Microbial Association-Microbial Interaction

Microbial Association

Many microbial populations interact and establish associations with each other and with higher organisms. Usually the association is nutritional, although other benefits may accrue and the association can become crucial to the survival of one or both partners. In 1879, de Bary coined the term ‘symbiosis’ to describe any situation where two different organisms live together. Confusingly, some biologists then used the same term specifically to mean the association where both the partners benefited. The term ‘symbiosis’ will be used in its original non-specific sense in this text. There are many sorts of symbiotic relationship such as mutualism, parasitism, amensalism and competition, predation, protocooperation (synergism) and commensalism between the organisms.

Microbial Associations and Fundamentals of their Interactions

The interactions between the two populations are classified as above according to whether both populations or one of them benefit from the associationship, or one or both populations are negatively affected
Mutualism and parasitism have been most extensively studied in microbial relationships.In view of their enormous biological, medical, and agricultural implications, the mutualism and parasitism will attract greater concentration in this chapter though other will be considered briefly.

Mutualism

Mutualism describes a relationship in which both associated partners derive some benefit, often a vital one, from their living together. Attempt to summarise the main kinds of mutualistic associations; some of which are trivial and of scientific interest only but others such as Rhizobium legume association, mycorrhizae, coral-microbial association, herbivore-microbial association and lichens are very important, or indispensable, both to the local ecosystem and on a world scale.

Kinds of Mutualistic Associations Involving Microorganisms

Mycorrhizae (Sing. Mycorrhiza)
Mycorrhizae represent a mutualistic symbiosis between the root system of higher plants and fungal hyphae. Frank, who first noted the existence of such a characteristic association in the roots of Cupulifereae in 1885, coined the term ‘mycorrhiza’. Over the last 20 years, basic works conducted by hundreds of researchers from different countries has shown that this association is fundamental and universally occurring. Among the different symbiotic associations between the soil microorganisms and root of plants, mycorrhizae are the most prevalent as they occur on more than 90% of the vascular plants. However, Kumar and Mahadevan (1984) have studied a large number of mycorrhizal associations are found that they are highly influenced by the toxic substances that, when present, are essentially concentrated in the root of plants. Such substances may be alkaloids, phenolics, terpenoids, tannis, stilbenes, etc.
Advantages

1. The fungus derives nutrients via the root of the plant. Sugars formed in the leaves move down the stem as sucrose. Sucrose itself never accumulates in the fungus; it is converted into isomers such as ‘trehalose’ thus resulting in the low sugar concentration.
2. The fungal hyphae act like a massive root hair system, scavenging minerals from the soil and supplying them to the plant.
3. Because of this associationship the plant partner, in addition to the nutritional benefits, develops drought resistance, tolerance to pH and temperature extremes, and greater resistance to pathogens due to ‘phytoalexins’ released by the fungus.

Classification

Mycorrhizae are generally divided into two types, although a third type that is more or less a combination of the first two is recognized by some.
The two major types are termed Ectomycorrhizae and Endomycorrhizae while the third one, however, is referred to as Ectendomycorrhizae.

Ectomycorrhizae (Ectotrophic mycorrhizae) Ectomycorrhizae are common on many forest trees, particularly pines, beech and birch which are of much economic value.The fungal hyphae form a sheath over the outside of the roots which is generally called ‘mantle of hyphae’. From this mantle, a hyphal network called hartignet extends into the first few layers of the cortex or rarely deeper and then reaches the endodermis.Root hair formation is suppressed in the infected root and the root morphology is changed by the repeated formation of short branches with blunt tips and limited growth. Common ectomycorrhizal genera are Basidiomycetes, particularly Agaricales such as Amanita, Tricholoma, Russula, Lactarious, Suillus, leccinum and Cortinarius; some Ascomycetes such as the truffles have also been reported.

Ectotrophic Mycorrhiza

The fungi of Ectomycorrhizae secrete various growth promoting substances such as auxins, cytokinins and gibberellic acids. Nevertheless, they produce some antimicrobial substances which protect the host plant against soil-borne pathogens. Fungi derive their carbon from the host in the form of glucose, fructose or sucrose which is ultimately converted to mannitol, trehalose, and glycogen. These mycorrhizae are known to stimulate plant growth and nutrient uptake in soils of low to moderate fertility.

