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The electrical potential across the bacterial cell envelope indicates when bacteria no longer operate as individual cells but as a collective. Researchers from the University of Cologne have discovered this connection between the electrical properties and the lifestyle of bacteria. Although bacteria are single cellular organisms, they form spatially structured communities, so-called biofilms. Within biofilms, bacteria behave as a collective and can protect themselves better against stresses like antibiotics.
Single bacteria build up an electric potential across their envelope (the membrane), and thus are electrically polarized. For the cell, this polarization is an important energy source for respiration, the uptake of nutrients and for the export of toxins. Recent methodological advances have enabled researchers to examine the dynamics of the membrane potential at the scale of single bacterial cells. These studies revealed that the membrane potential of single cells fluctuates independent of their neighbouring cells.
How does the potential change during biofilm development, and which environmental factors influence the potential? How does the potential relate to growth behaviour of cells and their tolerance to antibiotics?
These questions have now been posed by a team of researchers at 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 gonorrhoea, one of the most common sexually transmitted diseases, which can cause ectopic pregnancies and infertility. Within a few minutes, gonococci self-assemble into spherical colonies that comprise thousands of bacteria. "Using advanced light-microscopy and image analysis, we can measure the dynamics of the membrane potential of single cells in these colonies," the first author Dr Marc Hennes explains. "The potential is uncorrelated within fresh colonies in bacteria. When the colony reaches a critical size, we observe something completely unexpected: All the cells in the centre suddenly increase their potential; they hyperpolarize." Eventually, a shell of hyperpolarized cells occurs at the colony centre and travels through the colony. Behind this shell, the potential at the centre is lower. The researchers have interpreted this phenomenon of spatiotemporal correlated polarization patterns as the transition towards collective behaviour, indicative of biofilm formation. A combination of computer simulations and wet lab experiments showed strong evidence that this polarization pattern is linked to a change in the availability of oxygen. This pattern exists because cells at the centre deplete the oxygen faster than diffusion resupplies it.
An important question, therefore, was whether the pattern of membrane polarization correlated with the well-known functional heterogeneity of biofilms. Indeed, bacteria decreased their growth rate after they had gone through the process of hyperpolarization, whereas the growth rate of bacteria residing on the surface of the colony remained high. In addition, bacteria in the centre 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 subject of a project funded by the Center for Molecular Medicine Cologne at the UoC.
The future goal is to better understand the molecular mechanisms that underlie the formation of polarization patterns and their relation to antibiotic tolerance.
See:
Marc Hennes, Niklas Bender, Tom Cronenberg, Anton Welker, Berenike Maier. Collective polarization dynamics in bacterial colonies signify the occurrence of distinct subpopulations. PLOS Biology, 2023; 21 (1): e3001960 DOI: 10.1371/journal.pbio.3001960
Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)
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