Cell envelope growth of Gram-negative bacteria proceeds independently of cell wall synthesis

All bacterial cells must expand their envelopes during growth. The main load-bearing and shape-determining component of the bacterial envelope is the peptidoglycan cell wall. Bacterial envelope growth and shape changes are often thought to be controlled through enzymatic cell wall insertion. We investigated the role of cell wall insertion for cell shape changes during cell elongation in Gram-negative bacteria. We found that both global and local rates of envelope growth of Escherichia coli remain nearly unperturbed upon arrest of cell wall insertion-up to the point of sudden cell lysis. Specifically, cells continue to expand their surface areas in proportion to biomass growth rate, even if the rate of mass growth changes. Other Gram-negative bacteria behave similarly. Furthermore, cells plastically change cell shape in response to differential mechanical forces. Overall, we conclude that cell wall-cleaving enzymes can control envelope growth independently of synthesis. Accordingly, the strong overexpression of an endopeptidase leads to transiently accelerated bacterial cell elongation. Our study demonstrates that biomass growth and envelope forces can guide cell envelope expansion through mechanisms that are independent of cell wall insertion.

The role of cell-envelope synthesis for envelope growth and cytoplasmic density in Bacillus subtilis

All cells must increase their volumes in response to biomass growth to maintain intracellular mass density within physiologically permissive bounds. Here, we investigate the regulation of volume growth in the Gram-positive bacterium Bacillus subtilis. To increase volume, bacteria enzymatically expand their cell envelopes and insert new envelope material. First, we demonstrate that cell-volume growth is determined indirectly, by expanding their envelopes in proportion to mass growth, similarly to the Gram-negative Escherichia coli, despite their fundamentally different envelope structures. Next, we studied, which pathways might be responsible for robust surface-to-mass coupling: We found that both peptidoglycan synthesis and membrane synthesis are required for proper surface-to-mass coupling. However, surprisingly, neither pathway is solely rate-limiting, contrary to wide-spread belief, since envelope growth continues at a reduced rate upon complete inhibition of either process. To arrest cell-envelope growth completely, the simultaneous inhibition of both envelope-synthesis processes is required. Thus, we suggest that multiple envelope-synthesis pathways collectively confer an important aspect of volume regulation, the coordination between surface growth, and biomass growth.

Molecular Motors Govern Liquidlike Ordering and Fusion Dynamics of Bacterial Colonies

Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluidlike order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquidlike behavior whereby motor activity strongly affects surface tension and viscosity.

Differential interaction forces govern bacterial sorting in early biofilms

Bacterial biofilms can generate micro-heterogeneity in terms of surface structures. However, little is known about the associated changes in the physics of cell–cell interaction and its impact on the architecture of biofilms. In this study, we used the type IV pilus of Neisseria gonorrhoeae to test whether variation of surface structures induces cell-sorting. We show that the rupture forces between pili are fine-tuned by post-translational modification. Bacterial sorting was dependent on pilus post-translational modification and pilus density. Active force generation was necessary for defined morphologies of mixed microcolonies. The observed morphotypes were in remarkable agreement with the differential strength of adhesion hypothesis proposing that a tug-of-war among surface structures of different cells governs cell sorting. We conclude that in early biofilms the density and rupture force of bacterial surface structures can trigger cell sorting based on similar physical principles as in developing embryos.

DOI:http://dx.doi.org/10.7554/eLife.10811.001

Communities of bacterial cells can live together embedded within a slime-like molecular matrix as a biofilm. This allows the bacteria to hide from external stresses. A single bacterium can replicate itself and develop into a biofilm, and over time the bacterial cells in specific regions of the biofilm will start to interact with their neighbors in different ways. These interactions occur via structures on the surface of the bacterial cells, and the differences in these interactions resemble those that occur as cells specialize during the development of animal embryos. Previous research into embryonic development has shown how differences in the physical interactions between embryonic cells are essential for sorting the cells into their correct locations and shaping the embryo. However, little is known about which processes govern the development of biofilms.

Now, Oldewurtel et al. have asked whether differences in the physical interactions between bacteria trigger cell sorting during the early stages of biofilm development. The experiments involved measuring the force required to break the cell–cell connections (called the ‘rupture force’) in biofilms of a bacterium called Neisseria gonorrhoeae. Oldewurtel et al. found that, in agreement with previous predictions, physical interactions were important for sorting bacterial cells into clusters based on the structures on their surfaces. Bacterial cells actively pull on the surface structures of their neighbors, which allows the cells to sort themselves in a tug-of-war fashion. This means that a cell will move in the direction where it can pull the strongest (i.e., in the direction where the rupture force is highest).

While bacteria and embryos use different molecules to generate these pulling forces, these findings indicate that the basic physical principles are similar in both systems. One of the next challenges will be to evaluate how biofilms might benefit from the structures that develop due to cell sorting.

DOI:http://dx.doi.org/10.7554/eLife.10811.002

The effect of bacterial signal indole on the electrical properties of lipid membranes

Indole is an important biological signalling molecule produced by many Gram positive and Gram negative bacterial species, including Escherichia coli. Here we study the effect of indole on the electrical properties of lipid membranes. Using electrophysiology, we show that two indole molecules act cooperatively to transport charge across the hydrophobic core of the lipid membrane. To enhance charge transport, induced by indole across the lipid membrane, we use an indole derivative, 4 fluoro-indole. We demonstrate parallels between charge transport through artificial lipid membranes and the function of complex eukaryotic membrane systems by showing that physiological indole concentrations increase the rate of mitochondrial oxygen consumption. Our data provide a biophysical explanation for how indole may link the metabolism of bacterial and eukaryotic cells.

Three-dimensional obstacles for bacterial surface motility

Twitching motility enables bacteria to move over surfaces using type IV pili as grappling hooks. Here it is shown that the motility of the round Neisseria gonorrhoeae as well as of rod-shaped Myxococcus xanthus is guided by elevations with dimension and depth corresponding to the size of the bacteria.