Previously unknown mode of bacterial growth discovered

Bacteria, as single-celled organisms, reproduce by dividing their entire organism. In this way, they can multiply particularly rapidly, allowing for the exponential growth of bacterial populations, also known as pathogens. Population growth is based on the growth of individual bacterial cells. It is also characterized by a rapid increase in size and, depending on the species, limited by a certain maximum cell size of the bacteria. Until now, research has been based on the assumption that a bacterial cell, like its population as a whole, grows exponentially until it reaches this final size.

At the University of Kiel, the research group for microbial biochemistry and cell biology headed by Professor Marc Bramkamp studies, among other things, the mechanisms of organization and reproduction of bacteria, whose general principles are also important for the development of complex multicellular organisms. In a new research paper, Bramkamp and his team at the Institute of General Microbiology were able to show, using the bacterium Corynebacterium glutamicum as an example, that bacteria can also grow in a different mode, namely in two stages: Corynebacteria grow by synthesizing the cell wall at the poles of the cell. In this process, the growth rate of the cell poles is not identical at the beginning after a cell division. The youngest cell pole grows more slowly right after the division. Due to this special pattern of pole elongation, cell growth accelerates at first and then changes to a steady, linear growth.

Using special imaging techniques and a recently developed analysis method, the researchers were able to precisely quantify the increase in bacterial cell size in the living organism. These data were integrated into a theoretical model, which revealed that the two-step mode is the result of cell wall formation at the poles, which is the limiting process for growth in C. glutamicum. The Kiel researchers published their findings today in the scientific journal eLife, in collaboration with their partners at the Universities of Amsterdam and Munich.

Unusual bacterial growth observed and modeled

The regulation of cell growth and size is crucial for the functions of bacterial cells. Since it was previously assumed that single cells should increase their cell volume in proportion to the increase in their protein content, growth was assumed to be mainly exponential. This rapid increase in size goes hand in hand with the fact that bacteria must strictly control their cell size to ensure morphological uniformity of cells in a population. In fact, many bacterial species have complex mechanisms to regulate their size, but such mechanisms have not yet been identified in C. glutamicum and in Mycobacterium tuberculosis, a closely related pathogen.

To investigate the specificity of this bacterial model organism, the Kiel research team observed its growth in vivo, i.e. on the living object. "An exciting aspect of Corynebacteria, which is already visible under the microscope, is that the cells do not always divide exactly symmetrically, as we know from many other bacteria," explains Fabian Meyer, a PhD student in Bramkamp's group. "Another peculiarity is that the new cell wall does not grow on the long side of the cell, but inserts itself exclusively at the poles of the cell," Meyer explains. As a result, these bacteria only elongate at the ends of the cell and do not increase their volume evenly along their entire length like many other bacteria.

To characterize this unusual growth behavior of C. glutamicum, the research team developed a theoretical model of size development based on the Kiel data. Using biophysical methods, biophysicists Joris Messelink and Professor Chase Broedersz of the Vrije Universiteit Amsterdam and Ludwig-Maximilians-Universität München were able to extract average single-cell elongation patterns over time from the data, despite measurement noise and variability between cells.

"The calculations showed that the cells initially grow faster and faster, but then the increase in size slows down. Somehow, a speed limit is built into the growth dynamics of C. glutamicum," Broedersz explains. In contrast to the normally exponential evolution, the model here shows a two-step growth of bacterial cells, which the researchers describe as asymptotically linear. "The limiting factor for the occurrence of linear growth is the type of cell wall formation that distinguishes C. glutamicum from other bacteria," Broedersz continues. The research team was then able to confirm this theoretical prediction using live bacteria: In bacteria in which a central but redundant growth enzyme was artificially deactivated, slowing down the limiting factor for cell growth, the researchers repeated their growth inference procedure. The observed growth curves showed an overall decrease in elongation rate while maintaining both growth stages, consistent with theoretical predictions.

Conclusions on bacterial evolution

"Cell growth modeling accurately predicts the results of in vivo experiments, recapitulating the specifics of individual cell growth in C. glutamicum. Furthermore, the results provide an evolutionary interpretation of the growth mechanisms of C. glutamicum. As with any organism, the bacterial progeny must be similar to the parental generation in size and shape, and bacteria have apparently evolved different growth strategies to achieve this goal," summarizes Bramkamp in presenting the main results of the interdisciplinary cooperation between cell biology and biophysics.

In the case of bacteria, this primarily concerns cell length, which is expected to be relatively narrowly distributed in a population. "Our results show that the distribution of cell length is narrower under asymptotic linear growth than under exponential growth. The particular mode in C. glutamicum that transitions to linear growth could serve as a surrogate for tight regulation of division length, since this linear growth is less sensitive to fluctuations. Therefore, it may not need its own growth control mechanism as we know it in other bacteria," Bramkamp continued.