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 Staphylococcus aureus (MRSA). A metagenomic DNA soil library
containing 19 200 recombinant Escherichia coli 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 (Cm) derivatives, including three new compounds whose structures were
established as 1-acetyl-3-propanoyl­chloramphenicol, 1-acetyl-3-butanoyl­chloramphenicol,
and 3-butanoyl-1-propanoyl­chloramphenicol. Cm was used as the selectable antibiotic for cloning, suggesting that
heterologously expressed enzymes resulted in derivatization of Cm into new chemical entities with biological activity.
An esterase was found to be responsible for the enzymatic regeneration
of Cm, and the gene trfA 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-p-nitrobenzoyl­chloramphenicol
the most active against Mycobacterium intracellulare and Mycobacterium tuberculosis, with MICs of 12.5
and 50.0 μg/mL, respectively.