MICROBIAL INTERACTIONS
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.
References
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