When One becomes Two
Research interview with MPI-TM microbiologists on their latest findings on bacterial cell division
Splitting into two may sound simple. But if we take a closer look, bacterial cell division resembles a complex choreography: to ensure that each daughter cell is identical and complete, specific motions take place in a precisely defined sequence. But how do the cells do this? And what is the key to understanding these processes in detail? Here, postdoctoral researcher Dr. Ismath Sadhir and research group leader Dr. Seán Murray from the Department “Systems & Synthetic Microbiology” provide some answers.
Ismath, as Doctoral Student of the International Graduate School `IMPRS µ-life`, you have studied bacterial chromosomes in Dr. Seán Murray`s team. Which aspects of your work do you find most fascinating?
One reason why I find this research so exciting is its central importance for life. Studying bacterial chromosome organization sheds light on fundamental processes such as cell division and DNA replication. In bacteria, the precise organization and segregation of chromosomes during cell division are vital for their survival.
Another aspect is the importance of this basic research. Understanding these processes can reveal insights into bacterial growth, adaptation, and response to environmental changes. This knowledge is not only pivotal for basic science but also has practical implications in many fields, like antibiotic development, treatment of bacterial infections, and biotechnology. If we comprehensively understand these mechanisms, we will be able to better tackle challenges in medicine, agriculture, and industry, where bacteria play a central role.
You are lead author of a study recently published in the journal Nature Communications. Can you tell us in a few words what it is it about?
Bacterial chromosomes are dynamically and spatially highly organised within cells. In slow-growing Escherichia coli, the chromosomal terminus is initially located at the new pole. During replication, therefore, it must migrate to midcell to reproduce the same pattern in the daughter cells. In our study, we used high-throughput time-lapse microscopy to quantify this transition, its timing and its relationship to chromosome segregation.
Seán, what is your methodological approach?
Seán Murray: Traditional methods often rely on snapshots. But here we have a highly dynamic process that we wanted to monitor continuously. We therefore use high-throughput time-lapse microscopy with a microfluidic device called ‘mother-machine’. This technique not only allows us to simultaneously observe/image tens of thousands of cell cycles in a single experiment, but also continuously over several days.
This technique offered us a more detailed and dynamic view of bacterial chromosome organization. We were able to capture the precise timing and sequence as different regions of the chromosome migrated during cell division. And importantly, our method reveals details that traditional methods might miss.
So what did you find out?
Ismath Sadhir: Our key finding was the previously unknown intricate coupling between two regions of the chromosome: the origin (ori) and the terminus (ter) regions, the first and last part of the E. coli chromosome to be replicated and segregated in a cell cycle. We found that the movement of ter to mid-cell from the new pole of the cell is so tightly linked with the completion of ori segregation, that when ori segregation is impaired, ter cannot stably localize to mid-cell. So the coupling is obiously essential for ter`s stable positioning.
Secondly, and this was rather exciting, our results clearly show that during slow growth, the E. coli chromosome is (or also can be) longitudinally organized. This goes against the prevailing understanding that the E. coli chromosome is transversely organized. We would not have seen this without this new methodology.
What were the main challenges here?
Ismath Sadhir: Well, initially, setting up the microfluidics system was a hurdle. But with the support of our colleagues in the department, we were able to find a solution. And optimizing the imaging conditions for long-term continuous imaging of bacteria was a trial-and-error process, as existing protocols were typically for short-term imaging.
Another challenge was setting up the automated image analysis platform for analysing time-lapse images. But thanks to the highly interdisciplinary nature of our group, this was managed without much difficulty.
Were the results in line with what you had expected??
Ismath Sadhir: We were surprised to find that cells with impaired chromosome segregation still behaved similarly to normal cells, with each daughter cell inheriting a complete chromosome but with an inverted orientation within the cell. This challenges the standard assumption that segregation of the origin, which leads the chromosome segregation process, is essential for segregating the entire chromosome.
What are your future plans?
Seán Murray: We are currently using the methodologies from our E. coli study to explore chromosome organization in Bacillus subtilis, which is believed to have a different chromosomal arrangement. Moreover, we are expanding the use of the 'mother machine' to study various microbes, not just for chromosome organization. The technique's capability for extended imaging of numerous cell cycles enables us to analyze smaller subpopulations that are often overlooked. Understanding these subpopulations is crucial as they can offer insights into genetic variability, resistance mechanisms, and adaptive strategies within microbial communities, potentially leading to breakthroughs in microbial ecology, pathogenesis, and treatment strategies.
