Bacteria are the most dominant form of free-living life on Earth, and represent a major part of its genetic diversity. Some bacteria are closely adapted to a single environment in which they reside, whereas many others are capable of thriving across multiple environments. They range from the most benign inhabitants of the Earth to being deadly, multi-drug-resistant human pathogens. The success of bacterial life is exemplified by the wide variation in their genetic content, not just across phyla, but even among members of the same species. In addition is the plethora of gene regulatory mechanisms that—to state a most cliched phrase—ensure that appropriate genes are ex pressed when required. These two features of bacterial biology form the crux of this book.

It will not be wrong to state that the advent of genomics has considerably advanced our knowledge of both evolutionary and gene regulatory mechanisms in bacteria. The ambitious—and now historical—genome sequencing projects, driven by whole genome shotgun sequencing using automated Sanger sequencers, taught us many things about bacterial genetic diversity and the mechanisms underlying its generation. For example, by sequencing the genomes of many members of a species, such as Escherichia coli, we learnt that some bacteria have what is called an open pan genome: every new genome of a member of that species will invariably identify many genes hitherto un known in that species. Genome sequencing projects also stoked a controversy on the importance of horizontal gene transfer to genome growth and bacterial evolution itself: in the face of rampant horizontal transfer, does the concept of a bacterial species have any meaning? At the other end of the scale, genome reduction emerged as an important phenomenon under lying the evolution of obligate parasites, including those of major human pathogens such as the Rickettsia and Mycobacterium leprae.