Applied Microbiology

Microbial Interactions and Bio-films

Organisms rarely live in isolation. Many rely on other creatures as sources of food or nutrients. Photosynthetic plants and microbes provide oxygen that humans need to live. Trees offer shelter to other plants and animals. Some relationships between different organisms, though, are more involved. One organism may depend on another for its survival. Sometimes they need each other. This is called symbiosis.

Often, especially with microbes, one organism lives inside another — the host. When both organisms benefit from the relationship, it is called mutualism. When only one organism benefits, but the other one is not harmed, it is called commensalism. Microbial symbiosis occurs between two microbes. Microbes, however, form associations with other types of organisms, including plants and animals. Bacteria have a long history of symbiotic relationships and have evolved in conjunction with their hosts. Other microbes, such as fungi and protists, also form symbiotic relationships with other organisms. Bacteria form symbiotic relationships with many organisms, including humans. One example is the bacteria that live inside the human digestive system. These microbes break down food and produce vitamins that humans need. In return, the bacteria benefit from the stable environment inside the intestines. Bacteria also colonize human skin. The bacteria obtain nutrients from the surface of the skin while providing people with protection against more dangerous microbes. Fungi and plants form mutually-beneficial relationships called mycorrhizal associations. The fungi increase the absorption of water and nutrients by the plants and benefit from the compounds produced by the plants during photosynthesis. The fungus also protects the roots from diseases. Some fungi form extensive networks beneath the ground and have been known to transport nutrients between plants and trees in different locations. Lichens are an example of a symbiotic relationship between two microbes, fungi and algae. So far, around 25,000 lichens have been identified. They grow on rocks and tree trunks, with colours ranging from pale whitish green to bright red and orange. The lichens grow in several forms: thin and crusty coverings; small branching strands; or flat, leaf-like structures. They are usually the first plants to grow in the cold and dry habitats that they favour. Certain protists and algae form a symbiotic relationship known as living sands. This type of association occurs in tropical and semitropical seas and appears as green, orange, brown or red deposits containing calcium carbonate. Living sands were used in the construction of the Egyptian pyramids. Many different types of algae combine with their protist hosts. Without the algae, the protists cannot survive very long. Similar to living sands, some protists extract chloroplasts from diatoms, a type of algae. The chloroplasts provide the protists with the ability to convert sunlight to chemical energy through photosynthesis. Eventually, the chloroplasts break down and stop functioning.