Symbiotic associations of bacteria

Symbiotic association involves living together of two or more different organisms, usually for some benefits. Symbiotic relationships can be obligatory or facultatively mutualistic, commensalistic or parasitic biological interactions. Two types of symbioses are recognized depending upon occurrence of symbionts: in ectosymbiosis, one of the organisms resides externally of another and in endosymbiosis one of them lives inside the other. It would be very interesting to know about symbiotic associations of bacteria with different animals including human beings, protozoa, algae, fungi and plants. You will see nature's very complex but highly coordinated interactions of different living forms.

Symbiosis of bacteria-animals: Bacteria are known to be very common endo and ectosymbiotic microorganisms of humans, insects and other animals. All human beings acquire specific bacteria at the birth as normal flora which remain associated throughout the life span. Animals such as cattle also bear specific bacteria as resident microflora. Normal flora bacteria attach and colonize epithelial mucus lining of organs of gastrointestinal, genitourinary and respiratory tract of animals and humans. They represent mutualism as well as commensalism with the host. Normal flora receives protective environment, nutrition and transport from host while as host gets supply of certain B vitamins, development and activation of immune system and protection from pathogenic microbes. Lactobacilli residing in vagina produce lactic acid which protects female genitourinary tract from infectious yeasts like Candida and other pathogenic microbes. Rumen bacteria such as Propionibacterium spp. residing in cattle intestine are cellulolytic and actually digest cellulosic food ingested by animal; cattle don't have other means of cellulose utilization and cellulosic content of vegetable matter would otherwise impossible to degrade without aid by rumen bacteria. Cooperative interaction between aphids and bacteria Buchnera aphidicola is one of the best example of insect and bacterial symbiosis. Bacteria provide all enzymes necessary for the synthesis of essential amino acids which are lacking in aphid food (plant sap); in return aphids synthesize enzymes required to build up bacterial cell wall. Another example of bacteria-animal symbiosis is of gutless worms of deep hydrothermal vents. They have sulfur oxidizing bacteria as endosymbionts; since they don't have digestive tract, they are completely dependent on nutrition provided by these bacteria. Association of squids, jelly fish and fishes like angler fish and lantern fish with luminescent bacteria is interesting example of symbiotic interaction. Bioluminescence is acquired by luminescent bacteria like Vibrio fischeri which harbor inside the light organs of these animals; it is very useful for location of prey, as path finder in deep sea regions, to attract mate or to warn off or repel the predators.

Association of bacteria-protozoa: Complex interactions among termite, bacteria and protozoan Trichonympha is good example of mutualistic association. Cellulose in the form of plant fiber is the principle food of termite but it is not able to digest without digestive help from Trichonympha. Trichonympha on the other hand relies on bacteria present on its surface for particular lytic enzymes fir complete digestion of cellulosic material. Neither termite nor Trichonympha and associated bacteria live in absence of any of the organism. One more example is relationship between protozoan Mixotricha and bacteria. This protozoan lacks mitochondria but it harbors bacteria that fulfill the vital functions of mitochondria.

Bacteria-algae symbiosis: Obligate relationship is observed between some higher algae and bacteria synthesizing vitamin B12. Some algae cannot produce cobalamin on their own and hence dependent on exogenous supply of cobalamin which is fulfilled by bacteria associated with algal cells. Bacteria in turn derive nutrition from algae produced during photosynthesis. Algae-bacteria associateship is also evident in sewage with high phosphorus concentration and during activated sludge process.

Bacteria-fungi symbiosis: Lichens, which is very unique mutualistic association between fungi and algae is in fact influenced by presence of endosymbiotic alpha-proteobacteria. Lichen thallus consists of blue green algae or cyanobacteria, fungi and functional bacterial communities. Their functional, physiological and morphological status is however still unknown. In one example of bacteria-fungi associations, fungus growing ants harbor special Nocardia-like bacteria to protect fungal crop of ants from infection. These bacteria are present on cuticle of worker ants and engaged in their role of infection inhibition and protection.

Symbiotic associations of bacteria-plant: Relationship between rhizobia and leguminous plants is very well known and intensely studied symbiosis. It represents an obligate endosymbiosis in which rhizobia of Rhizobium spp. and Bradyrhizobium spp. carry out unique biological nitrogen fixation process inside root nodules of legume host plants. Bacteroids or nodule rhizobia provide fixed nitrogen to fulfill nitrogen requirement of host plant and in turn receives protection, oxygen and photosynthetic nutrients for their growth. Free living nitrogen fixing bacteria like Azospirillum, Frankia and Azotobacter also fix nitrogen but ectosymbiotically with grasses, trees and crop plants respectively. Nitrogen fixing root nodule and free living bacteria are integral living functional components of terrestrial ecosystems. Interaction between aquatic fern, Azolla and cyanobacteria Anabaena spp. is also excellent example of obligatory symbiotic association. Anabaena fix nitrogen into utilizable form for fern and in turn gets protective habitat, oxygen and nutrients formed during Azolla photosynthesis. Different species of indigenous rhizosphere and soil bacteria such as Pseudomonas, Micrococcus, Bacillus, Proteus, Sarcina and Actinomyces also live in association with plant species. They mobilize insoluble phosphate compounds in utilizable form, sequester iron and zinc essential for plant growth and plant host in turn enhance the growth of these bacteria by provision of nutritive root exudates as carbon and energy source. Rhizobacteria protect plant host from deleterious soil microbes and also degrade recalcitrant soil pollutants like hydrocarbons, pesticide residues or heavy metals found near plant rhizosphere. Plant-rhizobacteria association is very beneficial as it forms the basis of phytoremediation and biofertilization applications.

Bacteria-bacteria relationships: Cyanobacterial blooms, formation of biofilms such as dental plaque, opportunistic infections and involvement of native bacteria or consortia in biodegradation of petrochemicals or pesticides represent classical examples of interactions between different bacteria. Soils and aquatic environments contaminated by oil spills have been found to contain indigenous bacteria like Aeromonas, Pseudomonas, Sphingomonas, Acinetobacter, Brevibacterium, Micrococcus, Arthrobacter and Bacillus actively degrading aromatic and aliphatic components of oil. It was also investigated that rate of degradation was higher when these bacteria worked as consortium than individual strain. Many bacterial species of Pseudomonas, Deinococcus and Actinobacteria have been found to be associated with cyanobacteria forming harmful blooms. Some of these bacteria were also found to degrade algal toxins to receive their nutrient requirements. Bacteria in consortia are able to communicate by quorum sensing mechanism. Bacteria in consortia or biofilms function in coordination and get advantages like protection, nutrition and resistant to antimicrobial agents.

There could be more examples of bacterial associations with other organisms; only few of them are described in this article to get an idea about versatility of bacterial interactions. There could also be a possibility of existence of novel interactions which we still have not discovered so this would be one of the areas of investigation for enthusiastic researchers.

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