ABSTRACT
Over the past 40 years, the study of antibiotic resistance has revealed a new world of genes that are part of what can best be described as a “floating” or “itinerant” genome that is shared by a vast number of bacterial species and indeed genera. The floating genome contains genes that are not essential to the basic life of a bacterium but are “optional extras” that allow adaption to new conditions. Movement of these genes from one bacterium to another occurs by the process of horizontal gene transfer and the mechanisms by which this is achieved are the subject of a separate chapter (Chapter 3 a). Genes that are part of mobile elements that can translocate have the capacity to board the vehicles that traffic the highways and byways of horizontal gene transfer. The main forces that drive the evolution of multidrug resistant bacteria are thus horizontal gene transfer and translocation and, though these two types of gene movement are inextricably intertwined, for clarity it is important to clearly separate them conceptually. The difference between these two fundamentally distinct types of gene movement is illustrated in Figure 1.
