Study swimming patterns of bacteria near surfaces

A team of researchers from the Côte d 'Azur University and the Monaco Scientific Center recently conducted a study to better understand swimming patterns of bacteria near the surface. Their article, published in Physical Naturecould shed light on how bacteria explore surfaces, how host cells are infected and how they infect these cells

Bacteria often move near the surface of water or aqueous substances, which is explained by a number of reasons. First, nutrients in aqueous environments usually accumulate on their surface. In addition, host cells, which are particularly susceptible to infection by pathogenic bacteria, are also on, or part of, a surface (i.e., cellular tissue).

Researchers have been studying swimming patterns near the surface of bacteria for several years. Previous studies suggest that these patterns are determined by the hydrodynamic interactions between bacteria and the surface on which they navigate, which ultimately traps bacteria in smooth circular trajectories leading to inefficient surface exploration.

Physics research on swimming patterns near the surface of bacteria suggests that an individual bacterium is attracted to the surface, as well as an effective torque caused by the rotation of the flagellar beam, which forces it to turn in circles. This well-documented observation can be explained by the fundamentals of physics.

However, when one looks at the picture painted by these observations, it is difficult to understand how bacteria can survive, as their hydrodynamic interactions near the surface would seem to be a serious obstacle to their survival. What makes their endurance in such unfavorable circumstances even more disconcerting is the fact that in terms of evolution, bacteria should be able to easily explore the surfaces in order to find nutrients and / or locate colonization sites.

"We were very intrigued by these questions and suspected that this reductionist fluid mechanics approach could not be the whole story," Fernando Peruani, one of the researchers who led l & # 39; study. "We thought bacteria should be able to overcome this handicap: staying stuck in a circular orbit is certainly not an effective way to explore a surface." It is in this light that we decided to Study how different bacterial species move on surfaces for the purpose of understanding how surface exploration is done.

The work of Peruani and his colleagues is part of a broader project to better understand how pathogenic bacteria infect host cells. In their recent study, they used video microscopy and monitored bacteria in a relatively large window of observation to obtain long bacterial trajectories. They then analyzed the statistics of these trajectories to closely observe the swimming patterns near the surface of the bacteria.

"Sudden changes in the speed of the bacteria, which indicated that the bacteria were stopping intermittently, immediately intrigued us," said Peruani. "We then looked at the distribution of periods when the bacteria moved and did not move and we understood that if a Markov chain formalism was used to describe the data, three states were needed." This observation played a key role in our research. "

Subsequently, the researchers re-examined the data collected and analyzed the periods during which the bacteria had "stopped". They observed that bacteria were often attached to the surface and circled around one of the ends of the cell body.

"The evidence was clear: the bacteria were exploring the surface by performing transient adhesion events," Peruani said. "The next step was to build a theory for a swimmer with an internal state, controlled by a Markov chain, that takes three possible values, each associated with a different movement equation.This was a technical challenge, but Paid. "

The theory developed by Peruani and his colleagues allowed them to conclude that the frequency with which the observed "stops" occurred was far from random. Rather than hindering the activity of the bacteria, this frequency seemed to maximize its surface exploration.

The study conducted by this team of researchers led to two very important observations. First of all, the researchers understood that bacteria use transient adhesion as a regulatory mechanism of surface exploration. Secondly, they observed the existence of an optimal stopping frequency optimizing the surface exploration. Enterohemorrhagic E. coli (EHEC) and other pathogenic bacteria seem to be able to match this frequency to its optimal value.

"These two observations provide a better understanding of how bacteria explore surfaces, which is a necessary step in elucidating the way they look for host cells and their infection by bacteria," said Peruani. "An important message of this study is that a physical understanding of how bacteria move on surfaces can not be based solely on hydrodynamic interactions.Activity interactions also play a crucial role. In addition, allows bacteria to reorient themselves and escape the circular traps imposed by hydrodynamic interactions. "

Observations collected by Peruani and his colleagues offer valuable new information on the patterns of well-documented bacteria that swim near the surface. Researchers are currently planning further studies to understand how pathogenic bacteria are looking for and infecting host cells. For different species of bacteria, they expect to observe different search and colonization strategies. However, they also suspect that the number of strategies they will observe will be significantly lower than the number of existing pathogenic bacteria species.

"A quantitative physical understanding of bacterial infections, which is still lacking, could provide guidance on how to prevent bacterial infections," Peruani added. "Our study, for example, indicates that surface adhesion plays a crucial role in surface exploration.On the other hand, surface adhesion depends on the specific adhesins of the bacterium , as well as the physical properties of the surface, and we will certainly try to think of ways to modify these physical properties. "

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