Phylogenetic analysis of secretion systems in Francisellaceae and Legionellales : Investigating events of intracellularization

Sammanfattning: Host-adapted bacteria are pathogens that, through evolutionary time and host-adaptive events, acquired the ability to manipulate hosts into assisting their own reproduction and spread. Through these host-adaptive events, free-living pathogens may be rendered unable to reproduce without their host, which is an irreversible step in evolution. Francisellaceae and Legionellales, two orders of Gammaproteobacteria, are cases where host-adaptation has lead to an intracellular lifestyle. Both orders use secretion systems, in combination with effector proteins, to invade and control their hosts. A current view is that Francisellaceae and Legionellales went through host-adaptive events at two separate time points. However, F. hongkongensis, a member of Francisellaceae shares the same secretion system as the order of Legionellales. Additionally, two host-adapted Gammaproteobacteria, Piscirickettsia spp. and Berkiella spp., swaps phylogenetic positions between Legionellales and Francisellaceae depending on methods applied - indicating shared features of Francisellaceae and Legionellales. In this study, we set up a workflow to screen public metagenomic data for candidate host-adaptive bacteria. Using this data, we attempted to assert the phylogenetic position and possibly resolve evolutionary events that occurred in Legionellales, F. hongkongensis, Francisellaceae, Piscirickettsia spp. and Berkiella spp. We successfully acquired 23 candidate host-adapted MAGs by (i) scanning for genes, among reads before assembly, using PhyloMagnet, and (ii) screening for complete secretion systems with MacSyFinder. The phylogenetic results turned out indecisive in the placement ofBerkiella spp. and Piscirickettsia. However, results found in this study indicate that, contrary to previous beliefs, it is possible that it was one intracellularization event of a common ancestor that gave rise to the intracellular lifestyle of Francisellaceae and Legionellales.