A century of typhus, lice and Rickettsia

Section snippets
Historical introduction

The typhus Rickettsia, once strictly associated with wars and human disasters, has been a major focus of research during the last century. The clinical syndromes of epidemic typhus were first described during the 16th century in the Mediterranean area. From here the disease followed each army moving up through Europe, reducing their numbers and spreading into the civilian populations through their survivors. The term typhus was first used in 1760 and was distinguished from typhoid fever in 1836.
Phylogeny, epidemiology and clinical features

Historically the name `rickettsiae' has been used for small rods that could not be cultivated. The obligate intracellular lifestyle and the pathogenicity of many species have made it very difficult to study the biology, epidemiology and phylogeny of these bacteria. Initially, the rickettsiae were grown directly in cultures with lice or by serial infections of guinea-pigs. The development of cultivation methods for rickettsiae in the yolk sacs of hen's eggs in the 1940s represented an important
The genome sequence of R. prowazekii

The 1.1-Mb genome sequence of the aetiological agent of epidemic typhus, R. prowazekii, was published in 1998 [8] (figure 2). One of the most controversial aspects of this genome sequence was its low coding content. Many scientists found it hard to accept that as much as 24% of this extremely small genome would be nothing but junk DNA. In total, only 834 protein coding genes were identified and these were sorted into their functional categories to provide a description of the R. prowazekii
Degradation of the Rickettsia genomes

It seems reasonable to assume that the massive losses of genetic information in Rickettsia might have left traces in the modern genome sequences, particularly if genes are still being discarded from the Rickettsia genomes. Nevertheless, it was at first a surprise to find that the metK gene, which codes for AdoMet synthetase, and is essential in all other species, contained a termination codon at a central position of the gene [1]. Even more surprising, a second pseudogene was found downstream
Rickettsia and mitochondria

Two different models have been suggested for the origin of mitochondria and chloroplasts. One model suggests that mitochondria are derived from the inside, i.e. of autogenous origin [16]. The other model suggests that they are derived from the outside, i.e. of xenogous origin [13]. The latter model, the so-called endosymbiont theory, suggests that mitochondria originated from a eubacterium that gained access into an early version of the eukaryotic cell. Subsequent to its integration, it is
Conclusions

The complete R. prowazekii genome sequence has confirmed that there is a close phylogenetic relationship between Rickettsia and mitochondrial proteins. However, a majority of the nuclearly encoded mitochondrial proteins seems to be derived from within the nuclear genome. Based on these findings we have suggested a new hypothesis for the origin of mitochondria. The ox-tox hypothesis suggests that the selective pressure driving the acquisition of mitochondria was aerobic respiration, but that the