Behind the scenes: How bacteria use tailocins to kill rival cells
Jordan Vacheron and Clara Heiman
A discussion between Jordan Vacheron and Christoph Keel about the results of bacterial kin competitions involving phylogenetically very closely related strains is at the origin of this study. Although environmental Pseudomonas are known to rely on large-spectrum antimicrobials to target distant competitors, kins are generally insensitive to these toxic molecules. Therefore, competition against these close relatives requires highly specialized weaponry with a narrow activity spectrum. Although we had initially suspected the type VI secretion system to be the cause of the difference in competitiveness, our results showed that even without this structure, the outcome of the competition was the same. Thus followed a brainstorming session on how space and nutrients were shared by kins within the same ecological niche. We decided to focus on viral particles encoded inside the bacterial genomes as potential weapons against kins. When we analyzed the viral sequences inside the genome of our bacterial model strain Pseudomonas protegens CHA0, we discovered that one of these viral clusters coded for contractile structures resembling headless phages, also-called tailocins.
The research work for this paper started with the Master internship of Clara Heiman in 2018. We obtained promising results that encouraged us to deepen our understanding of the role of tailocins in competition between kin bacteria. In the literature, tailocins are hypothesized to be evolved from phages. Indeed, they share many similarities: induction following a stress, and partly similar morphology and activity mechanism. Despite all this, the production dynamic of these structures at single-cell level had never been studied using live-imaging microscopy. The results we obtained were spectacular. The tailocins were produced at the center of the bacterial cell before migrating to the pole. After the lysis of the producing bacterial cell, the tailocins were propelled at several tenths of micrometers towards targeted kin! Moreover, we showed that the activity spectrum of the tailocins is highly specific and narrow, mostly restricted to phylogenetically kin bacterial cells.
The results of our study were encouraging and we believed that they brought novelty to this expanding field. Although we entered the publication process confidently in December 2019 and targeted prestigious generalist journals, we did not pass the editor statement and never went through to the reviewing process. After trying three different journals, we were finally accepted in Communications Biology one year after the first submission. Once published, our paper elicited great attention on social media and the number of accesses to the paper was phenomenal from the first days after appearance.
The research performed on tailocins is flourishing with an increasing number of studies performed in many different countries. Indeed, the ability to specifically target and kill a bacterial strain through the usage of tailocins opens new possibilities in different domains such as medicine and agriculture to specifically target multidrug resistant pathogens as well as phytopathogens, and the potential to edit microbiomes with high specificity. On our side, this paper opened a new research direction for our lab: we currently have another tailocin study under review that focuses on understanding the role of these viral particles in more natural systems… so stay tuned!
Image: © Noémie Matthey.