Short History of Theories and Discoveries

The complex mutual relationship between intracellular microbes and their host cells is a challenging field of research and requires the perspective of evolution biology. The individual host-microbe interactions covered in this book all raise the following questions: how do microbes enter, survive and proliferate in, and how do they exit host cells ? And how can intracellular niches be characterized and what are the benefits of intracellular life for the microbes and its consequences for the host cell? The question, however, is how and under what selective pressure did these interactions evolve? The year 2009 marks the 200th birthday of Charles Darwin (1809-1882; 12th February 1809), and, more importantly, the 150th anniversary of the publication of his most important book The Origin of Species by Means of Natural Selection (24th November 1859) [1]. In this eminent and highly disputed and provocatively revolutionary work, Darwin outlined the concept of evolution by natural selection in the struggle of life. The concept of interspecies competition as the driving force for the evolution of all bacterial, animal and plant species laid the basis for modern day biology.

Louis Pasteur (1822-1895) and others proved that microbial life did not arise spontaneously and miraculously, but rather due to the omnipresence of microorganisms, an important fact for food preservation and the consequential establishment of sterilization techniques. The seminal work of the nineteenth-century microbiologists set the path to study the novel complexity of interspecies interactions in natural science and medical research. Although infectious diseases were an important determinant for human history, causing migration, settlement and conflict behavior, it was not until the nineteenth century that infectious agents were identified as causative agents for certain diseases rather than the diseases being of mysterious origins. The time between the end of the nineteenth and the beginning of the twentieth century was the high season of bacteriology, during which a huge number of microbial species were identified using newly developed culture techniques. Many of these microbes were associated with humans, animals or plants, and they were either pathogens, beneficial symbionts or commensals. A number ofthose microbes had chosen other unicellular or multicellular organisms as their ecological niches. Finally, infectious diseases were recognized as the driving force for the evolution of the innate and, in vertebrates, the acquired immune systems (Chapter 12).

Robert Koch (1843-1910) and his colleagues identified the first intracellular pathogenic bacterium, the tubercle bacillus (Mycobacterium tuberculosis). In the late nineteenth century tuberculosis was the prime cause of death in the metropolitan areas of Europe and North America, stirring up intensive medical and scientific interest. At around the same time, an important virulence trait of the tubercle bacillus, that is, living in macrophages, was described by Elie Metchnikoff (1845-1916), the founder of phagocyte biology. This is still a prime topic in tuberculosis research today (see Chapter 19). Metchnikoff was the first to observe the phagocytosis of bacteria by phagocytes in 1883 during his time at the Viennese Institute of Zoology and he also pointed out the importance of these cells in host response and inflammation [2,3]. The term macrophage was attributed to him and made him the founder of innate immunity. In 1908, he received the Nobel Prize for his achievements. Metchnikoff was also the first to observe tubercle bacilli thriving intracellularly in macrophages (Figure 1.1) [4]. However, it was not until the last quarter of the twentieth century that scientists started to study the virulence factors of pathogens, and that intracellular pathogens (and symbionts) were highlighted for their unique capabilities to survive within and manipulate their host cells.

The identification of intracellular survival mechanisms was made possible by novel techniques in cell biology and the arrival of modern molecular genetics. J. A. Armstrong and Philip D'Arcy Hart [5, 6] were the first to show inhibition of phagolysosome fusion by the tubercle bacillus. Similar peculiarities of Toxoplasma gondii- and Chlamydia psittaci-containing vacuoles were published in 1979 and 1981, respectively [7, 8]. In the last decade of the twentieth century, many virulence traits of intracellular microbes were elucidated. Genome analyses and molecular techniques, paired with novel model systems such as yeast two-hybrid screening technology, uncovered pathogenicity islands and plasmids, virulence factors, as well as host cell target structures. It was discovered that throughout evolution there must have been a tremendous horizontal gene transfer between different microbes as well as between bacteria and eukaryotes (Chapter 2). Many of those pathogens and their virulence traits will be covered in this book. Some important intracellular microbes, such as M. leprae, Chlamydia and Rickettsia, are not yet accessible to

1.2 A Look Through the Microscope of Evolution j 5

1.2 A Look Through the Microscope of Evolution j 5

Clamydia Microscope

Figure 1.1 (a) Elie Metchnikoff (1845-1916) in (b) MetchnikofPs depiction of a pigeon his later life. He was the founder of phagocyte macrophage containing mycobacteria.

biology and coined the term "macrophage." (c) Macrophage culture infected with myco-Metchnikoffwas also the first scientistto observe bacteria as observed by Elie Metchnikoff.

tubercle bacilli within macrophages and These pictures were kindly provided by Stefan H.

suggested they be able to survive within these E. Kaufmann, Max-Planck-Institute of Infection cells, which otherwise are able to kill microbes. Biology, Berlin, Germany.

Figure 1.1 (a) Elie Metchnikoff (1845-1916) in (b) MetchnikofPs depiction of a pigeon his later life. He was the founder of phagocyte macrophage containing mycobacteria.

biology and coined the term "macrophage." (c) Macrophage culture infected with myco-Metchnikoffwas also the first scientistto observe bacteria as observed by Elie Metchnikoff.

tubercle bacilli within macrophages and These pictures were kindly provided by Stefan H.

suggested they be able to survive within these E. Kaufmann, Max-Planck-Institute of Infection cells, which otherwise are able to kill microbes. Biology, Berlin, Germany.

manipulation by molecular genetics and future attempts will focus on generating targeted mutants in such organisms. A peek into the book of evolution of intracellular microbe genomes suggests that many of these virulence traits were established early in evolution, though probably not exactly for the purposes they are used for today.

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