The electrical potential across the bacterial cell envelope indicates when bacteria not operate as individual cells but as a collective. Researchers on the University of Cologne’s Institute for Biological Physics have discovered this connection between the electrical properties and the approach to life of bacteria. Although bacteria are single cellular organisms, they form spatially structured communities, so-called biofilms. Inside biofilms, bacteria behave as a collective and might protect themselves higher against external stresses like antibiotics.

Until now, it was largely unknown how the transition from a single bacterium to such a posh community works. The researchers examined how the electrical properties of bacteria change during biofilm formation and discovered characteristic patterns of the electrical potential that evolve in space and time. These patterns correlated with the event of recent habitats with various degrees of tolerance to antibiotics. The researchers describe their findings within the article ‘Collective polarization dynamics in bacterial colonies signify the occurrence of distinct subpopulations’ within the Journal PLOS Biology.

Single bacteria construct up an electrical potential across their envelope (the membrane), and thus are electrically polarized. For the cell, this polarization is a vital energy source for respiration, the uptake of nutrients and for the export of poisons. Recent methodological advances have enabled researchers to look at the dynamics of the membrane potential at the dimensions of single bacterial cells. These studies revealed that the membrane potential of single cells fluctuates independent of their neighboring cells.

How does the potential change during biofilm development, and which environmental aspects influence the potential? How does the potential relate to growth behavior of cells and their tolerance to antibiotics? These questions have now been posed by a team of researchers on the Institute for Biological Physics led by Professor Berenike Maier. They examined early stages of biofilm formation of Neisseria gonorrhoeae (aka gonococcus), the causative agent of gonorrhea, some of the common sexually transmitted diseases, which could cause ectopic pregnancies and infertility. Inside just a few minutes, gonococci self-assemble into spherical colonies that comprise 1000’s of bacteria.

Using advanced light-microscopy and image evaluation, we are able to measure the dynamics of the membrane potential of single cells in these colonies. The potential is uncorrelated inside fresh colonies in bacteria. When the colony reaches a critical size, we observe something completely unexpected: All of the cells in the middle suddenly increase their potential; they hyperpolarize.”

Dr Marc Hennes, First Creator

Eventually, a shell of hyperpolarized cells occurs on the colony center and travels through the colony. Behind this shell, the potential at the middle is lower. The researchers have interpreted this phenomenon of spatiotemporal correlated polarization patterns because the transition towards collective behavior, indicative of biofilm formation. A mixture of computer simulations and wet lab experiments showed strong evidence that this polarization pattern is linked to a change in the supply of oxygen. This pattern exists because cells at the middle deplete the oxygen faster than diffusion resupplies it.

A vital query, due to this fact, was whether the pattern of membrane polarization correlated with the well-known functional heterogeneity of biofilms. Indeed, bacteria decreased their growth rate after they’d undergone the means of hyperpolarization, whereas the expansion rate of bacteria residing on the surface of the colony remained high. As well as, bacteria in the middle of the colony showed more tolerance towards antibiotics. Increased tolerance to antibiotics is an acute medical problem when treating biofilms. The molecular mechanisms of tolerance are the topic of a project funded by the Center for Molecular Medicine Cologne on the UoC.

The long run goal is to higher understand the molecular mechanisms that underlie the formation of polarization patterns and their relation to antibiotic tolerance. This research will probably be carried out inside a recent priority program funded by the German Research Foundation (Deutsche Forschungsgemeinschaft).

Source:

Journal reference:

Marc Hennes, M., et al. (2023) Collective polarization dynamics in bacterial colonies signify the occurrence of distinct subpopulations. PLOS Biology. doi.org/10.1371/journal.pbio.3001960.