MICROBIAL TALK IN THE ENVIRONMENT
Padmavathi and C.S.V. Ramachandra Rao
DVR & Dr. HS MIC College of Technology
Kanchikacherla 521 180, Krishna Dist, Andhra Pradesh.
Email Id: firstname.lastname@example.org
is the way of expression and transfer of information
from one individual to another. It has always
been the essence of life on earth. Living organisms
communicate by signals. The higher organisms constantly
signal and communicate in the form of words, symbols,
colours, gestures and sounds to each other. The
process carries out many diverse functions, to
survive, in a social environment. Communication
is the method by which living beings gain access
to information regarding their surroundings (Dusenberg,
1996). In the natural environment the tiny living
cells establish effective communication network
to interact with the same species and with other
species.Earlier scientists believed that communication
is the exclusive property of higher organisms.
However, the recent data reveals the tiny organism
too have extensive communication network. Within
the world of microorganisms signaling, communication
and information flow occur. One of the areas of
microbial research that has advanced considerably
in recent years is that of bacterial cell-to-cell
communication. The ability of bacteria to communicate
with each other changed our general perception
about these single, simple organisms inhabiting
our world. Bacteria sense each other and their
numbers by means of cell to cell communication
mechanism called 'Quorum Sensing'(QS) that are
mediated by secretions of small diffusible substances
such as 'pheromones' or 'auto inducers' (Joyce
., 2004; Fuqua et al
2001) which are volatile substances. Quorum Sensing
is linked to the cellular processes with gene
expression. Accumulation of low molecular weight
diffusible molecules (Auto Inducers) like Acyl
Homoserine Lactones (AHL) in the environment,
regulate bacterial gene expression in a concentration
dependent manner (Charu & Srivastava, 2006). Quorum
sensing enables bacteria to coordinate their behaviour.
As environmental conditions often change rapidly,
bacteria need to respond quickly in order to survive.
These responses include adaptation to availability
of nutrients, defence against other microorganism
that may compete for the same nutrient and the
avoidance of toxic compounds potentially dangerous
for the bacteria. Different bacterial species
use different molecules to communicate.
In some cases a single bacterial species can have
more than one quorum sensing system and therefore
use more than one signal molecules. There is evidence
that interspecies communicate via quorum sensing.
This quorum sensing cross talk has implications
in many areas of microbiology as in nature bacteria
almost always exist in mixed species population
such as biofilms. In nature biofilms contribute
to the formation of mats of microorganisms with
complex interacting communities (Stal 1994) (Fig.3).
Quorum sensing is believed to regulate competence
development, sporulation, antibiotic synthesis,
virulence factor induction, cell differentiation
and nutrient flux along with other physiological
events and pathogenic bacterial influence (Cvitkovitch
et al., 2003; Greenberg 2003; Yarwood et al
In nature, bacteria regulate their cell numbers
by modifying their behaviour through intercellular
communication mechanisms. This is achieved by
production of signaling molecules called as autoinducers.
By reacting to these molecules the bacterial cells
sense their population and adjust their metabolic
activities much in the style of multicellular
organisms (Charu & Srivastava, 2006).
The bacterial based Quorum Sensing molecules belong
to two categories, 1. Aminoacids and short peptide
Pheromones 2. Fattyacid derivatives such as Acyl
Homo Serine Lactones (AHL). The former molecules
are utilized by +ve bacteria. The later are utilized
by -ve bacteria. The Quorum Sensing activities
are quite diverse and include gene expression,
biofilm formation, DNA uptake from the environment,
Virulence, toxin production & Biolumniscence.
In -Ve bacteria AHL's induce the synthesis of
compounds like toxins,antibiotics or exoenzymes.
AHL signal molecules from various bacteria are
related in structure(Fig.1). The molecules differ
only in the acyl side chain moiety( Charu & Srivastava,2006).
In Rhizosphere the microorganisms use Homo Serine
Lactone (HSL) based QS to communicate with plants.
A number of bacteria have been identified that
use HSL based QS. These are Aeromonas hydrophila,
Aeromonas salmonicida, Agrobacterium tumefaciens,
Chromobacterium violaceum, Erwinia caratovora
subsp. Caratovora, Erwinia stewartii, Nitrosomonas
europaea, Pseudomonas aeruginosa,, Pseudomonas
aureofaciens, Vibrio(Photobacterium) fischeri
. In Rhizosphere the microorganisms use
Homo Serine Lactone (HSL) based QS to communicate
Various Homo Serine Lactones used by bacteria
for cell to cell communication (Reproduced
from Charu & Srivastava, 2006 Current
In adverse conditions, within
the biofilms microorganisms produce and maintain
chemical conditions that favors the growth of
specific populations that otherwise might not
survive. Chemical variations facilitates the survival
of diverse fastidious bacteria with a unique range
of metabolic disorders (Wimpenny 1992).
