Gene replacement. (a) Schematic representation of the strategy used to construct mutants by gene replacement. Small, red and shaded arrows represent the primers, the target gene, and the kanamycin (Km) resistance cassette, respectively. The three PCR products obtained (PCR1, PCR2, and PCR3) were mixed at equimolar concentrations and subjected to a nested overlap-extension PCR to generate the desired linear DNA (see Materials and Methods for details). (b) Diagram showing the integration of the linear DNA via two recombination events. (c) Representation of the original genetic material replaced by the recombinant DNA on the A. baumannii chromosome.