Here is a list of four major microbial interactions:- 1. Clay-Humus-Microbe Interaction 2. Plant-Microbe Interactions 3. Animal-Microbe Interactions 4. Microbe-Microbe Interactions.
Contents
1. Clay-Humus-Microbe Interaction:
Clay mineral (and humic substances) affects the activity, ecology and population of microorganisms in soil. Clays modify the physicochemical environment of the microbes which either enhance or attenuate the growth of individual microbial population.
After release from clays, the organic material is either degraded by microorganisms or again bind to clays. Microorganisms have a negative charge at the pH of most microbial habitats. The magnitude of electronegativity on cell walls of bacteria and fungi is regulated by pH, amino acid residues and changes in wall composition.
Clay minerals get adsorbed and bind with proteins, amino acids, small peptides and humic substrates. Microorganisms utilize the nutrients for their growth and activity directly from clay- protein, clay-amino acids or peptides, and clay-humic substrate complexes.
Moreover, high levels of clay (e.g. montmorillonite) soil interferes and restricts infection of banana rootlets by Fusarium oxysporum f.sp. cubense, and thus exerts natural biological control of panama disease.
The clays and humic colloids influence the distribution and activity of Streptomyces, Nocardia and Micromonospora. Clay particles (e.g. kaolinite) is known to reduce the toxicity of cadmium (Cd) on Macrophomina phaseolina.
2. Plant-Microbe Interactions:
The above ground (foliage) and below ground (roots) portions of plants are constantly interact with a large number of microorganisms (e.g. bacteria, actinomycetes, fungi, amoebae, nematodes, and algae) and viruses, and develop several types of interrelationships.
Microbial interactions with both above ground and below ground parts of plants are briefly discussed in this section. Moreover, considering the result of interactions, it may develop destructive, neutral, symbiotic or beneficial association with plants.
Interactions on Above Ground Parts:
Microbial interactions on above ground part of plant occur in a varieties of ways where the foliage especially leaf surface (phyllosphere and phylloplane) acts as microbial niche.
i. Destructive Associations (Diseases):
Plants provide a substantial ecological niche for microorganisms. However, the abundance of this potential niche with respect to any individual microbe is more apparent than real, since a few are able to grow on a wide range of plant species.
Microorganisms show specificity with the hosts, organ, tissue and age of plants. The microorganisms that lead to destructive association are called pathogens. Example of some of pathogenic microorganisms is given in Table 28.1.
Disease development is governed by the resultant of three important factors:
(a) Host susceptibility,
(b) Congenial environment, and
(c) Virulent pathogen.
In the presence of resistant host, unfavourable environment, or a virulent pathogen, disease will not develop.
Plant-microbe interaction occurs at molecular level. In this interaction ‘gene-for-gene relationship’ of H.H. Flor (1940) implies. A gene-for-gene relationship exists when the presence of a gene in one population is contingent on the continued presence of a gene in another population and where the interactions between the two genes lead to a single phenotypic expression by which the presence or absence of the relevant gene in either organism may be recognised.
ii. Beneficial Association (Symbiosis):
The excellent example of plant-microbe interaction resulting beneficial association visualised on above ground part is the development of stem nodules. There are three known genera of legumes which are known to bear stem nodules are Aeschynomene, Sesbania and Neptunia. The stem nodules develop as a result of interaction between these plants and Azorhizobium species.
Rhizobia develop symbiotic association with hosts, fix atmospheric nitrogen and benefit the plants. S. aculeata is the most popular green manure in north India which contributes about 70 kg of nitrogen and 15-20 tonnes/ha wet biomass to the soil. A. americana is a wild annual legume which is also used as green manure. S. rostrata bears both stem as well as root nodules.
In addition, Anabaena azollae establishes symbiotic association with Azolla which is a member of pteridophyta. Species of Nostoc establishes symbiotic relationship with Anthoceros and Blasia, members of Bryophyta.
Interactions on below Ground Ports:
Similar to above ground part, plant root-microbe interactions occur in soil as well which lead different types of associations, e.g. destructive, associative or symbiotic. One of the interesting points is that the microbe has to pass the rhizosphere’ region before the start of interaction with plant roots.
i. Destructive Associations:
Like destructive association of above ground parts, the roots also result in a destructive associations. The symptoms developed by the pathogens on root are damping off, wilt, rot, knot, scab, etc. Root diseases caused by different groups of pathogens are listed in Table 28.2.
