Comparative in-vitro activity of penicillin alone and combined with gentamicin against clinical isolates of Streptococcus pneumoniae with decreased susceptibility to penicillin.

The worldwide emergence of Streptococcus pneumoniae with decreased susceptibility to penicillin has led to the suggestion that drug combinations might be used. The aim of this study was to determine possible synergy using a combination of penicillin with sub-inhibitory doses of gentamicin against 26 clinical isolates of S. pneumoniae with decreased susceptibility to penicillin, using half-chequerboards and killing curves. Synergy was demonstrated for ten of the 26 isolates with the combination of penicillin with gentamicin at 1 mg/l and for 22 isolates with penicillin and gentamicin at 2 mg/l. Killing curves on three isolates showed synergy and confirmed the chequerboard results. Further synergy studies using penicillin or cefotaxime/ceftriaxone, plus low dose gentamicin against penicillin-resistant pneumococci are indicated.