Vesicular-arbuscular (VA) mycorrhizae
Vesicular-arbuscular (VA) mycorrhizae represent associations between fungi, mostly the members of Zygomycetes, and a great number of angiosperms such as trophical forest trees, almost all agricultural crops (except rice in paddy fields) and most of the herbs and grasses of trophical and temperate natural ecosystems. Fungi forming VA mycorrhizae are restricted to only one family, Endogonaceae, of Zygomycetes with two genera, Endogone and Glomus, forming associations with a huge variety of distantly related plants. VA mycorrhizae are especially important because of their widespread occurrence and association with agricultural crops. In VA mycorrhizae the fungal hyphae develop some special organs, called vesicles and arbuscules, within the root cortical cells. Vesicles are thick walled, spherical to oval in shape, borne on the tip of the hyphae either in intercellular spaces or in the cortical cells of the root. These vesicles are food storage organs of the fungus. However, the arbuscules are brush-like dichotomously branched (extensively) haustoria developed within the cortical cells. Though widely distributed geographically, the VA mycorrhizae are not of usual occurrence in continuously flooded sites (Keeley, 1980).

VA Mycorrhiza The importance of VA mycorrhizae is in the effects that they have on plant nutrition, especially the immobile elements such as phosphorus. The external hyphae greatly increase the volume of soil and translocated the phosphorus to the roots. Plants are heavily infected with VA mycorrhizal fungi in phosphorus-deficient soils and myucorrhizae are poorly developed when the phosphorus supply is adequate. It is thus a self-regulating system, increasing, phosphorus uptaken when this element is in short supply. The phosphorus so absorbed is converted into polyphosphate granules in the hyphae and passed to the arbuscules for ultimate transfer to host plant.

Orchidaceous mycorrhizae
Orchidaceous mycorrhizae are very different from VA mycorrhizae. Here the higher plant is temporarily or permanently parasitic on the fungus; the later are mostly from the genus Rhizoctonia with the perfect stages occurring in basidiomyetes and ascomycetes. Orchid seeds are minute (0.3-14 mg), without any significant food reserve. Some fail to germinate at all unless infected by fungus; others germinate, but, development soon ceases unless the seedling becomes infected by the fungus (Hardley, 1975). The hyphae of the fungus penetrate the cells of the cortex and form coils within the cortical cells. These nutrient rich hyphal coils, generally called peletons, then break down making food available to the plant. How the orchid persuades the fungus to undergo this bizarre self-sacrifice is not known, though the role of “Phytoalexin orchinol” has been implicated. However, the degenerating peletons supply the orchid with carbohydrate and probably vitamins and hormones obtained by saprophytic action of fungus outside the root. most orchids eventually become green, and so the association between the orchid and the fungus may shift from parasitism to mutualism. Other orchids, e.g., Neottia nidus-avis (bird’s nest orchid) remain acholorophyllus and parasitic on the fungus throughout their life

Ericaceous mycorrhizae
Ericaceous mycorrhizae are associated with the two families, ericaceae and epacridaceae, in which the fungus forms dense intracellular coil in the outer cortical cells.Earlier, it was believed that the fungus was Phoma sp. but cultural studies proved that it is Pezizella ericae (an ascomycete). This fungus has trememdous capacity of mineralization and absorption of organic nitrogen, thus it greatly stimulates nitrogen uptake and plant growth even in infertile peat soil. Englander and Hull (1980) have suggested Clavaria spp. as the mycorrhizal fungus of Rhidodendron spp. and Azalea spp. However, this mycorrhizal association result in no development of root hairs as well as the absence of epidermal cells of the root.

Ericaceous Mycorrhiza; Ectendomycorrhizae (Ectendotrophic mycorrhizae) The mycorrhizae which bear the characteristic of both, ecto- and endomycorrhizae, are categorised by some as Ectendomycorrhizae. The fungal partner established mantle of hyphae on the surface of the root as well as hyphal coils and haustoria within the invaded cortical cells of the root. In the forests, ‘Conifer-Boletus-Monotropa’ association represents a well studied example of Ectendomycorrhizae. Monotropa, a nonchlorophyllous plant, usually grows near the roots of the conifers in the forests. It is now well established that a fungus called Boletus forms a common mycorrhizal association between the conifer and Monotropa, though the nature of the association differs with the two plants. Boletus forms ectomycorrhiza with Monotropa and endomycorrhiza with the conifer. The fungus forms a bridge between the two plants.

