Susceptibilities to ceftriaxone of streptococcal strains associated with infective endocarditis

We determined the bactericidal activity of ceftriaxone on 20 streptococci isolated from patients with infective endocarditis and that of penicillin G on 5 strains. The MICs of ceftriaxone were less than or equal to 2 micrograms/ml and the MBCs were low for 5 nontolerant strains (less than or equal to 2 micrograms/ml) and high for 15 tolerant strains (greater than or equal to 16 micrograms/ml). The maximal reduction of the viable bacterial counts after 24 h of exposure to antibiotic was achieved for a concentration of ceftriaxone of 4, 32 and 256 micrograms/ml, respectively for 5, 10 and 19 strains. The activity of penicillin G was similar.