There have been many important recent developments in our knowledge of the breadth of prokaryote diversity, our understanding of the driving forces behind that diversity, and of its significance for our lives and for fundamental processes upon Earth. It has become clear that the microbes we know about are actually just the tip of a biological iceberg. In fact, the majority of microbes are unculturable on laboratory media at present. Much of our attention has been focused on pathogens, understanding their interaction with the host and how to prevent disease. However, there is a growing appreciation that without microbes fundamental ecological processes would not be balanced. For example, microbes in the ocean have a direct influence on the composition of the atmosphere we breathe.

A major advance in allowing us to understand the extent and nature of microbial diversity has been the development of genome sequencing. In parallel, there has been the development of tools to allow whole-genome comparisons. This has facilitated the study of microbial diversity and evolution, such as allowing the tracking of unculturable organisms, the study of organisms from extreme environments, and of medical and environmental bacteria and interactions between them. It has given us insights into the exchanges of genes between organisms, resulting in an understanding of the emergence of pathogens, a process which involves both gene acquisition and gene loss. Genomic comparison has helped to identify core genes, to the point where we can predict a minimal genome needed for life, which can be supplemented by the horizontal transfer of genomic islands, phenotypic innovation and catabolic pathway evolution.

From birth we are protected from bacterial infections by the complex system of cells and cell products, which have functional and signalling properties, known collectively as immunity. The immune system has three major functions: (1) the ability to recognise infectious agents such as bacteria; (2) the capacity to kill these infecting organisms; and (3) the integration of (1) and (2) through specific cell–cell signalling. It is now recognised that the nature of our immune systems has been shaped in the crucible of evolution by interactions with infectious agents. It is also emerging that the various organisms that can infect us have evolved multiple mechanisms to evade both arms of our immune system – innate and adaptive immunity.

This book describes some of the emerging mechanisms employed by bacteria to evade both humoral and cellular immunity. The first section deals with novel aspects of the recognition of, and the response to, bacteria by a key cell population – dendritic cells (e.g., through Toll-like receptors), and by lymphocytes via the nonpolymorphic CD1 MHC molecules that recognise nonpeptidic antigens. The final chapter in this section describes natural resistance-associated macrophage protein (NRAMP), a metal ion transporter important in susceptibility to infection by mycobacteria. Mycobacteria also encode NRAMP-like proteins revealing another twist in the ongoing battle between bacteria and their hosts for essential metal ions such as iron and zinc.

In the second section attention switches to the ability bacteria have to evade humoral immunity.