Syntrophy

Syntrophy (Gr. syn=together; trophe-nourishment) is such mutualistic interrelationship between two different microorganisms which together degrade some substances (and conserve energy doing it) that neither could degrade separately. In most cases of syntrophism the nature of a syntrophic reaction involves H2 gas being produced by one partner and being consumed by the other. Thus, Syntrophy has also been called interspecies hydrogen transfer. Following are some examples of syntrophic associations:

(i) Ethanol fermentation to acetate and eventual production of methane is a good example. Ethanol oxidizing bacterium ferments ethanol producingH2 which is a valuable electron donor for methanogenesis hence used by a methanogen. When both these reactions are summed, the overall reaction is exergonic (i.e., energy releasing). Actually, the oxidation of ethanol to acetate plus H2 is energetically unfavourable, the reaction becomes favourable when H2 produced during it is consumed by the methanogens. In this way, both partners thus use the energy released in the coupled reaction of syntrophic association.
Ethanol Fermentation to Methane and Acetate By Syntrophic Association

(ii) Oxidation of butyric acid to acetic acid plus H2 by the fatty acid-oxidizing syntroph Syntrophomonas is another good example. Syntrophomonas does not grow in a pure culture on butyric acid as the energy released during butyric acid oxidation to acetic acid is highly unfavourable to the bacterium. But, if the hydrogen produced in the reaction is immediately utilized by a syntrophic partner (e.g., methanogen), Syntrophomonas grows luxuriantly in mixed-culture with the H2 consumer.

(iii) Syntrophobacter degrades low molecular weight fatty acids (e.g., propionic acid) to produce H2 which, if not consumed, inhibits the growth of the producer. When this hydrogen is immediately consumed by Methanospirillum, the syntrophic partner, the growth rates of both Syntrophobacter and Methanospirillum is stimulated, i.e., both partners are benefited.

Microbial Action in the Rumen of Ruminants

The cellulases hydrolyse the cellulose to glucose, and the microorganisms then ferment the glucose to a variety of organic acids such as acetic acid (ethanoate), butyric acid (butyrate) and propionic acid (propionate), so providing energy for their own growth. About 103 dm3 per day of Co2 and CH4 are produced as waste products, which are burped out by the animal. Thus the microorganisms present in rumen convert largely indigestible plant material into low molecular weight carbon compounds which can be utilized by the herbivore.

Microbial Action in the Rumen of the Ruminants

Microbes in herbivore-rumen
Though less in number than the bacteria, the protozoa may be 50% of the biomass and they include isotrichia, Diplodinium, Dasytrichia and Epidinium. Generally, these protoza have been assigned a role in increasing bacterial turn over rate by their feeding; they remove about 1% of the bacteria per minute. Some of the protozoa can also degrade cellulose but probably not significantly when compared with the bacteria.Many bacteria have been isolated from the rumen but the main ones are Rumenococcus flavifaciens. R. albus, Bacteroids succinogenes. B. amylophilus and Butyrovibrio. These and others degrade cellulose and starch and generate, as earlier described, organic acids. Methane is formed by Methanobacterium ruminnantium, using hydrogen and carbon dioxide produced by the degradation.The fungi in rumen have only been reported recently; they include some yeasts but more especially there are anaerobic chytrid-like fungi with multiflagellate zoospores which are apparently widely distributed. These fungi may be significant in the digestion of lignocellulose.

Herbivore-Microbial Association
Plants contain about 30% cellulose (dry weight), the large insoluble inert polysaccharide. It would be very much to the advantage of any herbivore to digest the chemical but, however, the only herbivore to possess the appropriate digestive enzyme, cellulose, is snails. All others, from insects to mammals, do not possess this enzyme and they establish mutualistic associationship with cellulose splitting bacteria and protozoa. These microorganisms generally occupy one of the several sites in the gut, the most advanced condition being that in ruminants. Ruminants, such as cow and sheep, have evolved a unique four chambered ‘stomach’ that has helped establish them as extremely successful herbivores. The rumen volume is large compared with size of the mammal (in the cow it is 80-100 L) so that there being a long resistance time for cellulose decomposition. Plant material is chewed, mixed with saliva and passed to rumen. The rumen contains very numerous microorganisms of which about 90% are cellulose secretors. These may be 104 to 105 protozoa. 1010 – 1011 bacteria and 4 × 104 fungi. The contents of the rumen are continually mixed by slow contractions of the wall at 1-2 minute intervals.