The pathogens infect roots. Entry of pathogens takes place through wounds caused by fungi or nematodes, cracks or root hairs. In most of the cases penetration is preceded by the formation of a specific cushion like structure (appressorium) which exerts mechanical pressure on root surface. Some pathogens directly penetrate the root tissues. In Rhizoctonia solani multicellular cushions are seen on the roots or hypocotyl of infected plants.
Nematodes directly inflict a slight mechanical injury on plant root. Their saliva is toxic for host tissues which results in cellular hypertrophy and hyperplasia, suppression of mitosis, cell necrosis and growth stimulation.
Second stage larvae of Meloidogyne and Heterodera normally enter the root at or just behind the root tip. Meloidogyne larvae enter through the ruptures made by emerging roots cracks on root surfaces, nodular tissues, etc. and results in development of root knots.
Certain wilt causing species of Fusarium (e.g. F. udum, F.oxysporum f. sp. cubense, F. oxysporum f sp. lycopersici, etc.) infect root, enter in vascular supply i.e. xylem bundles and produce mycelia that block the xylem vessels. These act as mechanical plug for xylem vessels. Consequently plants show wilting symptoms.
Interestingly, Macrophomina phaseolina enters in roots and gets established in root tissues. It produces intraxylem sclerotia. Sclerotia are produced in such a high amount that impart sprinkling charcoal like symptoms.
Therefore, root rot caused by this pathogen is called charcoal-rot. Certain fungi such as Pythium, Rhizoctonia, etc. cause damping-off of seedlings of several crop plants. Synchytrium endobioticum causes wart of potato tubers.
A member of actinomycetes (e.g. Streptomyces scabies) causes scab disease of potato. Agrobacterium tumifaciens, a soil- borne bacterium, causes crown gall of fruit trees including roots. Affected plants become stunted with restricted growth of plant part and poor fruit set. Pseudomonas solanacearum causing brown-rot and bacterial wilt of tomato, potato and other solanaceous plant is a well known pathogen.
After cutting open the affected tubers, and creamy, viscous exudation from open surface is observed and the dark brown discolouration of the vascular region becomes distinct. Consequently, tuber formation is affected and size of tubers is greatly reduced.
3. Animal-Microbe Interactions:
There are many kinds of microorganisms that interact with different groups of animals and develop a variety of relationships.
Some of the relationships have been discussed in this section:
i. Destructive Associations:
Pathogenic microbes interact with animals including man and cause many kinds of disease.
ii. Neutral Association (Nutralism):
Normal microbiota of human body:
There is a large number of microorganisms that normally act as the resident of different body organs of humans such as skin, nose and nasopharynx, oropharynx, respiratory tract, mouth, eyes, external ears, stomach, small intestine, large intestine (colon), and genitourinary tract (Table 28.4).
Reasons of having information’s about the normal human microbiota are:
(a) To have an understanding of microorganisms at specific site so that greater in site into the possible infections can be provided,
(b) To help the physician investigator so that he can understand the causes and consequences of overgrowth of microorganisms normally absent at a specific body site, and
(c) To increase awareness of the role of indigenous microbiota that stimulates host immune response
iii. Symbiotic Associations:
Symbiotic associations of bacteria, fungi and protozoans with insects, birds and herbivorous mammals are discussed below:
(i) Ectosymbiosis of Protozoa, Bacteria and Fungi with Insects and Birds:
Most of the animals such as insects (termites and cockroaches) cannot utilize the cellulose and lignin components of woody tissues of tree due to lack of cellulose and lignin degrading enzymes. Therefore, several insects develop ectosymbiotic association with cellulose- and lignin-decomposing microorganisms that can degrade these substrates.
All termites and cockroaches that eat upon wood, harbour flagellated protozoa in their guts. These protozoa digest cellulose. In turn the protozoa develop symbiotic association with certain N2-fixing bacteria and spirochetes which perhaps also help in cellulose degradation. In addition, during moulting season of cockroaches hormones (e.g. ecdysone) are secreted which induce cyst formation in symbiont protozoan.
