MECHANISMS OF ACTION OF ANTIBIOTICS I.Escherichia coli

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Abstract
The action of sub-bacteriostatic concentrations of chloramphenicol, chlortetracycline, and oxytetracycline on the logarithmic growth rates of E. coli, strain B, was determined for the single antibiotics and for the 3 possible combinations in pairs. Reductions in growth rates were expressed as per cent inhibition, and linear dosage-response curves were obtained by plotting the probits of per cent inhibition against the logarithms of antibiotic concentrations. Fifty per cent inhibitory concentrations, ED50, were determined from these curves for the antibiotics, singly or in series of pairs. "Fractional inhibitory concentrations" were calculated by dividing the partial antibiotic concentrations in each pair, representing one ED50, by the ED50 for the single antibiotics. The sums of the fractional inhibitory concentrations in such pairs were close to 1.0, indicating a strictly additive action of the 3 antibiotics in the test system employed. Equimoiar concentrations of chlortetracycline and oxytetracycline gave identical inhibitory effects while, molecule for molecule, 5.5 times as much chloramphenicol was required to duplicate the action of each of the tetracyclines.