Alimentary Canal of Ruminants

Bioluminescence by Marine Invertebrates and Fish
Some luminescent bacteria (e.g., Photobacterium, Beneckea) establish mutualistic association with marine invertebrates and fish (e.g., Anomalops katoptron). If oxygen is available, these bacteria that normally inhabit some specific organ of the partner, emit blue-green light. The production of light is due to luciferase enzyme that mediates the reaction of reduced flavin mononucleotide (FMNH2), molecular oxygen, and a long-chain aldehyde that produces flavin mononucleotide (FMN) in an electronically excited state. The return of the excited FMN to its ground state results in the emission of light. In turn, the animal partners supply the bacteria with nutrients and protein from competing microorganisms. However, the light emitted by bacteria is used in various ways like sexual mating rituals, search of food sources, warding-off predators, etc.

Coral-Microbial Association
Corals are highly productive and yet live in waters that are very poor in nutrients; the open ocean may have a net productivity of 50 g cm-2 year-1 whereas coral reefs may produce up to 2500 g cm-2 year-1. The reasons for this are still not clear. It is considered that the dinoflagellate symbionts, Gymnodinium microadriaticum and Amphidinium species, are ubiquitous in reed-building corals and they pass atleast 25%, and probably as much as 60 to 70% of their fixed carbon to the animal as glycerol and glucose. They may also take up nitrate from the water and pass it to the coral in a utilizable form alanine. Cyanobacteria are important on reefs in fixing nitrogen, and they may be free living or symbiotic. Bacterial symbionts living on the outside of the coral in mucilage layer have also been implicated in the conservation and rapid recycling of phosphorus and nitrogen to the corals. There are, therefore, a number of potential or actual symbiotic microorganisms which could account for the productivity of coral reefs. Apart from the productivity aspects, Gymnodinium is also very important in depositing skeletal calcium as a result of photosynthesis. The dinoflagellates apparently have to reinfect each generation of corals for they are not passed on during reproduction. How they do this is not known for they have never been found free living in the environment, though they can grow independently in culture.

Lichens

Lichens are remarkable in that under natural conditions the algal-fungal or cyanobacterial-fungal association behaves as a single organism. The fungus (mycobiont) is usually an ascomycete and about 20,000 lichen fungi have been described which is approximately 25% of all known fungi.
There are only some 30 genera of algae (Phycobiont) and cyanobacteria (cyanobiont) known to form lichens. The relationship between the two associates of the lichen thallus is still not fully confirmed, though lichens have been the classic material for the study of microbial mutualistic symbiosis. The Phycobiont/cyanobiont supplies carbohydrate to the mycobiont and the latter may supply minerals to the former. We have no experimental confirmation that the mycobiont supplies minerals to its associates; also, the Phycobiont may be able to absorb its own minerals from the substrate. ‘Good’ laboratory conditions cause the association to break down, whilst adverse conditions help to maintain it. This indicates that the association probably enables the associates to exploit habitat which would be unsuitable when they grow apart. Lichens are considered the ‘pioneer organisms’. They have been claimed to be important in increasing the rate of soil formation from bare rock (Seaward, 1977). They may accelerate physical destruction of the rock by shrinkage and expansion of the thallus, may decompose the rock by wide range of chemical substances such as carbon dioxide (acting as H2CO3), various organic acids, and chelating agents. Lichens may accumulate minerals and nitrogen which are eventually released to the primitive soil when the lichen thallus is decayed. Lichens are greatly affected (even killed) by the level of SO2 present in the atmosphere; their abundance can be used as an indicator of atmospheric pollution. They or their products may be used as food dyes, and indicators (litmus).
Lichens Classifications
Taking mycobionts into consideration, the lichens are classified into three categories:

(a) Ascolichens: Those having ascomycetous mycobiont. Examples: Paltigera, Parmelia, Collema, Graphis, Physcia, Cladonia, etc.
(b) Basidiolichens: Those having basidiomycetous mycobiont. Examples: Cora, Omphalina, etc.