(ii) Endosymbiosis of Bacteria and Fungi with Birds and Insects:
Moreover, there is a group of birds belonging to the genus Indicator which are commonly known as honey guides. These birds are found in Africa and also in India. These birds eat upon remnants of exposed honey comb but cannot digest bees wax. Therefore, they harbour in their intestine the two microbes. Micrococcus cerolyticus and Candida albicans for carrying out the digestion of bees wax.
Except carnivorous insects, the others that live upon blood or plant sap develop symbiotic association with bacteria such as coryneforms and Gram-negative rods, and Nocardia (a member of actinomycetes). These microsymbiont are present in insect hosts in specialised cells.
The cells that contain fungi are called mycetocytes, and those that contain bacteria are called bacteriocytes. These microsymbionts provide to the insects with some growth factors (that are lacking in insects) and some essential amino acids. Also the microsymbionts assist in breakdown of certain waste products.
(iii) Ruminant Symbiosis:
The herbivorous mammals (e.g. catties, sheep, goats, camels, etc) are known as ruminants because they have a special region of gut which is called rumen. These animals use plant cellulose as the source of carbohydrate which is not digested in normal gut. The cellulosic material is digested in rumen which acts as incubation chamber teeming with protozoa and bacteria. In some animals like cow, the size of rumen is very large.
Some of anaerobic cellulose-digesting bacteria (e.g. Bacteroides succinogens, Ruminococcus flavofaciens, R.albus and Botryovibrio fibrisolvens) develop mutualistic symbiosis, and hydrolyse cellulose and other complex polysaccharides to simpler forms which in turn are fermented to fatty acids (.g. acetic acid, propionic acid, butyric acid) and gases (methane and carbon dioxide).
Some of the bacteria are capable of digesting proteins, lipids and starch as well. Lignin fraction of plant remains undigested.
The rumen bacteria ferment proteins and lipids and produce hydrogen and carbon dioxides gase, which in turn is converted into methane by Methanobacterium ruminantium. The bacteria of rumen multiply into a large population. However, most of them are passed into stomach along with undigested material where they are killed by proteases and other enzymes. The fatty acids in rumen are absorbed and gases are passed out.
4. Microbe-Microbe Interactions:
Different types of beneficial and harmful interrelationships between microorganisms, and plants/animals have been discussed earlier. Similarly, microorganisms interact themselves and lead to beneficial and harmful relationships.
Some of the interactions and interrelationships have been discussed in this connection:
i. Symbiosis between Alga and Fungus (Lichens):
Lichen is a thallus of dual organism i.e. a fungus and an alga that form a self supporting combination. The fungal component is called mycobiont and the algal partner as phycobiont. The two groups of organisms live in close proximity and appear as a single plant. The fungus forms the thallus of the lichen, whereas the alga occupies only 5-10% mass of the thallus.
Mycelium of the fungal partner forms a close network that appears as tissue. Inside this compact mass of mycelium algal cells are embedded. Generally, fungi derive nutrition saprophytically from dead organic materials, or parasitically from a living host. But in lichen fungal mycelium derives nutrition from the alga. The algal cells form food by themselves and/or fix N2 from the atmosphere which then are diffused into fungal hyphae.
This type of mode of nutrition is called biotrophic nutrition which is seen in lichen. The members of algae forming lichen belong to Cyanophyta or Chlorophyta. However, it may be unicellular or filamentous forms. The genera of blue-green algae are Nostoc, Gloeocapsa, Rivularia and Stigonema. Of the green algae, species of Trebouxia are the most common unicellular green algae.
The fungal partners forming lichen are mostly the members of Ascomycetes, and 2-3 genera of Basidiomycetes. No fungus of Phycomycetes enters into lichen formation. Symbiosis is based on the facts that alga provides food to fungus, and fungus provides shelter to alga.
(i) Classification:
On the basis of nature of fungal partner and fructification types lichen are divided into two groups: ascolichens (in which fungal component is an Ascomycete), and basidiolichens (in which the fungal component is a Basidiomycete).
However, on the basis of the habitat lichens are divided into three groups: saxicolous (growing on rocks or stones), corticolous (growing on leaves and bark of trees epiphytically) and terricolous (growing on soil).
(ii) Lichen Thallus:
As in lower plant, in lichens also the plant body is known as thallus. Lichen thalli are grey or greyish green in colour.
