Microorganisms - ENVIS Centre

Communication 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 et al., 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., 2003).

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 etc. In Rhizosphere the microorganisms use Homo Serine Lactone (HSL) based QS to communicate with plants.

Fig: 1. Various Homo Serine Lactones used by bacteria for cell to cell communication (Reproduced from Charu & Srivastava, 2006 Current Science: 90,666-678).

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 balance.

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 1985).

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).



Figure 2: Root nodule formation by Rhizobium. Root nodule formation on legumes by Rhizobium is a complex process that produces the nitrogen-fixing symbiosis. a) The plant root releases flavonoids that stimulate the production of various Nod metabolites by Rhizobium b) Attachment of Rhizobium to root hairs involves specific bacterial proteins called rhicadhesins and host plant lectins that affect the pattern of attachment and nod gene expression. c) Structure of typical Nod factor that promotes root hair curling and plant cortical cell division d) Initiation of bacterial penetrate into the root hair cell and infection thread growth coordinated by the plant nucleus 'N' e) Cell-to-cell spread of Rhizobium through transcellular infects threads followed by release of rhizobia and infection of host cells f) Formation of bacteroids surrounded by plant divided peribacteroid membranes and differentiates of bacteroids into nitrogen fixing symbiosomes. (Source : Prescott et al., 2000, Microbiology, Int ed. 5th ed. McGraw Hill)

Figure 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)

Another important 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; 1973).

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 (Lockwood 1964).

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 organisms.