SELECTION OF ENTEROCOCCUS-FAECIUM STRAINS WITH STABLE AND UNSTABLE RESISTANCE TO THE STREPTOGRAMIN RP-59500 USING STEPWISE IN-VITRO EXPOSURE

Therapy of vancomycin-resistant enterococcal infection is an increasin g problem for many large medical centers. One promising agent for use against Enterococcus faecium is RP 59500 (Quinupristin/Dalfopristin). To assess the potential for emergence of resistant strains during clin ical trials with this new compound, we collected anti tested 11 vancom ycin-resistant and three vancomycin-susceptible E. faecium strains. Th e strains were exposed to doubling dilutions of RP 59500, beginning at drug concentrations ranging from 0.25 to 16 mu g/ml agar. A saturated swab with approximately 3 x 10(7) organisms/ml runs spread as a lawn on agar containing XP 59500 and incubated at 35 degrees C for 48 h. Gr owth on the highest drug-containing plate was used to prepare the inoc ulum for the next series of resistance-selection experiments. After th e final passage, the range of the highest RP 59500 plate concentration with growth was 32-512 mu g/ml (initial resistance frequency ranging from 1 x 10(-6) to > 1 x 10(-4)). After the selection of resistant str ains, the organisms were passed twice weekly on antimicrobial agent-fr ee media to determine resistance stability in the absence of drug. Res istance in strains with an RP 59500 MIC of greater than or equal to 16 mu g/ml was stable after repeated passage for 4 weeks. These results indicate that stable resistance to RP 59500 can be selected in E. faec ium when the organism is exposed to increasing drug concentrations onl y slightly above the minimum inhibitory concentration (MIC). This stab le resistance is seen in strains exhibiting an MIC greater than or equ al to 16 mu g/ml, and unstable in those with resistance where the XP 5 9500 MIC is less than eight times the original drug MIC. Our observati on suggests more than one mechanism of resistance is likely present wh en stable resistance to RP 59500 develops.