A method for producing a recombinant polypeptide of interest comprising: a) obtaining a population of cells auxotrophic for a first metabolite and a second metabolite, wherein said second metabolite is a natural amino acid; b) introducing into cells of said population a first nucleic acid construct comprising an auxotrophic selection marker, wherein said auxotrophic selection marker comprises a first nucleic acid sequence encoding at least one polypeptide active in the biosynthesis of said first metabolite, and wherein expression of said auxotrophic selection marker restores prototrophy for the first metabolite; c) introducing into cells of said population a: (i) a second nucleic acid sequence encoding said recombinant polypeptide of interest; and (ii) a promoter operably attached to said second nucleic acid sequence so as to direct expression of the second nucleic acid sequence; d) subjecting said population of cells to a first medium lacking said first metabolite under conditions such that transformed cells having restored prototrophy for the first metabolite are obtained; and e) subjecting said transformed cells to a second medium comprising a non-natural amino acid correlating to said second metabolite under conditions such that said second nucleic acid sequence is expressed to produce said recombinant polypeptide of interest having said non-natural amino acid incorporated therein; wherein said first medium and said second medium are the same or different; wherein said second nucleic acid sequence is provided on said first nucleic acid construct and wherein said host cell is selected from Gram-negative Proteobacteria Subgroup 18. a) obtaining a population of cells auxotrophic for a first metabolite and a second metabolite, wherein said second metabolite is a natural amino acid; b) introducing into cells of said population a first nucleic acid construct comprising an auxotrophic selection marker, wherein said auxotrophic selection marker comprises a first nucleic acid sequence encoding at least one polypeptide active in the biosynthesis of said first metabolite, and wherein expression of said auxotrophic selection marker restores prototrophy for the first metabolite; c) introducing into cells of said population a: (i) a second nucleic acid sequence encoding said recombinant polypeptide of interest; and (ii) a promoter operably attached to said second nucleic acid sequence so as to direct expression of the second nucleic acid sequence; d) subjecting said population of cells to a first medium lacking said first metabolite under conditions such that transformed cells having restored prototrophy for the first metabolite are obtained; and e) subjecting said transformed cells to a second medium comprising a non-natural amino acid correlating to said second metabolite under conditions such that said second nucleic acid sequence is expressed to produce said recombinant polypeptide of interest having said non-natural amino acid incorporated therein; wherein said first medium and said second medium are the same or different; wherein said second nucleic acid sequence is provided on said first nucleic acid construct and wherein said host cell is selected from Gram-negative Proteobacteria Subgroup 18.