Novel pathway for curcumin utilization found in diverse Gram-negative soil-dwelling bacteria with evidence of horizontal gene transfer

Bacteria are known to contain complex pathways to metabolize a plethora of recalcitrant compounds, especially in soil and gut microbiomes. Plant-derived polyphenols are often considered to be non-pharmaceutical antimicrobials that exist within both the soil and gut. Curcumin is the polyphenol active compound in turmeric root ( Curcuma longa ) and demonstrates anti-inflammatory, antioxidative, and anti-cancer properties. The NADPH-curcumin/dihydrocurcumin reductase gene (curA), yielding curcumin-reducing enzymatic activity, was previously discovered in the genomic DNA of Escherichia coli strains isolated from stool samples. We hypothesized that curA may also be present within soil-dwelling bacteria. In the current study, we isolated 15 Gram-negative soil-dwelling bacteria with the highly conserved curA gene, using growth selection strategies including Winogradsky columns and Rapid Flask Cultures containing turmeric powder. Using in silico synteny analysis, we found that curA is one gene within a highly conserved synteny cluster of 13 genes (ydcU, ydcV, patD, yncL, ortT, ydcY, ydcZ, mnaT, curA, mcbR, yncD, yncE, and ansP) encoding metabolic-related proteins including enzymes, transport systems, and genetic regulators. Reverse-transcriptase quantitative PCR (RT-qPCR) analysis showed curA and 12 genes within the synteny cluster were significantly upregulated during 48 h growth with 1% turmeric in E. coli str. K-12 substr. DC10B (positive control strain) and soil-isolated Chryseobacterium sp. Notably, ATR-FTIR data supported preferential cell-associated accumulation/retention of curcuminoid-derived material consistent with uptake. Metatranscriptomics analysis of soil-turmeric growth slurries identified active transcription of genes associated with central carbon metabolism and amino acid biosynthesis pathways, consistent with the transcriptional activation of the curA synteny cluster observed in vitro. In addition, within the same soil-turmeric slurries, we observed transcriptional activation of putative type IV and type VI secretion system genes, consistent with potential mobility of the synteny cluster. Collectively, the present study provides substantial genomic, transcriptional, and biochemical evidence consistent with a candidate curcumin utilization pathway in Gram-negative bacteria.