In situ recombination assay based on colony colour. (A) Schematic illustration of the recombination assay, which leads to an insertion of the blasticidin resistance gene (bsd) into the genomic malK locus. Insertion mutates the malK gene, so recombinant colonies are white when plated on MacConkey agar. (B) Schematic representation of the reverse recombination event, which restores the functionality of the malk gene by deleting bsd, which was inserted in (A). The linear substrate was a 400 bp dsDNA fragment of the malK gene. (C) Picture of red (non-recombinant) and half white/red (recombinant) colonies obtained using the design shown in (A). (D) Picture of white (non-recombinant) and half red/white (recombinant) colonies obtained using the recombination design shown in (B).