Exploring the spatiotemporal regulation of the predatory bacterium Bdellovibrio bacteriovorus cell division

Details

Supervisors

Laloux, Géraldine ; Hallet, Bernard

Faculty

Faculté des sciences

Degree label

Master [60] en sciences biologiques

Abstract

More than 60 years after its serendipitous discovery, the obligate predatory bacterium Bdellovibrio bacteriovorus still fascinates scientists due to its unique lifestyle. This bacterium preys upon Gram-negative bacteria by invading their periplasm, digesting their cytoplasmic content to elongate, dividing into a variable number of progeny and eventually escaping their prey to resume their cell cycle. Although recent findings allowed to shed light on some fundamental mechanisms of its non-canonical cell cycle, many secrets remain to be unveiled. Among them, the B. bacteriovorus cell division has remained largely unexplored. As such, this M.Sc. thesis aimed at investigating the B. bacteriovorus cell division at the molecular level. The first part of my project explored the stringent response, triggered in bacteria during metabolic stress including amino acids starvation, for the first time in B. bacteriovorus. It was indeed hypothesised that the amino acids depletion in the prey’s cytoplasm might elicit the stringent response, resulting in the predator growth arrest and cell division initiation to exit the bdelloplast. The results showed that the stringent response is not essential for the B. bacteriovorus cell cycle. Moreover, the addition of SHX, known to artificially elicit the stringent response in some bacteria by mimicking amino acids starvation, stopped the growth of the predator in the bdelloplast, which was consistent with the reported effect of SHX on bacterial growth. Intriguingly, SHX addition also arrested the growth of a B. bacteriovorus strain mutant for the stringent response, therefore indicating that SHX-associated growth arrest was not only due to the stringent response. Taken together, the results suggest that the stringent response is not fundamental for the B. bacteriovorus cell cycle although it may still play a role in fine-tuning the duration of its growth phase. The second part of my project focused on characterising the spatiotemporal organisation of the protein ZapT, hypothesised to be part of the B. bacteriovorus division machinery. The results revealed that ZapT followed a tightly regulated pattern both in space and time, highly similar to that of ZapA, which belongs to the B. bacteriovorus divisome. A translational ZapT-mCherry fusion highlighted the sequential appearance of ZapT foci at the future division sites prior division, which strongly suggests a role in division. Furthermore, the constriction of the mother cell as well as the progeny release also appeared to be sequential processes. Altogether, the results indicate that the divisome recruitment and assembly, the constriction and progeny release in B. bacteriovorus are asynchronous, which differs from previous findings. Lastly, a positive interaction between ZapT and ZapA from B. bacteriovorus was assessed in E. coli. Considering that ZapT homolog in Caulobacter crescentus was proposed to partially coordinate chromosome organisation and cell division, this interaction might suggest a role for ZapT in mediating these two closely connected physiological processes, thereby ensuring that each daughter cell inherits a copy of the chromosome.