On the basis of structure of thalli, lichens are of three main types (Fig. 28.3):
(a) Custose lichens (flat thalli, without any lobe, growing on stones, rocks, bark or any hard substrata, and appears like crust, for example Haemmatomma puniceum and Graphic scripta),
(b) Foliose lichens (thalli are flat, much lobed and leaf-like appearing as twisted leaves, have distinct lower and upper surface, attached to substrate with rhizoid-like structure called rhizinae, for example Chaudhuria, Cetraria, Parmelia, Peltigera, Physcia and Xanthoria), and
(c) Fruticose lichens (thalli are most conspicuous, most complex, and slender and freely branched, the branches are cylindrical, flattened and form thread like tuft, thalli not differentiated into upper and lower surfaces, for example Cladonia, Ramalina and Usnea.
ii. Antagonistic Interactions (Antagonism):
The composition of the microflora/microfauna of any habitat is governed by the biological balance created through interactions and associations of all individuals present in a community.
However, the environmental conditions upset the equilibrium. Any inhibitory effect of an organism created by any means to the other organism(s) is known as antagonistic interaction, and the phenomenon of this activity is called antagonism. Antagonism is the balancing wheel of the nature.
Through this mechanism some sorts of biological equilibrium is maintained. Antagonism has three facets, amensalism, competition, and parasitism and predation.
(i) Amensalism (Antibiosis and Lysis):
Amensalism is the phenomenon where one microbial species is adversely affected by the other species, whereas the other species is unaffected by the first one. Generally, amensalism is accomplished by secretion of inhibitory substances such as antibiotics, etc. Antibiosis is a situation where the metabolites secreted by organism A inhibits the organism B, but the organism A is unaffected (Photoplate 28.2).
Metabolites penetrate the cell wall and inhibit its activity by chemical toxicity. Generally, antimicrobial metabolites produced by microorganisms are antibiotics, siderophores, enzymes, etc. The potent antagonists e.g. Trichoderma harzianum and T. viride are known to secrete cell wall lysing enzymes, β-1, 3-glucanase, chitinase, etc. Lysis of fungal mycelium occurs due to secretion of enzymes.
Siderophores:
Siderophores are the other extracellular secondary metabolites which are secreted by bacteria (e.g. Aerobacter aerogenes, Arthrobacter pascens, Pseudomonas cepacia, P.fluorescens), Actinomycetes (Streptomyces spp.), yeast (Rhodotorula spp.), fungi (Penicillium spp.), and dinoflagellates (Prorocentrum minimum).
Siderophores are commonly known as microbial iron-chelating compounds because these have a very high chelating affinity for Fe3+ ions and very low affinity with Fe2+ ions. Siderophores are low molecular weight compounds. These after chelating iron (III) transport it into bacterial cells. Kloepper (1980) were the first to demonstrate the importance of siderophore production by PGPR in enhancement of plant growth.
Siderophores chelate Fe2+ and make Fe3+ deficient condition for other microorganisms. Consequently growth of microbe is inhibited. When the siderophore producing PGPR is present on root surface, it supplies iron to plant.
Therefore, plant growth is stimulated. For example, secretion of siderophore by Pseudomonas fluorescens and inhibition in growth of Macrophomina phaseolina (forming a clear zone) is shown in Photoplate 28.2. Role of siderophores in biological control of plant pathogens is of much importance in recent years.
(ii) Competition:
Among the microorganisms, competition exists for nutrients, including oxygen and space but not for water potential, temperature or pH. Success in competition for substrate by any particular species is determined by competitive saprophytic ability and inoculum potential of that species.
Garrett (1950) has suggested four characteristics which are likely to contribute to the competitive saprophytic ability:
(a) Rapid germination of fungal propagules and fast growth of young hyphae towards a source of soluble nutrients,
(b) Appropriate enzyme equipment for degradation of carbon constituents of plant tissues
(c) Secretion of fungistatic and bacteriostatic growth products including antibiotics, and
(d) Tolerance of fungistatic substances produced by competitive microorganisms.
Thus competition exists for limiting resources. The inadequate quantity of readily available carbon compounds is a more likely basis for competition. At low level of carbon, the fast growers will often hold slow growers in check when both are added to sterilized soil.
