Using Pseudomonas aeruginosa as an example, the researchers show how an aggregate-seeded biofilm outcompetes one arising from single cells.
Image credit: CDC/ Janice Haney Carr
Researchers estimate that 60-80% of microbial infections in the body are caused by bacteria growing as a biofilm rather than as free-floating or single cells.
Biofilms are densely packed communities of microbial cells that surround themselves with secreted polymers and grow on surfaces, both living (such as heart and lung tissue or skin) and non-living (such as medical devices).
Studies show the bacterial communities within biofilms are complex and diverse, giving rise to the idea that they are coordinated and cooperative groups, analogous to multicellular organisms.
Traditional models of infection assume bacteria enter the body individually, as single cells, and only then form biofilms.
The new study, led by the University of Copenhagen in Denmark and published in the journal mBio, suggests the traditional models should be revised to consider the difference between biofilms that arise from single cells and biofilms that arise from pre-formed clumps or aggregates of bacteria.
The researchers note how in natural environments and during infection, bacterial cells tend to clump together to form aggregates, and biofilms can also shed aggregates as part of the dispersal process.
"This makes it likely," they note, "that biofilms are often seeded by aggregates and single cells, yet how these aggregates impact biofilm initiation and development is not known."
Aggregate versus single-cell biofilm
So, for their study, the team compared the relative fitness of biofilms formed from single-celled and aggregate forms of Pseudomonas aeruginosa during early development.
Pseudomonas aeruginosa is a strain of bacteria found widely in the environment that causes infection in humans. It can lead to serious infections in patients in hospitals or with weakened immune systems. In healthy people, it usually only causes mild illness.
The researchers found that the relative fitness of the two forms depends on the level of competition for growth resources.
For example, when competition for nutrients is high, the aggregates have an advantage over single-cell forms because they can extend vertically above the surfaces they colonize and gain better access to resources.
The team also notes other differences, including the fact that biofilms seeded from aggregates have much stronger resilience toward antibiotics and immune response. The researchers conclude:
"Our findings show that an aggregate landing on a surface will eventually outcompete the biofilm population arising from single cells attached around the aggregate and dominate the local biofilm development."
Senior author Thomas Bjarnsholt, a professor in Copenhagen's Costerton Biofilm Center, says:
"This is something we have to pay far greater attention to in trying to prevent infections, for example in connection with operations."
He explains that we have a lot of bacteria on and in our skin that are clustered as biofilm. When the surgeon makes an incision, the biofilm is pushed into the body.
Lead author Kasper Kragh, a postdoctoral researcher at the Costerton Biofilm Center, says we have to take a step back and, with an open mind, look again at how bacteria cause infection and how to fight them. He notes:
"Antibiotics are not designed to fight biofilm. Often antibiotics are not sufficient to fight chronic infections. This may partly be because antibiotics are to a large extent designed to fight single-celled bacteria, not biofilm."
Meanwhile, Medical News Today recently learned how another team of researchers is suggesting that antibiotic-resistant bacteria may be defeated by "breaking down their walls."