Heikkinen E, Kallonen T, Saarinen L, Sara R, King AJ, et al (2007) Comparative Genomics of Bordetella pertussis Reveals Progressive Gene Loss in Finnish Strains. PLoS ONE 2(9): e904. doi:10.1371/journal.pone.0000904
Nous avons ici un exemple d’évolution d’une bactérie pathogène qui est loin de la résistance aux antibiotiques, même si c’est aussi un effet secondaire, et indésirable, en réponse à une pression de sélection iatrogène. C’est l’immunisation de la population (vaccination) qui est ici l’élément sélectif, agissant sur une période allant de 1953 à 2004.
Il est intéressant de voir comment le pathogène est modifié progressivement (sélection cummulative, isn’t it?) à travers cette période, y compris en perdant des fragments génomiques, ce qui pourrait être considéré comme négatif si l’approche était naïve.
Background. Bordetella pertussis is a Gram-negative bacterium that infects the human respiratory tract and causes pertussis or whooping cough. The disease has resurged in many countries including Finland where the whole-cell pertussis vaccine has been used for more than 50 years. Antigenic divergence has been observed between vaccine strains and clinical isolates in Finland. To better understand genome evolution in B. pertussis circulating in the immunized population, we developed an oligonucleotide-based microarray for comparative genomic analysis of Finnish strains isolated during the period of 50 years.
Methodology/Principal Findings. The microarray consisted of 3,582 oligonucleotides (70-mer) and covered 94% of 3,816 ORFs of Tohama I, the strain of which the genome has been sequenced. Twenty isolates from 1953 to 2004 were studied together with two Finnish vaccine strains and two international reference strains. The isolates were selected according to their characteristics, e.g. the year and place of isolation and pulsed-field gel electrophoresis profiles. Genomic DNA of the tested strains, along with reference DNA of Tohama I strain, was labelled and hybridized. The absence of genes as established with microarrays, was confirmed by PCR. Compared with the Tohama I strain, Finnish isolates lost 7 (8.6 kb) to 49 (55.3 kb) genes, clustered in one to four distinct loci. The number of lost genes increased with time, and one third of lost genes had functions related to inorganic ion transport and metabolism, or energy production and conversion. All four loci of lost genes were flanked by the insertion sequence element IS481.
Conclusion/Significance. Our results showed that the progressive gene loss occurred in Finnish B. pertussis strains isolated during a period of 50 years and confirmed that B.pertussisis dynamic and is continuously evolving, suggesting that the bacterium may use gene loss as one strategy to adapt to highly immunized populations.