But there is no such check on the less active heterotroph when carbon supply is adequate. Under these conditions, competitiveness is directly correlated with growth rate.
(iii) Parasitism and Predation:
Parasitism is a phenomenon where one organism consumes another organism, often in a subtle and non-debilitating relationship. Predation is an apparent mode of antagonism where a living organism is mechanically attacked by the other with the consequences of death of the former.
It is often violent and destructive relationship. These phenomena are dealt with the example of fungi, amoebae and nematodes (Table 28.5).
Table 28.5 : Examples of Predation and parasitism.
(a) Mycoparasitism (Fungus-Fungus Interaction):
When one fungus is parasitized by the other fungus, this phenomenon is called mycoparasitism. The parasitizing fungus is called hyper parasite and the parasitized fungus as hypoparasite (Fig. 28.4). Mycoparasitism commonly occurs in nature.
As a result of inter-fungus interaction, several events take place which lead to predation viz., coiling, penetration, branching, sporulation, resting body formation, barrier formation to check the entry of pathogen, and lysis of host cell(s) (Fig. 28.4).
In coiling event (A) the hyperparasite i.e. antagonist (a) recognises its host hypha i.e. hypoparasite (h) among the microbial community, comes in its contact and coils around the host hypha. Host recognition by the antagonist has been discussed on molecular basis. Manocha (1985) has given the basis of host-recognition by mycoparasites.
Cell wall surface of host and non-host microbes contains D-glucose and N-acetyl-D-galactosamine residues as lectins present on the cell wall, an antagonist recognises the suitable sites (lectin residues) and binds the host hypha. As a result of coiling the host hypha loses its strength. Antagonist dissolves cell wall of host and enters inside the lumen of the later (Fig. 28.4B).
Sometimes host develops a resistant barrier (Fig. 28.4C) to prevent the penetration and proliferation inside the lumen. Host’s cytoplasm accumulates to form a spherical, irregular or elongated structure, so that the hypha of antagonist could not pass towards the adjacent cells of the hypha (C).
Depending on nutrition, the antagonist forms branches and sporulates (s) inside the host hypha (D). Until the host’s nutrients deplete, the antagonist produces resting bodies (the survival structures), for example chlamydospores (c) inside the host hypha (E) due to loss of nutrients and vigour for survival (Table 28.5; Fig. 28.4F).
(b) Mycophagy:
Mycophagy is the phenomenon of feeding upon fungi by amoebae. Many amoebae are known to feed on pathogenic fungi. The antagonistic soil amoebae are Arachnula, Archelle, Gephyramoeba, Geococcus, Saccamoeba, Vampyrella, etc.
These amoebae interact with fungal hyphae and make perforations. The fungi on which perforations have been observed are Cochliobolus sativus, Gaeumannomyces graminis var. tritici, Fusarium oxysporum, Phytophthora cinnamomi. On the lysed hyphae of these fungi amoebae develop round cysts.
Chakraborty (1983) have described the following three major steps of feeding on fungal propagules by soil amoebae:
Attachment:
As a matter of chance trophozoites of amoebae attach to fungal propagules i.e. conidia, hyphae, etc. The attachment occurs by chemotaxis or thigmotaxis.
Engulfment:
The fungal propagules according to its size are fully engulfed by amoebae. But the small trophozoites attached to the hyphal wall or spore make perforations on it.
Digestion:
The completely or partially engulfed propagules/cytoplasm of the host fungi are digested in a large central vacuole formed inside the cysts.
(c) Nematophagy:
The phenomenon of eating upon nematodes by fungi is known as nematophagy and the fungi as predaceous fungi. Fungi are mechanically involved in attacking and killing the nematodes resulting in consumption of nematodes.
The predaceous fungi are widely distributed in the surface litter and decaying organic matter. Over 50 species of fungi are known that attack nematodes. Different developmental stages of nematodes are susceptible to attack by different types of fungi.
As early as 1869, for the first time M.S. Woronin established the fact that the predaceous fungi capture and destruct the nematodes with certain specialised trapping organs.
During 1930s, C. Drechsler added greatly to the list of predaceous fungi and unravelled the mechanism of trapping. Duddington (1957) reviewed the work of fungi that attack microscopic animals and contributed significantly to the knowledge of nematophagous fungi.