The exploitation of bacterial genome sequences has so far provided a wealth of new general information about the genetic diversity of bacteria, such as that of many pathogens. Comparative genomics uncovered many genome variations in closely related bacteria and revealed basic principles involved in bacterial diversification, improving our knowledge of the evolution of bacterial pathogens. A correlation between metabolic versatility and genome size has become evident. The degenerated life styles of obligate intracellular pathogens correlate with significantly reduced genome sizes, a phenomenon that has been termed "evolution by reduction". These mechanisms can permanently alter bacterial genotypes and result in adaptation to their environment by genome optimization. In this review, we summarize the recent results of genome-wide approaches to studying the genetic diversity of pathogenic bacteria that indicate that the acquisition of DNA and the loss of genetic information are two important mechanisms that contribute to strain-specific differences in genome content.