Chloramphenicol Derivatives with Antibacterial Activity Identified by Functional Metagenomics

A functional metagenomic approach identified novel and diverse soil-derived DNAs encoding inhibitors to methicillin-resistant <i>Staphylococcus aureus</i> (MRSA). A metagenomic DNA soil library containing 19 200 recombinant <i>Escherichia coli</i> BAC clones with 100 Kb average insert size was screened for antibiotic activity. Twenty-seven clones inhibited MRSA, seven of which were found by LC-MS to possess modified chloramphenicol (<i>Cm</i>) derivatives, including three new compounds whose structures were established as 1-acetyl-3-propanoyl­chloramphenicol, 1-acetyl-3-butanoyl­chloramphenicol, and 3-butanoyl-1-propanoyl­chloramphenicol. <i>Cm</i> was used as the selectable antibiotic for cloning, suggesting that heterologously expressed enzymes resulted in derivatization of <i>Cm</i> into new chemical entities with biological activity. An esterase was found to be responsible for the enzymatic regeneration of <i>Cm</i>, and the gene <i>trfA</i> responsible for plasmid copy induction was found to be responsible for inducing antibacterial activity in some clones. Six additional acylchloramphenicols were synthesized for structure and antibacterial activity relationship studies, with 1-<i>p</i>-nitrobenzoyl­chloramphenicol the most active against <i>Mycobacterium intracellulare</i> and <i>Mycobacterium tuberculosis</i>, with MICs of 12.5 and 50.0 μg/mL, respectively.