(c) Deuterolichens: Those having deuteromycetous mycobiont. These lichens are very few in number and are mostly sterile in the sense that their fungal associates do not produce spores

Taking, phycobionts into consideration, the lichens are classified into two categories:

(a) Chlorophycophilous: Those having green algal Phycobiont.
(b) Diphycophilous: Those having green algae as well as cyanobacteria in the same thallus.Those lichens that have only blue green bacteria (cyanobacteria) and fungal partner in their thalli are called cyanophillous.
Taking morphology into account, lichens are classified into three forms:

(a) Crustose: Crust-like, the crusts are so closely attached to the substratum by the whole of its lower surface that is very difficult to dissociate them without breaking. Example – Lecidea, Graphis, Verrucaria, etc.
(b) Foliose: Leaf-like; adhere to the substratum only at definite points by means of certain outgrowths called ‘rhizines’. Ex-Gyrophora. Peltigera, Parmelia, Physcia, Collema, etc.
(c) Fruticose: Shrubby, generally upright in habit; some are pendent, i.e., hang from the twigs and branches of the trees. Examples – Cladonia, Ramalina, Evernia, etc. 9shrubby) and Usnea barbata (pendent).

Anatomically, lichens can be classified into two types:

(a) Homolomerous: When algal/cyanobacterial counterpart scattered irregularly but uniformly among the fungal counterparts within the thallus; no any definite cortical layer.
(b) Heteromerous: Thallus with remarkable differentiation into zones: algal/cyanobacterial counterpart contained to a particular zone or layer.

Parasitism

Parasitism represents the symbiotic associationship between two living organisms and is of advantage to one of the associates (parasite) but is harmful to the other (host) to a greater or lesser extent. The parasites may be destructive or balanced. The former destroy the host cells in their later stages of development whereas the latter fulfil their demands from the host in such a way that the host cells are not destroyed but continue to live.
Facultative and Obligate Parasites
Associations would be easy to describe if organisms always behaved in the same way. Unfortunately, they do not. Many microorganisms, for instance, can survive as both parasites and saprophytes. The fungus Ceratocytstis ulmi, which causes Dutch elm disease, kills the tree and then lives saprophytically on its dead remains. Such an organism which mostly lives as saprophyte but seldom holds the charge of a parasite is referred to as facultative parasite. In contract, downy mildews, powdery mildews, etc. only grow on live protoplasm of the host plant in nature. Such as organism which cannot live elsewhere except on the living protoplasm of its host in nature is called obligate parasite (biotroph). Facultative and obligate parasites often differ in their pathogenic effects, i.e., in their ability to injure the host. Since obligates are restricted to living organisms, their effects on the host are often less severe, although the host may show less vigorous growth. In contrast, facultative parasites which have only recently acquired a host, tend to be more damaging.

Mycoparasitism

When one fungus parasitizes the other, the act is referred to as ‘mycoparasitism’. This term has been generally used interchangeably with ‘hyperperasitism’. ‘direct parasitism’ or ‘interfungus parasitism’. This incitant is generally called ‘mycoparasite’ or ‘hyperparasite’. Mycoparasitism has been classified into two main groups on the basis of nutritional relationship of parasite with host : necrotrophic and biotrophic.

(a) Necrotrophic Mycroparasitism
The necrotrophic (destructive) parasite makes contact with its host, excretes a toxic substance which kills the host cells and utilizes the nutrients that are released.

(b) Biotrophic Mycoparasitism
The Biotrophic (balanced) parasite is able to obtain its nutrients from the living host cells, a relationships that normally exists in Nature. The Mycroparasitism is of common occurrence and examples can be found among all the groups of fungi from chytrids to higher basidiomycetes (Sneh et al., 1977; Dwivedi and Mishra, 1982; El-Shafie and Webster, (1979).
(b) Few examples are as follows:
A three member mycoparasitic associationship has also been reported in which chytridium parasiticum is parasite on Chytridium subercrelatum which, too parasitizes Rhizidium richmondense, another chytrid (Willoughby 1956).
The biological control of plant diseases has recently become an area intensive research in view of the hazardous impact of pesticides and other agro-chemical on the ecosystem. Amongst the biological agents, the mycoparasites have attained a significant position. It has been suggested that efforts should be made to investigate the biological control of plant diseases through parasitism and predation. Therefore, the mycologists and plant pathologists are searching for new mycoparasites because the greater number of these the greater would be the chance of exploiting them as agents for biological control. Trichoderma is an important example.

