Small, prokaryotic organisms on the scale of micrometers, bacteria have always been difficult to study. While most of the cellular structures and functions have been discovered, the cytoplasm has often been overlooked. A recent study from Yale University dove into the mysterious nature of bacterial cytoplasm and how its properties determine cellular behavior and physiology. Dr. Christine Jacobs-Wagner, Professor of Molecular, Cellular, and Developmental Biology and of Microbial Pathogenesis, led a team that discovered that bacterial cytoplasm displays glass-like properties that depend on the activity or dormancy of the cell.
While most eukaryotes rely on active transport involving cytoskeletal filaments and motor proteins, bacteria were thought to rely mainly on diffusion. However, bacterial cytoplasm is much more complex than the simple liquids generally considered in diffusion due to the wide range of particle sizes, from just a few nanometers to hundreds of micrometers. The fact that metabolic activities end up disrupting the equilibrium of the cytoplasm made Jacobs-Wagner and her team very curious, and thus they set out to understand more about the properties of bacterial cytoplasm and how it affects cellular processes.
The scientists first noticed while studying the bacterial protein crescentin that in the its native condition, the protein was bound to other crescentin molecules and created a filamentous structure attached to the membrane. Yet, when a bulky molecule was attached, the crescentin strand detached from the membrane and moved randomly in the cytoplasm. What was even more intriguing was that when the cells generating these proteins stopped growing, the crescentin structures stopped moving. This led to an investigation of whether metabolic activities play a role in the motion of diffusing cytoplasmic components.
First, they tested to see if this metabolism-dependent motion was specific to the type of bacteria that produced crescentin. To do so, they used standard E. coli with engineered DNA molecules that were observed for movement through the cytoplasm. These molecules exhibited the same metabolism-dependent motion; when the bacterial cells were depleted of energy, the molecules did not move very far. This dependency was also confirmed when a protein probe that was specifically designed to not have any particular interaction with the bacterial cytoplasm exhibited the same movement characteristics.
Wondering what process drove this movement, the scientists investigated multiple alternatives. They found that it was not driven by any cytoskeletal “active diffusion” or by any motor-based actions of proteins. On the other hand, particle size did have an effect on this metabolism-dependent motion if the particles were larger and the cells were depleted of energy, then the particles would not move as far. The movement of the particles diffusing through the cytoplasm indicates a possible glass-forming liquid system: Larger particles “perceive” the cytoplasm as glassy because it is difficult for them to diffuse while smaller particles perceive the same environment as liquid because it is easy for them to move around. The reason why the cytoplasm exhibits this property is hypothesized to be because of the high concentration of relatively big molecules within the bacterium as compared to eukaryotes.
This study demonstrates that the cytoplasm has a greater role in the diffusion of particles than researchers had initially thought. The idea that metabolic activities can make the cytoplasm more fluid is very novel, and Jacobs-Wagner and her team are further exploring these properties by modeling the cytoplasm and predicting how molecules of different sizes move.
Cover Image: Once thought to not serve a crucial function in diffusion, bacterial cytoplasm has now been found to show glass-like tendencies which depend on the activity of the cell. Courtesy of Next Nature.