Molecular Characterization

In 1991, Wilson sequenced a fragment of 16S rRNA from a patient with WD.[3] This sequence was original and showed that, phylogenically, the bacterium was included to the Actynomyces clade. One year later, the sequencing of 1321 bp of the 16S rRNA from the biopsy of a patient confirmed these data;[21] the bacterium was named then Tropheryma whippelii. After the culture and the description of the bacterium, the bacterium was officially named Tropheryma whipplei.[4] At the time the genome sequencing of T. whipplei was initiated, only five genes were identified: 16S rRNA, 5S rRNA, 23S rRNA, rpoB, and hsp65.[22] Phylogenetic analysis classified T. whipplei within the high G+C content Gram-positive bacteria class (Actinobacteria, relatively near to two known species in human pathology, Actinomyces pyogenes and Rothia dentocariosa, and to bacteria principally found in the environment). The 16S-23S interregion of T. whipplei had been studied by PCR and sequencing from clinical samples from patients with WD.[11] The sequence analysis has revealed the existence of six different types of spacer.[11] The more frequent spacer types were types 1 and 2.[23]

Recently, the genome of two different strains of T. whipplei has been entirely sequenced.[22] Sequencing of T. whipplei Twist, propagated from the cardiac valve of a patient, revealed a 927,303-bp genome which encodes 808 predicted protein-coding genes and presents a GC% of 47%.[22] Sequencing of T. whipplei TW08/27, propagated from the cerebrospinal fluid of a patient, revealed a 925,938-bp genome which encodes 784 predicted protein-coding genes and presents a GC% of 46.3%.[24] T. whipplei presents a unique circular chromosome and is the only known reduced genome species (< 1 Mb) within the Actinobacteria.[22,24] The two genomic sequences of the two strains are mostly (>99%) identical at the nucleotide sequence level and encode quasi-identical gene complements. One of the specific genome features includes deficiencies in amino acid metabolisms. This observation points to a host-restricted lifestyle for T. whipplei. To compensate for the defective biosynthetic pathways, the missing amino acids must be obtained from the environment or the host. This information has allowed the design of a comprehensive medium, supplemented with amino acids, in which T. whipplei grows axenically.[10] Another specific characteristic of T. whipplei is the lack of clear thioredoxin and thioredoxin reductase genes. Thioredoxin reductase is a ubiquitous enzyme that reduces thioredoxin, which in turn acts as electron donor in various essential redox reactions in the cell.[22] A functional thioredoxin system has systematically been retrieved in all bacterial genomes sequenced so far. Thus T. whipplei might be the first example of a bacterium without the usually essential thioredoxin pathway, but this observation must be confirmed by further experiments. A mutation in DNA gyrase, which predicts a resistance to quinolone antibiotics, is also present. These data are consistent with the high minimal inhibitory concentration of ciprofloxacine observed in vitro for T. whipplei.[25] The genome of T. whipplei contains also a gene cluster that seems to direct the biosynthesis and export of extracellular polysaccharide. This observation could explain the fact that the T.whipplei cell wall includes, in addition to peptidogly-can, an unusual inner layer composed mainly of polysaccharides which accounts for the positive PAS staining reaction. The alignment of the two genome sequences revealed a large chromosomal inversion, the extremities of which are located within two paralogous genes.[24] These genes belong to a large cell-surface protein family defined by the presence of a common repeat highly conserved at the nucleotide level. The repeats appear to trigger fragment genome rearrangements in T.whipplei, potentially resulting in the expression of different subsets of cell-surface protein.[24] This phenomenon might represent a new mechanism developed to evade the host's immune response during the course of this chronic disease. It is also remarkable to note a high frequency of repeats in T. whipplei because usually repeated sequences are considered to be less frequent in the reduced genomes.[24] Besides, new primers were designed according to the sequence of this highly conserved domain, allowing the improvement of the current molecular diagnosis of WD.[26] The isolated lifestyle of parasitic bacteria has been proposed to induce ''ongoing genome degradation.'' However, T. whipplei genome exhibits a coding content (86%) comparable to the free-living bacteria and shows a few pseudogenes and a small sign of ''ongoing degradation.'' Finally, the important finding derived from T. whipplei genome sequences is the frequent genomic instability mediated by protein-coding repeats within genes of membrane proteins.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook


Post a comment