Biofilms
Multicellular communities of single-celled organisms attached to a surface are the predominant form of life on Earth. Development of these biofilms involves distinct stages of self-organization, starting with a single cell that senses and approaches a surface. ~ Swedish microbiologist Ute Römling
Microbes have been living together in biofilms for billions of years. This is how they have founded almost all of Earth’s ecosystems: as colonies, creating stable communities that share nutrients and other essentials.
Biofilms are often cooperative associations among several groups: bacteria, fungi, algae, protozoa, as well as their oversized descendants: plants and animals. Such colonies are capable of collated perception, distributed information processing, and collective gene regulation – essentially, transforming into a superorganism.
A quorum for biofilm creation begins with a chemical effusion among a population that signals congressional intent. Many bacteria have 2 or more quorum-sensing systems. This lets them know the nature of the engagement, which helps optimize group integration.
Chemical communication among bacteria involves complex interconnected regulatory networks that serve to fine-tune the expression of diverse group behaviors. ~ American microbiologist Michiko Taga & American molecular biologist Bonnie Bassler
The first step to colonization is adhering to a suitable surface. This is initially done via reversible van der Waals forces.
A firmer anchor is had by cell adhesion structures, such as pili: tiny fibrous appendages on bacteria that get a grip. A different type of pili – conjugative pili – is used by bacteria during conjugation, now better known as horizontal gene transfer (HGT).
HGT was identified in 1946, when American molecular biologist Josua Lederberg and American geneticist Edward Tatum found that the intestinal bacterium E. coli engaged in a process resembling sex to exchange circular gene-bearing plasmids – whence the term conjugation.
The decision to form a biofilm changes genetic expression among those involved, invoking a network of protein interactions different from those of solitary life.
As a biofilm evolves, it builds and adapts to its surroundings. The earliest colonists create microhabitats and contribute food, serving as a welcoming matrix for other microbes to attach and grow into a gleaming film, forming cosmopolitan communities.
The most successful colonies are thick microbial mats with dozens of dynamic interactive layers. Life is good.
Biofilms are highly resilient. They produce protective enzymes that coat the outer surface.
This has an element of altruism, as fitter bacteria help protect their weaker fellows. This fortifies the colony, which is much tougher than any individual ever could be.
Biofilms comprise members optimally juxtaposed. Individuals are invariably at different stages of their life cycle.
Many colonists may be killed by an antibacterial agent, such as penicillin, which attacks replicating cells. They become nutrition for those that survive by virtue of being inactive during the onslaught.
Bacteria within communities interact to organize their behavior. ~ Chinese microbiologist Jintao Liu et al
Diverse species of microbes communicate with each other and even feed each other. Interdependence and symbiosis take various forms.
Chlorobium aggregatum is a consortium of 2 bacteria species that feed each other. Peripheral bacteria oxidize sulfide into sulfate via a photosynthetic process. A central anaerobic heterotroph reduces the sulfate to sulfide. Consortium life is economical because it reduces reliance on the environment for crucial nutrients.
When a biofilm reaches a threshold size, it suddenly begins to oscillate in its growth pattern. These oscillations resolve inherent group conflict.
Bacteria on the outside of the biofilm are most vulnerable to chemical and antibiotic attacks. At the same time, they provide protection for interior community members. But outer bacteria are closest to the nutrients needed for growth. If outer cells grow unchecked, they will consume all the food, and starve the sheltered interior cells.
To gain equity, interior colony members produce a metabolite necessary for growth of the outside bacteria. This lets the inner cells periodically brake consumption by the outer members. By creating a rhythm of sharing throughout the colony, biofilms stay robust.
Cells that reside within a community cooperate and compete with each other for resources. This conflict between protection and starvation is resolved through emergence of long-range metabolic co-dependence between peripheral and interior cells. Collective oscillation in biofilm growth benefits the community. Oscillations support population-level conflict resolution by coordinating competing metabolic demands in space and time. ~ American microbiologist Gürol Süel et al
Coordination becomes especially important when facing food shortages. When bacteria populations discern that nutrient supply is dwindling, they cooperatively respond to optimize consumption. Rather than trying to hog what is left, as humans would be prone to do, bacterial communities take turns.
Time-sharing enables biofilms to counterintuitively increase growth under reduced nutrient supply. Distant biofilms coordinate their behavior to resolve nutrient competition through time-sharing. ~ Jintao Liu et al
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Dental plaque is a biofilm. The plaque comprises bacteria embedded in an amorphous matrix secreted and shared by the colony. The matrix sticks to the teeth, hewing to its homestead to survive.
Such aggregation is a common technique for hanging together on a surface. Microbial communities reside on rocks, vegetation, and in the soil as adhesive aggregates.
Biofilms are a profoundly important force in the development of ecosystems, both aquatic and terrestrial. In sediment and bedrock, biofilms are essential in recycling elements and forming soils.
A microbe in water is a different creature than when in a biofilm. A biofilm becomes its own habitat, with different gradients of oxygen, pH, nutrient concentration, and other aspects. Biofilms have a wide variety of structures, some quite complex.
