variation between seasons.

"Nearly 300 of the almost 23,000 S pneumoniae isolates included in this analysis were resistant to the five non-fluoroquinolone classes of antibiotics tested, and 98% of those isolates were still susceptible to levofloxacin," Dr. Ta concluded.