Xer-cise enables bacteria to be genetically modified without leaving antibiotic resistance genes on their chromosomes. Xer-cise therefore overcomes the problems caused by the biosafety risk of potentially spreading antibiotic resistance to pathogens, the limited number of available antibiotic resistance genes and the competing use of these genes on plasmids.
Xer-cise utilises Xer recombinases, which are naturally present in virtually every bacterial species. Genetic modification is achieved by inserting a DNA cassette containing an antibiotic resistance gene, so that modified bacteria can be identified by their ability to survive in the presence of the antibiotic. By placing the sites recognised by Xer recombinases (dif sites) either side of the antibiotic resistance gene, the recombinases excise the gene when cells are grown later with no antibiotic present.
Xer-cise has been demonstrated in bacteria including Escherichia coli, Salmonella, Bacillus subtilis and Mycobacterium, is applicable to many other bacteria.
Simon Saxby, VP of RecipharmCobra Biologics, the specialist biologics process development and cGMP manufacturing division of the international contract development and manufacturing organisation, Recipharm AB, said: “Xer-cise has enabled molecular biologists at RecipharmCobra and elsewhere to rapidly construct bacterial strains that are not antibiotic resistant, and are therefore advantageous for the commercial production and delivery of biologics. We anticipate that this technology will greatly simplify and accelerate the genetic modification of many species of bacteria.”