Antimicrobial resistance (AMR) remains a major public health concern affecting across the globe. Specifically, lots of efforts have been undertaken to counteract the acquisition of antimicrobial resistance genes (ARGs) in foodborne pathogens. Spontaneous mutations and/or horizontal gene transfer leading to acquisition of ARGs can inadvertently cross-protect microbes to other, unrelated microbial control agents used throughout the agri-food chain (e.g., food preservation methods, biocides, etc.). Therefore, what should we expect from a non-AMR foodborne pathogen acquiring ARGs? An effective approach to address this challenge is to study the phenotypic profile of customized AMR variants derived from parental foodborne strains with well-described genomes. In this chapter, we describe two methodologies used to transform Salmonella Typhimurium LT2 (SeT) with either extrachromosomal plasmids carrying ARGs (Protocol No. 1—Plasmid Transformation), or linear DNA fragments carrying a specific mutation for stable incorporation into the genome by substituting parental loci of ARGs (Protocol No. 2—Gene Editing). The application of any of these two protocols facilitates the construction of strains with defined genotypic and phenotypic profiles, aiding in the characterisation of their resistance to antibiotics and microbial control agents used in the agri-food chain. Both protocols can be easily adapted to a wide range of Gram-negative bacteria.