“As humans, our environment consistently exposes us to a variety of dangers. Tornadoes, lightning, flooding and hurricanes can all hamper our survival. Not to mention the fact that most of us can encounter swerving cars or ill-intentioned people at any given moment.
Biofilms form when bacteria adhere to surfaces in aqueous environments and begin to excrete a slimy, glue-like substance that can anchor them to all kinds of material
Thousands of years ago, humans realized that they could better survive a dangerous world if they formed into communities, particularly communities consisting of people with different talents. They realized that a community is far more likely to survive through division of labor– one person makes food, another gathers resources, still another protects the community against invaders. Working together in this manner requires communication and cooperation.
Inhabitants of a community live in close proximity and create various forms of shelter in order to protect themselves from external threats. We build houses that protect our families and larger buildings that protect the entire community. Grouping together inside places of shelter is a logical way to enhance survival.
With the above in mind, it should come as no surprise that the pathogens we harbor are seldom found as single entities. Although the pathogens that cause acute infection are generally free-floating bacteria – also referred to as planktonic bacteria – those chronic bacterial forms that stick around for decades long ago evolved ways to join together into communities. Why? Because by doing so, they are better able to combat the cells of our immune system bent upon destroying them.
It turns out that a vast number of the pathogens we harbor are grouped into communities called biofilms. In an article titled “Bacterial Biofilms: A Common Cause of Persistent Infections,” JW Costerton at the Center for Biofilm Engineering in Montana defines a bacterial biofilm as “a structured community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living surface.” In layman’s terms, that means that bacteria can join together on essentially any surface and start to form a protective matrix around their group. The matrix is made of polymers – substances composed of molecules with repeating structural units that are connected by chemical bonds.
According to the Center for Biofilm Engineering at Montana State University, biofilms form when bacteria adhere to surfaces in aqueous environments and begin to excrete a slimy, glue-like substance that can anchor them to all kinds of material – such as metals, plastics, soil particles, medical implant materials and, most significantly, human or animal tissue. The first bacterial colonists to adhere to a surface initially do so by inducing weak, reversible bonds called van der Waals forces. If the colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion molecules, proteins on their surfaces that bind other cells in a process called cell adhesion.
A biofilm in the gut.
These bacterial pioneers facilitate the arrival of other pathogens by providing more diverse adhesion sites. They also begin to build the matrix that holds the biofilm together. If there are species that are unable to attach to a surface on their own, they are often able to anchor themselves to the matrix or directly to earlier colonists.
During colonization, things start to get interesting. Multiple studies have shown that during the time a biofilm is being created, the pathogens inside it can communicate with each other thanks to a phenomenon called quorum sensing. Although the mechanisms behind quorum sensing are not fully understood, the phenomenon allows a single-celled bacterium to perceive how many other bacteria are in close proximity. If a bacterium can sense that it is surrounded by a dense population of other pathogens, it is more inclined to join them and contribute to the formation of a biofilm.
Bacteria that engage in quorum sensing communicate their presence by emitting chemical messages that their fellow infectious agents are able to recognize. When the messages grow strong enough, the bacteria respond en masse, behaving as a group. Quorum sensing can occur within a single bacterial species as well as between diverse species, and can regulate a host of different processes, essentially serving as a simple communication network. A variety of different molecules can be used as signals.
“Disease-causing bacteria talk to each other with a chemical vocabulary,” says Doug Hibbins of Princeton University. A graduate student in the lab of Princeton University microbiologist Dr. Bonnie Bassler, Hibbins was part of a research effort which shed light on how the bacteria that cause cholera form biofilms and communicate via quorum sensing.
“Forming a biofilm is one of the crucial steps in cholera’s progression,” states Bassler. “They [bacteria] cover themselves in a sort of goop that’s a shield against antibiotics, allowing them to grow rapidly. When they sense there are enough of them, they try to leave the body.”
Although cholera bacteria use the intestines as a breeding ground, after enough biofilms have formed, planktonic bacteria inside the biofilm seek to leave the body in order to infect a new host. It didn’t take long for Bassler and team to realize that the bacteria inside cholera biofilms must signal each other in order to communicate that it’s time for the colony to stop reproducing and focus instead on leaving the body.
“We generically understood that bacteria talk to each other with quorum sensing, but we didn’t know the specific chemical words that cholera uses,” Bassler said.”