Microbial interactions include mutualism, commensalism, parasitism, competition, and predation. Mutualism is a positive interaction that benefits both species. There are different types of mutualism including obligate, where species depend on the interaction for survival, and facultative, where species benefit but can live independently. Examples of mutualism include lichens, where algae and fungi interact obligately, and honeybees and flowering plants, which interact facultatively. Microbial interactions are important for ecosystem function and evolution.

1. Interactions between
Microorganisms
Dr. P. Suganya
Assistant Professor
Department of Biotechnology
Sri Kaliswari College (Autonomous)

2. Introduction
• Biological interactions are the effects that the organisms in a
community have on one another.
• There are completely different kinds of microbial interactions which
incorporates interaction with different microbes, Plant-Germ
interactions promoting plant growth, interaction with animals,
interaction with humans, and interaction with water, etc.
• Microbial interactions are ubiquitous, diverse, critically important in
the function of any biological community, and are crucial in global
biogeochemistry.
• The most common cooperative interactions seen in microbial systems
are mutually beneficial.
• The interactions between the two populations are classified according
to whether both populations and one of them benefit from the
associations, or one or both populations are negatively affected.
• There are many sorts of symbiotic relationships such as mutualism,
parasitism, amensalism, commensalism and competition, predation,
protocooperation between the organisms.

4. Types of Microbial Interaction
• Positive interaction: Mutualism,
Syntrophism, Proto-cooperation,
Commensalism
• Negative interaction: Ammensalism
(antagonism), parasitism, predation,
competition

5. Mutualism

6. Mutualism Definition
• A mutualism is a form of interaction between organisms of
two or more species where all the species benefit from the
interaction.
• The benefits from the interaction can be protection,
pollination, dispersal, or provision of nutrients.
• In most cases, mutualism exists between a pair of species, but
some interactions might exist between more than two species.
• Mutualism can be obligate in some species where the species are
dependent on the interaction for their survival.
• In others, however, mutualism provides benefits, but the species
are not dependent on the interaction for survival; this is known
as facultative mutualism.
• Mutualism can also be species-specific where the interaction is
exclusive between two species, but some interactions are diffuse
and involve multiple interactions between different species.

7. • The number of species involved in the interactions
depends on how the species benefit from the interaction.
• Mutualistic interactions mostly exist between species
that have widely functional and living requirements.
• The term mutualism was used initially by Pierre-Joseph
van Beneden in 1876 in his book Animal Parasites
and Messmates to indicate the meaning ‘mutual and
among species’.
• Mutualism is an important interaction in ecology as well
as evolution. Currently, it is assumed that about 80% of
land species in a terrestrial ecosystem depend on
mutualistic interactions with fungi for nutrients.

8. • The term of interaction between the species in
mutualism can be short-termed or longer depending on
the nature of the interaction and the species involved.
• The smaller member of the interaction is termed the
mutualist, whereas the larger species is called the host.
• Mutualism involves provisioning a resource by one
species and receiving the resource by the other species.
During the interaction, providing the resource is costly,
whereas receiving the resource is beneficial.
• In order to maintain and evolve mutualism, the benefits
must outweigh the cost. If the cost outweighs the
benefits, mutualism breaks down and moves to a
parasitic association.

9. What is Symbiosis?
• Symbiosis is a closed and prolonged interaction between two
organisms of different species that benefit one or both of the species.
• Symbiosis has been traditionally confused with mutualism, but
symbiosis differs in that the species exist in close association.
Symbiosis is a broader category that consists of interactions like
mutualism, commensalism, and parasitism.
• Symbiosis is of two types depending on the proximity of the involved
species.
• Endosymbiosis is the interaction where one of the species is present
within the body of the other.
• Ectosymbiosis is the relationship where the species are present in
close proximity but not within each other.
• Symbiosis, like most ecological interactions, is recognized as one of the
essential forces behind evolution.
• In some species, symbiosis is essential for the survival of the species,
which is called an obligate symbiotic interaction. In others, it is
facultative, and the species can live on their own without the
interactions.
•

11. Types of Mutualism
• Mutualism is of different types depending on
various factors.
• Two important factors resulting in different
mutualistic systems are the number of species
and the degree of obligateness of the
interaction.
• The following are some of the types of
mutualism;

12. 1. Obligate Mutualism
• Obligate mutualism is the interaction between
different species where the interaction is
essential for their survival, and thus the species
are obligated or forced to depend on each
other.
• Most of the symbiotic, as well as non-symbiotic
interactions, occur through obligate mutualism.
• Obligate mutualism is also termed exclusive
mutualism as the interactions are very specific,
and the absence of the interaction results in the
death of one or both species.
• The benefits in obligate mutualism are usually
more than those in facultative mutualism.

