Penicillin Combined with Gentamicin or Streptomycin: Synergism against Enterococci

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Abstract
The action of penicillin in combination with gentamicin against enterococci was studied. One hundred strains of enterococci, 14 of which were recovered from blood cultures of patients with endocarditis, were studied for susceptibility to penicillin, gentamicin, and streptomycin. All strains were inhibited by ≤50 µg of gentamicin/ml. The majority were inhibited by ≤78 µg of streptomycin/ml, but 13 strains were not inhibited by 50,000 µg/ml. Thirty-three strains were studied for synergism of combinations of antibiotics. A combination of 20 µg of penicillin and 4 µg of gentamicin/ml was synergistic against all 33 strains, while 20 µg of penicillin combined with 20 µg of streptomycin/ml was synergistic against only 20 of the 33 strains. The minimal inhibitory concentration of streptomycin for four of these 20 strains was more than 50,000 µg/ml. The combination of penicillin and gentamicin may be considered an alternative for the treatment of enterococcal endocarditis, especially when penicillin and streptomycin are not synergistic.

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