INTRODUCTION

The evolution of bacterial pathogens is essentially the story of how all life evolves. It is a story of mutation and selection, of adaptation and survival, of incremental genetic changes and quantum leaps in genome content. All organisms evolve, but the evolution of microbes is the best studied. The short generation times of microbes and the ability to grow individual populations to large numbers allow researchers to study rare events over many generations – something that is next to impossible with larger, more complex organisms such as fruit flies, mice, and humans.

This chapter begins with a brief overview of the principles of mutation and selection in bacteria. This background will prepare the reader for the subsequent sections that will discuss the various forms of horizontal gene transfer (HGT) and how they each contribute to bacterial evolution. Specific examples are presented to give the reader insight into the enormous power of genetic selection as well as the great diversity of pathways that bacteria take in adapting to their environment. We also highlight some recurrent themes in bacterial pathogenesis. The chapter concludes with a discussion of a new paradigm of bacterial pathogen evolution that involves the loss of gene function as an adaptation to colonization of the host. The concept of pathoadaptation is introduced and expanded to include selection for gene loss as the pathogen improves its fitness within the host niche.