New terms for parasites
The belief that the obligate parasites cannot be grown in laboratory on artificial culture media came at stake when after reports poured in recent years in which there are claims to grow successfully some obligate parasites on culture media. This has prompted the biologists to propose new terminologies in this respect.
BlotrophA parasite which always obtains its food from living tissues regardless of the ease with which it can be cultured in the laboratory on artificial media, is referred to as a biotroph.The term ‘biotroph’ is now being used for obligate parasites because some of them have been successfully cultured in the laboratory during past few decades.

Hemibiotroph

A parasite is called Hemibiotroph if it attacks living in the same way as the biotroph but continues growing and reproducing even after the living tissues are dead. Hemibiotroph, infact represent facultative saprophytes.

Necrotroph

A parasite, when it kills living of the host in advance of penetration during infection and then obtains its food as a saprophyte, is called a nectotroph. Necrotrophs represent the facultative parasites and are also referred to as perthotrophs or perthophytes.

Amensalism

Amensalism (from the Latin for not at the same table) refers to such an interaction in which one microorganism releases a specific compound has a negative effect on another microorganism. That is, the Amensalism is a negative microbe-microbe interaction. Some important examples are:

(i) Antibiotic Production by a microorganism and inhibiting or killing of other microorganism susceptible to that antibiotic is the important example of Amensalism. Concentrations of such antibodies in the bulk of soil or water are certainly small, though there could be a large enough quantity on a micro-habitat scale to give inhibition of nearby microorganisms. The antibiotics reduce the saprophytic survival ability of pathogenic microorganism in soil. The attini ant-fungal mutualistic relationship is promoted by antibiotic producing bacteria (e.g., Streptomyces) that are maintained in the fungal gardens (see box). In this case, streptomyces produces an antibiotic which controls Escovopsis, a persistent parasitic fungus, which can destroy the ant’s fungal garden.
(ii) Production of ammonia by some microbial population is deleterious to other microbial population. Ammonia is produced during the decomposition of proteins and amino acids. A high concentration of ammonia is inhibitory to nitrate oxidizing populations of Nitrobacter.

A Schematic Diagram Showing the Use of Antibiotic Producing Streptomyces by Attini ants to control the growth of fungal parasite in their fungal Garden
Fungal Gardens
The fungal gardens of ants (e.g., attini ants, Acromyrmex), ambrosia beetles, and some termites are excellent mutualistic examples of an insect helping fungi to grow in pure culture.Attini ants macerate leaf material, mix it with saliva and faecal matter, and inoculate the prepared substrate with fungus. After the fungus flourishes, the ants harvest a portion of the fungal biomass and the byproducts they ingest. Ambrosia beetles and some termites develop fungal pure-culture in their habitat and rely on the cellulolytic enzymes of the microbial populations to convert plant remains into habitat sources that they can use. The insect provides an optimal habitat for growth of the fungus.

Competition

In contrast to the positive interactions of mutualism and synergism, competition represents a negative relationship between two populations in which both populations are adversely affected with respect to their survival and growth. In this case, the microbial populations compete for a substance which is in short supply. Competition results in the establishment of dominant microbial population and the exclusion of population of unsuccessful competitors.During decomposition of organic matter the increase in number and activity of microorganisms put heavy demand on limited supply of oxygen, nutrients, space etc. The microbes with weak saprophytic survival ability are unable to compete with other soil saprophytes for these requirements and either perish or become dormant by forming resistant structures.

