Ecology and interactions of Bacteria
Bacteria and other prokaryotes (archaea) have turned out to be social and cooperative, much more than biologists initially thought.
Some bacteria cooperate in groups, and distribute metabolic tasks and share products. Others form cooperative associations with a host organism (although some also form neutral or harmful associations).
Cooperation and «multicellularity» in bacteria
Bacteria often benefit from cooperating with each other. This cooperation can be superficial or it can be coordinated to the point that it begins to look a lot like eukaryotic multicellularity.
These are some examples of bacterial cooperation; you can judge for yourself if they really count as multicellularity or not (a controversial issue that is still ongoing in biology).
Cooperative bacteria: Chain-forming cyanobacteria.
Cyanobacteria of the genus Anabaena do not separate to live on their own when they reproduce by binary fission (bacterial cell division), but rather are held together in interconnected chains of cells, such as those shown in the image to the left.
Cyanobacterial cells have the ability to photosynthesize and fix nitrogen. However, a single cell cannot carry out both processes at the same time because the oxygen released during photosynthesis blocks nitrogen fixation (affects nitrogen fixing enzymes).
To solve this problem, when little nitrogen is available, some cells in the chain become heterocysts. Heterocysts specialize in nitrogen fixation, unlike the rest of the cells in the chain that carry out photosynthesis.
A biofilm is a set of microorganisms united by a viscous substance (made up mostly of carbohydrates) that they secrete themselves. In many cases, biofilms are formed through quorum perception. In quorum perception, bacteria exchange signals that allow them to detect population density and change their behavior when density exceeds a certain limit.
Biofilms are often made up of various types of bacteria or other microorganisms. In some cases, the different members of the biofilm are metabolically complementary: some produce the molecules that others can use. Biofilms generally have water-permeable channels that serve for the exchange of nutrients and wastes, and which some biologists liken to a «primitive circulatory system.
Most bacteria in nature probably live on surfaces, rather than free-floating, and biofilms are literally everywhere. They form on household surfaces such as kitchen counters, cutting boards, sinks, and toilets. Even dental plaque that your dentist removes is a biofilm!
Biofilms often attract attention when they cause problems. Pathogenic (disease-causing) biofilms like the Staphylococcus seen in the photo above can be a serious problem in hospitals. They are often difficult to remove with antibiotics and can lead to persistent infections if they contaminate medical equipment such as catheters. Other biofilms corrode metal pipes and damage industrial equipment.
Despite this, some biofilms have beneficial applications. For example, they are used in water treatment plants to remove organic matter from wastewater.