Susceptibility of Enterococci and Listeria Monocytogenes to N-Formimidoyl Thienamycin Alone and in Combination with an Aminoglycoside

The susceptibilities of 12 strains of enterococci and 10 stains of Listeria monocytogenes to N-formimidoyl thienamycin were determined by a standard broth dilution method. Minimal inhibitory concentrations for L. monocytogenes strains were less than 0.25 micrograms/ml. As a group, strains of Streptococcus faecium were less susceptible than Streptococcus faecalis strains, but even for the latter, the minimal inhibitory concentrations were slightly greater than those previously reported. Minimal bactericidal concentrations against all organisms were many-fold higher than the corresponding inhibitory concentrations. In time-kill studies, combinations of N-formimidoyl thienamycin with gentamicin were synergistic against (or they completely sterilized cultures of) all the enterococcal strains and nine to 10 strains of L. monocytogenes. The magnitude of killing by the combinations was comparable to that previously observed with penicillin-gentamicin combinations. N-Formimidoyl thienamycin-tobramycin combinations were synergistic against those strains of S. faecalis and L. monocytogenes tested, but not against those of S. faecium.