Abstract Inosine modification on tRNA anticodon (I34) is universally conserved in three kingdoms of life and critical to tRNA decoding capabilities. We found that tRNALeu(IAG) in commensal human bacterial families in Lactobaccilalles is concurrent with genome-wide synonymous leucine codon reprogramming. Pathway analysis reveals significant synonymous Leu codon changes in proteins in multiple KEGG pathways on cellular metabolism, where many genome-wide dominant UUA in families without tRNALeu(IAG