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Latest resistance trends in respiratory pathogens: mixed news: S. pneumoniae resistance leveling off. (Streptococcus).

CHICAGO -- There is both good and bad news regarding the latest trends in Streptococcus pneumoniae antimicrobial resistance, Daniel E. Sahm, Ph.D., announced at a satellite symposium held in conjunction with the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.

The good news is that S. pneumoniae antimicrobial resistance seems to have leveled off.

The bad news is that it has done so at unacceptably high nonsusceptibility rates for several widely used drugs, said Dr. Sahm, chief scientific officer at Focus Technologies Inc., Herndon, Va.

He presented the latest data from the Tracking Resistance in the United States Today (TRUST) study, the nation's largest longitudinal continuous surveillance program for susceptibility profiling of the major respiratory pathogens. Each year since 1998, TRUST researchers have analyzed at Focus Technology's centralized laboratory an average of more than 6,000 S. pneumoniae strains and 1,000 each of Haemophilus influenzae and Moraxella catarrhalis, the major pathogens in bacterial community-acquired pneumonia, collected at several hundred medical institutions.

Data from TRUST 7, the analysis covering the 2002-2003 respiratory illness season, offer reassurance regarding H. influenzae and M. catarrhalis. Even after more than 15 years of clinical use of the fluoroquinolones, 100% of tested strains of both pathogens remain susceptible to levofloxacin, moxifloxacin, and gatifloxacin.

Both bugs are also highly susceptible to ceftriaxone, amoxicillin/clavulanate, cefuroxime, and azithromycin. But nearly 30% of H. influenzae strains are resistant to ampicillin, and 18% are resistant to trimethoprim/sulfamethoxazole (TMP/SMX). Of all these antimicrobials, only ampicillin is not still highly active against M. catarrhalis, Dr. Sahm said at the symposium sponsored by Ortho-McNeil, which funds the TRUST program.

Resistance is far more problematic with S. pneumoniae, the most common cause of community-acquired pneumonia. Based on annual TRUST data from the 1998-1999 respiratory season through TRUST 7, resistance to penicillin, TMP/SMX, and macrolides has leveled off--but with fully 25%-35% of isolates being nonsusceptible to any one of those agents. Ceftriaxone's activity remains consistently high, with only about 5% of S. pneumoniae isolates being resistant in recent years. Resistance to levofloxacin or other fluoroquinolones remains even more uncommon; 99% of isolates are susceptible, and there is no suggestion of drift toward greater numbers of intermediate or resistant strains when comparing TRUST data for the past 3 years.

Multidrug resistance to S. pneumoniae is of particular concern to the Food and Drug Administration. The prevalence of strains with multidrug resistance (MDR) in the laboratory climbed from 1998-1999 through last year before tapering off slightly in TRUST 7 to about 12%. The most prevalent pattern of triple-drug MDR involves azithromycin, penicillin, and TMP/SMX.

Roughly 2% of S. pneumoniae strains have four-drug resistance. The most common profile involves azithromycin, penicillin, TMP/SMX, and ceftriaxone.

The thought of strains with five-drug resistance gives infectious disease specialists the willies. When these strains occur, the most common pattern involves the addition of levofloxacin to the most common four-drug MDR pattern. This leaves only vancomycin as an active agent against these strains.

Fortunately, TRUST 7 showed no isolates with five-drug resistance. There were four such isolates out of 9,499 strains analyzed in 1999-2000, none in 2000-2001, and eight in 2001-2002.

"So while these are very formidable strains, it's interesting to see that they don't seem to be expanding in terms of their numbers from one year to the next. It seems to be, at least at this point, somewhat sporadic," Dr. Sahm observed.

He added that the fluoroquinolones' extremely rare involvement in MDR phenotypes raises an intriguing possibility: Perhaps the use of this class of drugs works against selection of MDR clones.
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Title Annotation:Infectious Diseases
Author:Jancin, Bruce
Publication:Internal Medicine News
Geographic Code:1USA
Date:Nov 1, 2003
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