In vitro synergism of FR-31564, a new phosphonic acid antibiotic.

Against most test strains of Gram-negative bacilli, the in vitro effect of FR-31564 together with beta-lactam antibiotics or trimethoprim was strongly synergistic; with tetracycline and nalidixic acid the effect was additive; and with gentamicin and sulfamethoxazole the effect was additive or antagonistic. FR-31564 was markedly synergistic with beta-lactam antibiotics against beta-lactam antibiotic-resistant Gram-negative bacilli such as Klebsiella pneumoniae, Enterobacter aerogenes, E. cloacae, Citrobacter freundii and Serratia marcescens. The combination of FR-31564 with beta-lactam antibiotics effected a reduction of MICs against most of the test strains to clinically achievable concentrations in human serum.