Microbial Interactions in the Mycorrhizosphere
Soil microorganisms have an important influence on soil fertility and plant health (Gianinazzi and Schuepp, 1994). Symbiotic mycorrhizal fungi, such as arbuscular mycorrhizal (AM) fungi form a key component of the microbial populations influencing plant growth and uptake of nutrients. In addition to increasing the absorptive surface area of their host plant root systems, the hyphae of these symbiotic fungi provide an increased area for interactions with other microorganisms, and an important pathway for the translocation of energy-rich plant assimilates to the soil. Traditionally, the influence of plant assimilates on microbial communities has been defined in relation to the rhizosphere, the narrow zone of soil surrounding living roots (Hiltner, 1904). The natural roles of mycorrhizosphere organisms may have been marginalized in intensive agriculture, since microbial communities in conventional farming systems have been modified due to tillage (Sturz, et al. 1997; McGonigle, and Miller, 1996). and high inputs of inorganic fertilizers, herbicides and pesticides (Gianinazzi, and Schuepp, 1994; Gianinazzi, et al. 2002).
Fig. 1. Schematic view of possible interactions among different components of the mycorrhizosphere. The drawing is not to scale and underestimates the relative surface area of the external mycorrhizal mycelium.
Microbial Interactions in the Rumen
Cellulose and other plant fiber polymers are fermented in the rumen by a cellulolytic biofilm containing a complex mixture of microbial species. Polymer hydrolysis and fermentation are accompanied by the release of plant secondary “defense” chemicals such as tannins, saponins and coumarins. The release and microbial transformation of these compounds (Dawson, et al. 1997) modifies the gut environment and the processes therein. Although ruminants have traditionally been seen as “mobile fermentors” (Hungate, 1966), there is increasing awareness of their contribution to global warming through methane production and their possible role as reservoirs for pathogens such as Escherichia coli O157.
Nutritional interactions in the cellulolytic biofilm are based primarily upon interspecies competition for and transfer of nutrients and/or protons. Many saccharolytic bacteria and methanogens stimulate microbial growth and cellulolysis by removing products or supplying nutrients such as branched-chain VFA (Wolin, et al. 1997). For example the propionate producing bacterium Selenomonas ruminantium uses sugars and H2 produced by the anaerobic cellulolytic fungus Neocallimastix frontalis (Marvin, et al. 1990), and the presence of S. ruminantium enhances cellulolysis.
In contrast, co-culture of cellulolytic species reveals the existence of competitive and inhibitory interactions, co-cultures often demonstrating reduced fiber degradation compared to that achieved by the more active organisms grown axenically. This may be based on competition for cellulose, perhaps through competition for attachment sites as the bacteria spread from their initial point of attachment, or competition for soluble nutrients. Thus, cultures of Ruminococcus flavefaciens strains 007 and 17 were found to be more active in degrading fibrous substrates such as barley straw and clover than were cultures of Fibrobacter succinogenes strains S85 and BL2 or mixed cultures containing both species (Saluzzi, et al. 1993), possibly reflecting interactions of this type.
Methanogenesis in the ruminant gut is calculated to account for 15-25% of the global methane emission of around 7.5 x 1012 g yr-1 (Kirchgessner, et al. 1995). Methane is the major electron sink product of the fiber-degrading cellulolytic-methanogen consortium; by comparison the amylolytic-methanogen consortium produces relatively more propionate and less methane (Kirchgessner, et al. 1995). Classical two-component co-cultures are not an appropriate model for rumen methanogenesis. Thus, the addition of S. ruminantium to co-cultures of N. frontalis and Methanobrevibacter smithii reduced methanogenesis compared to that obtained in the 2- component cultures, presumably because the bacterium, which competes successfully for sugars released upon cellulose hydrolysis, produces less H2 than the fungus (Marvin, et al. 1990). The rumen microbes are seen as the “first line of defense” against toxic plant secondary compounds ingested by ruminants. In addition to effects on animals, plant defense chemicals may have significant effects on the composition and activity of the rumen microbiota. Tropical legumes such as Sesbania, Acacia and Calliandra have shown marked effects on some microbial species (Osuji and Odenyo, 1997; Salawu, et al. 1998).
Recently, concern has arisen about the emergence of human pathogens, such as the verocytotoxic Escherichia coli serotype O157, for which farm animals may act as a reservoir, shedding of the pathogen in faeces resulting in contamination of the human food chain. This serotype does not produce disease symptoms in adult animals, although neonatal cattle may be susceptible (Dean, et al. 1997). The survival of E. coli O157, was reduced in cattle dosed with commensal E. coli and Proteus mirabilis strains recovered from ruminant faeces (Zhao, et al. 1998). Pseudomonas aeruginosa from the rumen of sheep inhibited growth of E. coli O157 in vitro (Duncan, et al. 1997). The P. aeruginosa strains from different animals produced different bacteriocins (pyocins), which may have an ecological role in determining the dominant biotype present. Two pigments from P. aeruginosa (pyocyanin and fluorescein) were shown to inhibit both commensal and O157-serotype strains of E. coli. Pyocyanin had the greater effect but, as reported previously (Hassan and Fridovitch, 1980) was more inhibitory in the presence of oxygen than its absence. It seems that some of the bacteria interacting most directly with E. coli in the ruminant gut are other relatively minor members of the gut microbiota. Variations in the presence and numbers of these bacteria may contribute to differences between animals in the survival and shedding of E. coli. The co-excretion of such bacteria may also influence the survival of E. coli in shed faeces and in the soil. Pyocyanin would presumably be active if present in faeces exposed to air.
I would like to express on my humble opinion in microbial interactions topic. Microbial interactions in all environmental sites are either useful or harmful. Some microbes have intrinsic pathogenicity due to presence of certain genes encoded virulence and pathogenicity. These microbes are harmful and cause many diseases for human, animal, plants, protozoan and even for prokaryotes such as bacteria. Other microbes consider friends to environment because they live in the environment as symbiotic relationship. These useful microbes present in all environmental sources as microflora (microbes take their requirements from environmental source and give the later its requirements). Both of useful and harmful microbial interactions enter in the formation of environmental balance in nature.
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