Chloramphenicol damages bacterial DNA.

L(+)-$\text{threo}$-chloramphenicol induces reversion of His^?$\text{Salmonella typhimurium}$ strains TA100 and TA1535 in the conventional Ames' assay without microsomal activation. Any mutagenicity of D(?)-$\text{threo}$-chloramphenicol was masked by toxicity. Similarly, a sensitive fluctuation test showed mutagenesis with L(+)-$\text{threo}$-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 $\text{Escherichia coli}\text{B}\text{r}$ and $\text{Salmonella typhimurium}$ strains TA1535, TA100 and TA1976. The D(?) isomer caused breaks in $\text{Escherichia coli}\text{B}\text{r}$ and $\text{Salmonella typhimurium}$ TA1976 although it was less effective and it did not produce DNA breaks in TA1535 or TA100.