Chloramphenicol damages bacterial DNA

L(+)-threo-chloramphenicol induces reversion of His− Salmonella typhimurium strains TA100 and TA1535 in the conventional Ames' assay without microsomal activation. Any mutagenicity of D(−)-threo-chloramphenicol was masked by toxicity. Similarly, a sensitive fluctuation test showed mutagenesis with L(+)-threo-chloramphenicol at concentrations of 0.5 μM and above but the D(−) isomer proved to be toxic even at these low levels. The L(+) isomer caused single strand breaks in the DNA of Escherichia coliBr and Salmonella typhimurium strains TA1535, TA100 and TA1976. The D(−) isomer caused breaks in Escherichia coliBr and Salmonella typhimurium TA1976 although it was less effective and it did not produce DNA breaks in TA1535 or TA100.