Video: Disease-causing bacteria form swarming colony

Hyperswarming bugs are not as scary as they sound. Disease-causing bacteria in lab experiments have repeatedly evolved the ability to spread more rapidly than their wild counterparts – but the result could actually lead to new strategies for combating infections.

Pseudomonas aeruginosa is a common disease-causing bacterium that typically infects people with weakened immune systems, for example, people with cystic fibrosis or those undergoing cancer therapy. Given the chance, the bacteria will spread across surfaces such as human tissue to form a biofilm that may resist treatment with antibiotics.

Joao Xavier from the Memorial Sloan-Kettering Cancer Center in New York and colleagues placed colonies of the bacterium at the centre of nutrient plates and let them spread to form such biofilms. After 24 hours, the team scraped up all of the bacteria, took a small sample and placed it in the centre of a new plate.


The bacteria were left to multiply and spread again for 24 hours, before they were collected and a sample was placed at the centre of a third nutrient plate. The team continued this process, moving a sample of bacteria to a new plate each day for nine days.

After this time, around 20 of the colonies began to change the way they spread. Instead of swarming outwards to form a branching snowflake-like pattern, leaving parts of the plate uninfected, they spread evenly and rapidly to infect the entire area – a behaviour that Xavier’s team dubs “hyperswarming” (see movie).

“What we saw was quite unexpected: the evolution of hyperswarming bacteria in independent lineages,” says Xavier.

Fancy flagella

Further investigation explained why the behaviour evolved so often. It was tied to a single mutation that allowed the bacteria to develop several whip-like flagella that helped them to move around very rapidly. Normally the bacteria move more slowly because they have just one flagellum.

But although growing several flagella allowed the bacteria to efficiently cover the entire surface, the adaptation also left them less able to stick together tightly and form biofilms.

“We were surprised that the hyperswarmers were worse at forming biofilms than the wild type,” says Xavier. “Our data suggests that they do attach [to one another] but then have a much higher probability of detaching before they start growing and reproducing at the surface. The most likely reason for their early detachment is that they just move more.”

Figuring out a way to prevent the bacteria from forming biofilms could make infections easier to treat. The results suggest that a therapy that encourages the bacteria to hyperswarm could make them less harmful.

The experiment will also serve as a model for future evolutionary studies. “It is very rare to find such a reproducible example of experimental evolution,” says Xavier. “Therefore we want to make this into a prime model to investigate parallel evolution in the lab.”

Bacteria are becoming known for other forms of cooperative behaviour as well as swarming. Some can communicate amongst themselves and others have been harnessed to create mini-pumps.

Journal reference: Cell Reports, DOI: 10.1016/j.celrep.2013.07.026