Bacteria coexist in shared spaces by 'playing nice'

It is an unsettling yet undeniable truth that slimy biofilms – congregations of bacteria – do not discriminate when it comes to their settlement.

These bacteria colonies can be found in a diverse array of environments ranging from hot springs to plant roots, and even on your household surfaces like bathtub and fridge. They also lurk in impending wounds and on medical devices such as catheters.

Biofilms, although primarily composed of different bacterial species, have left scientists wondering how these species harmoniously coexist.

Bacteria in biofilms

In a new study, Dartmouth scientists have investigated this microcosm of bacterial biofilm species – primarily focusing on how three distinct bacterial species coexist. The researchers also explored a surprising dynamic – when do the bacteria choose to discontinue their shared living?

The researchers spotted an interesting pattern. One species, Pseudomonas aeruginosa, known for its versatility and formidable resistance to antibiotics, had an upper hand over two other bacterial species.

However, when the growth became overcrowded, instead of wrestling for domination, this species chose to relocate in search of less crowded areas. This strategic departure allowed the other members of the bacterial colony to flourish.

Biofilms as a community

“Pseudomonas’ dispersal behavior allows for the three species to coexist where otherwise they would not,” explained corresponding author Carey Nadell, an assistant professor of biological sciences at Dartmouth.

“This is the first case of showing explicitly that dispersal has very important ecological consequences when you’re thinking about biofilms as a community.”

The scientists scrutinized a triad of bacterial species, including P. aeruginosa, Escherichia coli, and Enterococcus faecalis.

These bacteria often act as opportunistic pathogens and are frequently isolated from catheter-associated urinary tract infections. This particular selection of species would help in understanding these infections better.

How biofilms support diverse bacteria

Uncovering the mystery of peaceful cohabitation in such a competitive environment spurred their experiment.

Study first author Jacob Holt is a graduate student in Nadell’s research group who led the study.

“We wanted to know how biofilms can support a diversity of species or strains because we know that bacteria are really good at killing each other,” said Holt.

“So that was a big motivation – if they’re so good at these antagonistic behaviors, how do they coexist in these tightly associated communities?”

Cycle of bacteria in biofilms

To explore this, the bacteria were grown on a glass surface viable for biofilm development and also in a liquid culture, with equal proportions of each species.

Utilizing fluorescence microscopy, the scientists tracked the change in population over time. When grown in the liquid culture, P. aeruginosa outcompeted the other two species after about three days. Fascinatingly, in a biofilm environment, the dynamics were atypical.

In the biofilm conditions, E. faecalis and E. coli initially propagated faster than P. aeruginosa. However, a few days later, P. aeruginosa drastically multiplied, displacing the other species.

Again, as the biofilm region became overly crowded, the P. aeruginosa numbers declined, allowing the resurgence of the other two species. After that, P. aeruginosa began its domination again. This cycle kept repeating.

Equation of coexistence

In their quest to decode these cycles, the team tested different theoretical models. The one that held water was surprisingly simple.

“The fundamental mechanism is very straightforward,” said Holt. “When a dominant species gets to a very high abundance, it selectively removes itself from the system, which permits the other species to stick around.”

This hypothesis was further tested by an experiment involving a genetically engineered P. aeruginosa strain, unable to disperse due to a lack of the required traits.

In this scenario, P. aeruginosa bacteria completely dominated the biofilm, resonating with the results seen in the liquid culture, thereby affirming their model’s findings.

Future research directions

The scenario-based approach of this study emphasizes the need for more realistic research settings.

“It’s important to push for more ecological realism. The inferences you gain from well-mixed liquid bacterial cultures often may not apply to biofilm environments, which are much more common in the real world,” said Professor Nadell.

“It points to the importance of studying these communities in a context with a little bit of added realism.”

Planning to step up the game of realism, the team is now working towards growing Vibrio cholera, the bacteria causing cholera, on shrimp shells, which is their natural habitat in the marine environment.

This might just be the first step in understanding the complex world of bacteria and biofilms, but it has certainly paved the way for future investigations.

After all, isn’t it fascinating to learn how even the tiniest of creatures display such complex survival strategies?

The study is published in the journal Current Biology.

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