INTRODUCTION

The planet Earth was formed, in a molten state, some 4.5 billion years ago. It cooled off sufficiently 4 billion years ago to allow the formation of the oceans. Cyanobacteria, found in rock fossil records that are approximately 3.6 billion years old, provide the earliest evidence of life in the form of complex unicellular organisms. Molecular phylogeny is the tool that enables us to understand life in all its complexity and recognise relations hips between organisms. In essence, using molecular techniques we are able to deter mine the evolutionary relationships of living creatures. By comparing the difference in sequences of homologous genes encoding ribosomal RNA (16S rRNA genes from prokaryotic cells and 18S rRNA from eukaryotic cells) we can measure the evolutionary distance between species of organisms. Computer analysis of rRNA gene sequences suggests that cellular life has evolved along three major lineages. Two of these, Bacteria and Archaea, are exclusively microbial and consist of prokaryotic cells. The third lineage, Eukarya, not only contains unicellular organisms but also myriads of multicellular organisms. Two important points have emerged from the study of molecular phylogeny: 1) unicellular organisms are the major and most diverse form of life, and 2) eukaryotes are not of recent origin, as previously thought, but as ancient as the Bacteria and Archaea lineages, all of which have emerged from a universal ancestor. Although the human race may live in harmony, and is subject to colonisation with many different prokaryotic (e.g. bacteria) and eukaryotic (e.g. fungi and parasites) organisms, this harmony is shattered from time to time when a relatively restricted number of microbial species enter our body and cause pathology – infection.

Antimicrobials, developed on a mass scale from the 1940s onwards, brought renewed hope that, to quote the UN Surgeon General in his address to Congress in 1969, ‘We can close the book on infectious diseases’. Unfortunately, nothing could have been further from the truth. With the continuous emergence of microbes that display a vast array of antimicrobial resistance mechanisms, the ‘golden era’ of antimicrobials is under serious threat.