13. Example- Lichen
• Lichens are the most important example of obligate
mutualism consisting of a relationship between a
fungus and algae.
• The interaction allows both of the species to survive in
conditions where they would not be able to survive
otherwise.
• The algae provide nutrients to the fungus by producing
organic matter by the process of photosynthesis. The
fungus, in turn, protects the algae from the environment
with the help of its filaments.
• The fungus also gathers moisture and nutrients from
the surrounding environment.
• Both the fungus and the alga cannot exist in such
environmental conditions unless present in the obligate
mutualistic interaction.

15. 2. Facultative Mutualism
• Facultative mutualism is the interaction between two or more
species where the species benefit from the interaction but can
also exist independently of each other.
• The benefits obtained from facultative mutualism are less than
that of obligate mutualism.
• Facultative mutualism is not as specific as obligate mutualism
and can thus exist between a variety of species.
• Facultative mutualism can be described in one of the three ways;
resource-resource mutualism, service-resource mutualism,
and service-service mutualism.
• In resource-resource mutualism, both of the species provide a
resource to the other species.
• In service-resource mutualism, one of the species provides a
service whereas the other provides a resource.
• In service-service mutualism, both the species provide service to
the other species.

16. Example- Honeybee and flowering
plants
• The mutualism between honeybee and flowering
plants represents a service-resource type of
facultative mutualism.
• In the interaction, the flowers of the plant provide
nectar to the bee, which acts as a source of nutrients
for the bee.
• The bees, in turn, provide a service of transferring
pollen grains from one flower to another to aid
the process of fertilization.
• It is an example of facultative mutualism as the
species can exist independently, but the interaction
provides benefits to each other.

18. 3. Trophic Mutualism
• Trophic mutualism is a type of ecological
interaction that involves the transfer of energy
and nutrients between two species.
• Trophic mutualism involves the resource-
resource interaction where the species provide
resources to one another in the form of
nutrients and energy.
• It exists between species of two distinct trophic
levels like autotrophs and heterotrophs.
Trophic mutualism can be both obligate and
facultative

19. Example- Mycorrhiza
• Mycorrhiza is the interaction between fungi and the
roots of plants where the plants provide carbon to
the fungi, and the fungi provide nutrients.
Mycorrhizas also improve water uptake and
resistance against pathogens.
• The plant is an autotroph that produces sugar
by photosynthesis whereas the fungi are
heterotrophy that depends on green plants for
nutrients.
• The fungi provide phosphorus as well as water to
the plant as a part of the interaction.

21. 4. Defensive Mutualism
• Defensive mutualism is a type of service-
resource relationship where one of the species
provides nutrients whereas the other provides
protection against predators or parasites.
• In simple words, defensive mutualism can be
explained as a species defending the other for a
reward.
• Most of the defensive mutualism exists
between insects and plants, especially fast-
growing plants with a continuous light source.
• Defensive mutualism is most obligate, but
some of the plant-fungi interactions are
facultative.

22. Example-
Acacia and Pseudomyrmex ants
• Acacia plants produce newly enlarged thorns
(also called bull’s horns) that contain loosely
arranged pith.
• The ants can easily chew and remove the pith
to create a hollow interior. The small hole in
the thorn is used by the ant as a home.
• The plants also provide extra-floral nectar to
the ants from the glands at the base of leaves.
• The ants protect the plants against various
predators as well as parasites as a part of the
interaction.

24. 5. Dispersive Mutualism
• Dispersive mutualism is the interaction between insects
or animals and plants where animals acquire nectar
from the flower while facilitating the transfer of pollen
grains.
• Dispersive mutualism is a service-resource mutualism
that enables the dispersal of the plant to new areas.
• Some animals help in the dispersal of seeds to suitable
habitats in exchange for nutrients from the fruit.
• The relationship between the plant and pollinator is
highly specific and usually exists between the same two
species.

25. Example- Ficus and fig wasps
• The flowers of Ficus are present on the inside
of the plant. The female wasps enter the fig
flowers and carry the pollen grains to other
flowers.
• The wasp lays eggs on the part of the flower
which is nourished by the galls present in the
flower.
• Thus, the plant helps in the larval development
of the wasp while the insect enables pollination
of the plant.

27. References
• oeschcke V., Christiansen F.B. (1990) Evolution and Mutualism.
In: Wöhrmann K., Jain S.K. (eds) Population Biology. Springer,
Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-74474-
7_14
• Denton K., Krebs D.L. (2016) Symbiosis and Mutualism. In:
Weekes-Shackelford V., Shackelford T., Weekes-Shackelford V.
(eds) Encyclopedia of Evolutionary Psychological Science.
Springer, Cham. https://doi.org/10.1007/978-3-319-16999-
6_3050-1
• Freedman H.I., Addicott J.F., Rai B. (1983) Nonobligate and
Obligate Models of Mutualism. In: Freedman H.I., Strobeck C.
(eds) Population Biology. Lecture Notes in Biomathematics, vol
52. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-
642-87893-0_4