Surface Tumbling Explains Bacteria’s Ability to Contaminate Many Surfaces

While tracing the movement of Escherichia coli, a team of French researchers noticed that near solid surfaces the bacteria run in circles. Breaking down E. coli's routine step by step, the scientists identified a signature move—surface tumbling.

Bacteria can live as individuals, swimming freely around the environment, but eventually, they settle down on surfaces to form colonies and biofilms. To do so, bacteria randomly tumble to slow down and re-orientate themselves three-dimensionally, to explore and find the ideal environment.

"Tumbles are very interesting. The bacteria itself does not know where the environment is preferential for them," says Laurence Lemelle, first author of a paper published today in the Biophysical Journal. "It doesn't know where to go, how to feel things. But it knows if the past environment was better or worse than the present." Bacteria use the gathered information to stop tumbling or lower the frequency of tumbling. "This means you swim more towards a direction. At the end, the population statistically swims towards the preferential conditions," she says.

Some studies claim that bacteria don't tumble and only swim when they're near a surface. But Lemelle and her colleagues say that claim sounds unlikely. Physics predicts that bacteria would get trapped, running in infinite circles on the surface if they only swam.

Propelling forward with their flagella, bacteria swim 20 times the length of their body in one second, and tumbles happen even quicker, at one-tenth of a second. To take a close look at how bacteria escaped from the surface, the team built a high-magnification, high-sensitivity, high-speed camera equipped with night vision.

To the bacteria, tumbling allows them to escape from the surface, enabling them to colonize other places and optimize the exploration of the surface itself. Bacteria can swim on cell surfaces until they get in contact with a specific receptor to optimize infections. They can also swim to settle down on surfaces that are difficult to clean to form bacterial biofilms, which can be antibiotic-resistant.

"Before the pandemia, the COVID pandemic, it was difficult to convince people that we need to anticipate and develop alternative approaches to reduce the surface biocontamination," said Lemelle. "People were like, 'We have plenty of antibiotics. There's resistance, but we have time.' From a medical standpoint, understanding the near-surface tumbling events of bacteria can help limit the biocontamination of surfaces and develop antibacterial methods."