In-vitro activity of RP 59500, a semisynthetic streptogramin, against staphylococci and streptococci

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Abstract
The in-vitro activity of RP 59500, an injectable streptogramin derived from pristinamycin, was determined by agar dilution and compared with that of pristinamycin. Two hundred and sixty-one recent clinical isolates of Gram-positive cocci were tested against both antibiotics. The two compounds displayed similar activities. The MIC 90 S of RP 59500 ranged from 0·5 to 2 mg/L in 114 strains of Staphylococcus aureus showing various phenotypes of antibiotic resistance (penicillin-susceptible; penicillin-resistant and methicillin-susceptible; methicillin-resistant; erythromycin-resistant, either inducible or constitutive; quinolone-resistant). Similar results were obtained with coagulase-negative staphylococci. RP 59500 was consistently active against streptococci, with MIC 90 s of 0·25, 0·25 and 0·50 mg/L for Streptococcus pyogenes ( n = 20), Streptococcus agalactiae ( n = 20) and Streptococcus pneumoniae ( n = 20), respectively. Enterococcus faecalis ( n = 20) appeared to be notably less susceptible (MIC 90 , 8 mg/L). In view of this consistent activity against all staphylococci and streptococci tested, including multiply resistant isolates, RP 59500 merits further investigation.

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