In natural environment, distinct microbial populations
frequently interacts with each other. In a global
context all symbiotic relationships can be viewed
as beneficial because they act to maintain ecological
Microbial population establish this kind of relationship
through cell to cell communications. One of the
well established mutualistic relationship between
microorganisms and plants is the symbiotic nitrogen
fixation by Rhizobium. This process involves a
two way communication between the leguminous plant
and the nitrogen fixing Rhizobium sps
The process of nodule formation is the result
of a complex sequences of interaction between
rhizobia and plant roots (Solheim 1984; Brewin
1991). Flavonoids or isoflavonoids secreted by
the host plants induce the expression of a number
of nodulation genes in the rhizobial bacteria.
The products of nod genes, also called Nod factors,
are the species-specific, lipooligosaccharides.
These signal compounds, which are released by
induced rhizobial cells, elicit the curling of
plant root hairs and division of meristematic
cells eventually leading to the formation of root
nodules (Fig.2). Rhizobia respond by positive
chemotaxis to plant root exudates and move towards
the localized sites on the legume roots. Rhizobium
species are attracted by aminoacids and dicarboxylic
acids present in root exudates, as well as by
very low concentrations of excreted compounds
such as flavonoids. Lectins, plant proteins with
high affinities to carbohydrate moieties on the
surface of appropriate rhizobial cells, have been
identified as specific mediators of the attachment
of rhizobia to susceptible root hairs (Dazzo and
Hubbell 1975; Dazzo and Brill 1979; Hubbell 1981).
During the nodulation process, tryptophan secreted
by the plant roots is metabolized to Indole Acetic
Acid (IAA) by the rhizobia. The IAA, with unknown
cofactors together initiates hair curling or branching.
The root hairs grow around the bacterial cells.
Polygalacturonase, secreted by the rhizobia or
possibly by the plant roots, depolymerizes the
cell wall and allows bacteria to invade the softened
plant root tissues (Hubbell 1981; Ridge and Rolfe
Within the infected tissue, rhizobia multiply,
forming unusually shaped and sometimes grossly
enlarged cell called bacteroids. Interspersed
with the bacteroid-filled cells of the nodule
are uninfected vacuolated cells that may be involved
in the transfer of metabolites between the plant
and microbial tissues. The communication between
the nitrogen fixing bacteria rhizobium and leguminous
plants are of great importance in global nitrogen
cycling and increase in soil fertility.
The communication between soil microorganisms
and plant roots satisfy important nutritional
requirements for both the plant and the associated
microorganism (Brown 1974). This is apparent by
the large numbers of microorganism found in the
rhizoplane defined as rhizosphere, the region
of soil directly influenced by the plant roots
(Campbell and Roviva 1973).
There is a communication between the microorganisms
in the adverse conditions of the environment.
The cooperative behaviour within a population
are evident in the case of slime mold, Dictyostelium.
When food sources become limited, the cells of
this species swarm together to a central organism.
This swarming action is in response to the chemical
stimulus of cyclic AMP being released, and it
occurs in a pulsating wave motion as the stimulus
to synthesize AMP is transmitted from proximal
to distal cells. The cells unite to form a fruiting
body and spores that subsequently disperse. Frequently,
some spores reach favourable habitat with a more
abundant food supply via this mechanism, germinate
and resume an amoeboid life stage.
Communication among members of this microbial
population allows for cooperative searching and
utilization of resources in the habitat. Similar
communication mechanism has also been seen in
myxobacterial population. (Shimket 1990: Shapiro
1991: Dworkin 1996).
3: The growth of Biofilms. Biofilms, or
microbial growths on surfaces such as
in freshwater and marine environments,
can develop and become extremely complex,
depending on the energy sources that are
available. a) Initial colonization by
a single type of bacterium b) development
of a more complex biofilms with layered
microorganisms of different types c) A
mature biofilms with cell aggregates,
interstitial pores, and conduits (Reproduced
from Prescott et al., 2000 Microbiology,
5th Int. Ed. Mc Graw Hill)
communication mechanism existing between microorganisms
is to overcome competition. Members of a microbial
population use the same substrates and occupy
the same ecological niche. If an individual
within the population metabolizes substrate
molecules then the molecule is not available
for other members of the population. Within
a high-density population, leaked metabolic
products may accumulate to an inhibitory level.
For example lactic acid accumulation can limit
the activity of lactobacillus (Atlas and Bartha;
In a similar way when microbial population produces
a substance i.e., antibiotics that is inhibitory
to other population. The communication mechanism
is called amensalism. There are cases of complex
amensalism between populations in natural habitat.
For example virucidal or fungistatic in soil
Communication between microorganisms is paramount
in the microbial world and many variety of signals
are used to communicate between cells. The microbes
are very simple living creatures leading simple
lives but it is a great deal that msicroorganism
can perfectly communicate with one another.
Even though they are tiny, they are capable
of doing things in the nature just like higher
is Professor & Head,
Department of Biotechnology at MIC College of
Technology, Kanchikacherla, Andhra Pradesh.
He has been actively involved in Teaching, Research
& Consultancy for the last 15 years. He is an
Accredited Specialist in Environmental Microbiology
at Canadian Colleges of Microbiologists, Canada
Dr.Rao's research interests are microbial diversity
in fresh and marine waters.
is an Assistant
Professor at Department of Biotechnology at
MIC College of Technology. She has been involved
in active teaching to the B.Tech Biotechnology
students at the above college.
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