Tracking resistance to Steptococcus pneumoniae

The following statement was given by Clyde Thornsberry, PhD, professor in the department of pathology at Vanderbilt University School of Medicine in Nashville and principal investigator of "Tracking Resistance in the U.S. Today." He spoke at the 38th Interscience Conference on Antimicrobial Agents and Chemotherapy on Sept. 25, 1998, in San Diego.

Pneumococcal resistance to several antimicrobial agents can develop rapidly. Therefore, the ability to monitor the activity of newly released agents is important. Since the release of levofloxacin (LEVO) in January 1997, we have systematically tracked pneumococcal susceptibility to levofloxacin and seven other antimicrobials against common respiratory pathogens.

In 1997-1998, penicillin (PEN) nonsusceptibility (MIC > 0.125 ug/ml) in 3,340 strains of Streptococcus pneumoniae (SP) was 36% [22% intermediate (I), 14% resistant (R)] compared to 34% nonsusceptible (20% I, 14% R) in 1996-1997. (See charts, p. 215.)

Clarithromycin resistance increased from 18% to 23% in SP. In 1997-98 blood isolates, 29% of SP was not susceptible to PEN (19% I, 10% R) compared with 39.8% (24% I, 15.5% R) in respiratory isolates.

For PEN-resistant SP (MIC > 2 ug/ml), nonsusceptibility (I+R) for other antimicrobials was 93% for amoxicillin-clavulantate (AC), 80% for ceftriaxone (CTX), 98% for cefuroxime (CUX), 71% for azithromycin (AZ) and clarithromycin (CLAR), and 93% for trimethoprim-sulfamethoxazole (SXT); all SP were susceptible to vancomycin (VAN) and LEVO. For AC, CUX, VAN, and LEVO, these rates among PEN-resistant isolates were comparable to those in 1996-1997, and there was an increase in CLAR resistance (10%) and CTX resistance (7%). AZ and SXT were not tested in 1996-1997.

Resistance in the other respiratory organisms, Heomophilus influenzae (HI) and Moraxella catarr halis (MC) has remained relatively constant between the 1996-1997 and 1997-98 studies. Both organisms are 100% susceptible to LEVO, CTX, and CUX, and the percentage of organisms producing beta-lactamase is comparable to the 1996-1997 study (33.2% vs. 33.4% for HI and 91% vs. 93% for MC). While pneumococcal resistance to many antimicrobials continues to increase, LEVO and VAN have maintained a high activity level.