Growth and septation of animal-attached bacteria

Up to now, the study of the molecular and cellular mechanisms of bacterial reproduction focused on model bacteria such as Escherichia coli. On the other hand, studies on the cell biology of bacteria in their natural environment - including those thriving on animal surfaces - are scarce. Four gammaproteobacterial phylotypes reproduce atipically: the rod-shaped Laxus oneistus and Robbea hypermnestra symbionts grow in width and set their constricting rings longitudinally. Eubostrichus fertilis symbiont cells can divide at virtually every length between 4 and 45 m resulting in an unprecedented 10-fold length variation within the same cell population. Finally, symmetric transverse fission of up to 120 m-long E. dianeae symbionts makes them the longest bacteria in which binary division has ever been observed. In this research proposal we want to determine the molecular and cell biological mechanisms underlying growth and division of non-model, animal-attached Gammaproteobacteria. The ultimate goal is the identification of the growth and septum positioning mechanisms conserved among all Gammaproteobacteria, a group of microorganisms of paramount interest, both ecological and medical. We will address these questions by studying cell division proteins in cell-free systems and by applying a wide palette of state-of-the-art and classic microscopic techniques (e.g. and Selective Plane Illumination Microscopy on live animal-baceria consortia, 3D structured illumination microscopy, cryo-EM and confocal laser scanning microscopy).

Bacterial cell growth and division have only been studied in a dozen of cultivable species in spite of the fact that millions of them are estimated to live on our planet. This knowledge gap must be urgently filled if we want to grasp the conserved fundamentals of cell reproduction. Therefore, we studied the reproductive strategies of Thiosymbion, a group of non-model bacteria, which exclusively occur on the surface of animals (ectosymbionts). In particular, in longitudinally dividing Thyiosymbion, we found that: 1) septation can start at one cell pole only, so that a ring of the tubulin homolog FtsZ is dispensable; 2) not only bacterial cell division but also cell growth can be host-polarised; 3) the actin homolog MreB is medial throughout the cell cycle and its polymerisation is required for medial FtsZ polymerisation and septation; 4) a bidimensional segregation mode maintains symbiont chromosome orientation toward it host. We hypothesise that these extraordinary cell biological features are adaptions to the symbiotic lifestyle. Establishment of symbiont cultures and development of gene manipulation/protein imaging techniques are ongoing to prove that cell biological adaptations, such as longitudinal division or fixed chromosome configuration, are required for symbiosis establishment or maintenance.