In vitro activity of citropin 1.1 alone and in combination with clinically used antimicrobial agents against Rhodococcus equi

OBJECTIVES: The aim of this study was to investigate the in vitro activity of citropin 1.1, an antimicrobial peptide derived from the Australian tree frog Litoria citropa, alone and in combination with ampicillin, ceftriaxone, doxycycline, netilmicin, ciprofloxacin, rifampicin, linezolid, vancomycin, clarithromycin and imipenem against 12 nosocomial isolates of Rhodococcus equi. METHODS: Antimicrobial activity of citropin 1.1 was measured by MIC, MBC, time-kill studies and chequerboard titration method. RESULTS: All isolates were inhibited at concentrations of citropin 1.1 between 2 and 8 mg/L. Combination studies demonstrated synergy only when the peptide was combined with clarithromycin, doxycycline and rifampicin. CONCLUSIONS: Our findings show that citropin 1.1 is active against R. equi and that its activity could be enhanced when it is combined with hydrophobic antibiotics.