Predation

Predation typically occurs when one microorganism, the predator, engulfs and digests other microorganisms, the prey, and the former derives nutrition from the latter. In microbial fraternity, however, the distinction between predation and parasitism is not sharp. The interaction between Bdellovibrio bacteria and other small gram-negative bacteria is considered by some as predation but by others as parasitism. Bdellovibrio is apparently quite widespread in aquatic habitats and attacks other bacteria, normally gram-negative ones by boring a hole in the wall, entering the bacterium and causing lysis with the eventual release of many small Vibrio-shaped bacteria. The major microbial predators are the protozoa which may engulf bacteria and more rarely algae and other protozoa. These systems have been used extensively in models and simulations of predator prey-relationship. In the simplest from the protozoan population (e.g., Tetrahymena) is limited by its bacterial food (e.g., Klebsiella) and numbers of both prey and predator show cyclic oscillations. Another such example is of Didinium-paramecium (both protozoa) relationships. Didinium preys on the paramecium until the population of the later becomes extinct. Lacking a food source, the Didinium population also becomes extinct. If a few members of the paramecium population are able to hide and escape predation by the Didinium, then the paramecium population recovers following the extinction of the Didinium. Thus a cyclic oscillation can occur in the population of these two protozoans. Predatory fungi exist and have been considered as possible biocontrol agents for some diseases of plants caused by soil microorganism. Nematodes and protozoa may be trapped by a variety of net-like-hyphae, sticky surface and nooses. The organism is the invaded by hyphae and digested.

Protocooperation (synergism)

Protocooperation (or synergism), like mutualism, represents an association between two microbial population in which both population benefit from each other, but it differs from the mutualism in that the association is not ‘obligatory’. Both synergistic populations of microbes are able to survive in their natural environment on their own. Protocooperation or synergism allows microbial population to perform metabolic activities such as synthesis of a product which neither population could perform alone.
Following are few examples:

(i) The Desulfolvibrio bacteria supply H2S and CO2 to Chlorobium bacteria and, in turn, the Chlorobium bacteria make sulphate (SO4) and organic material available to Desulfovibrio. Thus the mixture of the two bacterial populations produce much more cellular material than either alone.
(ii) Nocardia population metabolizes cyclohexane resulting in degradation products that are used by Pseudomonas population. The Pseudomonas population. The pseudomonas species produce biotin and growth factors that are required for the growth of Nocardia.

(iii) Azotobacter population present in soil fixes atmospheric nitrogen if they have a sufficient source of organic compounds. Other soil bacterial populations such as cellulomones are able to utilize the fixed form of nitrogen and provide the Azotobacter population with needed organic compounds.

Commensalism

Commensalism represents a relationship between two microbial populations in which one is benefit and the other remains unaffected (i.e., neither benefit nor harmed). Thus the commensalism is a unidirectional relationship between two microbial populations, It is quite common, frequently based on physical or chemical modifications of the habitat, and is usually not ‘obligatory’ for the two population involved. Commensalistic association is often established when one microbial population, during the course of its normal growth metabolism, modifications the habitat in such a way that the other population is benefited.
Following are some examples:
(i) A disease causing microbial population when a lesion on the surface, it creates an entry-passage for other microbial population that otherwise could not enter and grow in the host tissues. For convenience, Mycobacterium leprae, the causative agent of leprosy, open lesions on the body-surface and thus allow other pathogens to establish secondary infections.

(ii) When facultative anaerobes utilize oxygen and lower the oxygen content, they create anaerobic habitat which suits the growth of obligate anaerobes because the latter benefit from the metabolic activities of the facultative anaerobes in such a habitat. On the contrary, the facultative anaerobes remain unaffected. The occurrence of obligate anaerobes within habitats of predominantly aerobic character, such as the oral cavity, is dependent on such commensal relationship.
(iii) Population of Mycobacterium vaccae, while growing on propane cometabolizes (gratuitously oxidizes) cyclohexane to cyclohexane which is then used by other bacterial population, e.g., Pseudomonas. The latter population is thus benefited since it is unable to oxidise cyclohexane to cyclohexanone. Mycobacterium remains unaffected since it does not assimilate the cyclohexanone.

Cometabolism

Cometabolism is the process in which one organism growing on a particular substrate gratuitously oxidizes (i.e., oxidizes without any motive) a second substrate which is of no use for it. The oxidation products, however, are well used by other organism.
An Example of Commensalism based on cometabolism

Some microbial populations create Commensalistic habitat by detoxifying compounds by immobilization. Leptothrix bacteria deposite manganese on their surface. In this way, they reduce manganese concentration in the habitat thus permitting the growth of other microbial populations. If Leptothrix do not act so, the manganese concentration would be toxic to other microbial population.