I had the opportunity to attend for the first time the ISSSI conference, devoted to the study of Staphylococci in the beautiful Copenhagen. It was a fantastic conference in which I met lots of researchers with common interests and reunited with collaborators.
This occasion was special because I got to present for the first time results from my work on bacteria, from one of the projects I’m involved in. It was entitled “Massive gene decay and insertion sequence acquisition has shaped the evolutionary history of the host restricted Staphylococcus aureus subsp. anaerobius”.
Staphylococcus aureus subsp. anaerobius is the aetiological agent of Morel’s disease in sheep and goats, which causes very specific abscesses in lymph nodes. This bacteria only grows in microaerophilic conditions and is unable to infect other hosts. We applied whole-genome sequencing to a collection of isolates from several countries and aimed to examine its evolutionary history and to understand the molecular basis of its host adaptation and restricted metabolism.
Using phylogenetics and population genomics we inferred that anaerobius emerged at least a thousand years ago but it has had a very limited expansion. It seems to have evolved from a S. aureus subsp. aureus ancestor that jumped to a new host and underwent an extreme host-adaptation that changed its genome dramatically. Some of the genomic signatures of this process are a massive gene decay mediated through the accumulation of many pseudogenes and insertion elements and the existence of large chromosomal rearrangements. Similar evidence of restricted niche adaptation had been reported in other distantly related bacteria such as Mycobacterium or Yersinia, but it is unheard of in the Staphylococci context.
The presence of these abundant pseudogenes (that take up 10% of the genome) could explain why this bacterium has such a restricted metabolism (including being unable to grow in aerobic conditions) and reduced pathogenicity.
We also performed expression analyses that revealed that the insertion elements are being transcribed and this represses the transcription of the genes located next to those insertion elements. We suggest that these control of gene expression mediated by insertion elements underpins an orchestrated mechanism of host adaptation.
To sum up, Staphylococcus aureus subsp. anaerobius is remarkable example of drastic modifications that affect a bacterial genome during severe adaptation to a new host.
I got great feedback and comments which will hopefully help me finish up this study and aim for a publication soon!