Microbial Growth in Natural Environments:

Microorganisms in a particular location are exposed to many overlapping gradients of nutrients and various other environmental factors. Microorganisms will grow in ‘microenvironments' until an environmental or nutritional factor limits growth. Liebig's law of minimum states that the total biomass of an organism will be determined by the nutrient present in the lowest concentration relative to the organisms requirements. Shelford's law of tolerance states that there are limits to environmental factors, below and above which a microorganism cannot survive and grow, regardless of the nutrient supply. Each organism has a specific temperature, pH, oxygen level and hydrostatic pressure to grow. The growth of a microorganism depends on both the nutrient supply and its tolerance of the environmental conditions. In response to low nutrient levels (oligotrophic environments) and intense competition, many microorganisms become more competitive in nutrient capture and exploitation of available resources. Morphology of the organism can change in order to increase its surface area and ability to absorb nutrients. Microorganisms can also undergo step by step shut down of metabolism except for housekeeping genes. Natural substances can also directly inhibit microbial growth and reproduction in low-nutrient environments. These agents include phenolics, tannins, ammonia, ethylene etc. This may be a means by which microorganisms avoid expending limited energy resources until an adequate supply of nutrients becomes available.

Growth of natural prokaryotic populations outside the laboratory can be determined by counting the number of viable microorganisms present. A viable microorganism is one that is able to grow actively, resulting in the formation of a colony or visible turbidity in liquid medium. John R. Postgate of University of Sussex in England was one of the first to note that microorganisms stressed by survival in natural habitats were sensitive to secondary stresses. Such stresses can produce viable microorganisms that have lost the ability to grow on media normally used for their cultivation. To determine the growth potential of such microorganism, Postgate micro viability assay, in which microorganisms are cultured in the thin agar film under a cover slip. The ability of a cell to change its morphology, even if it does not grow beyond the single stage indicates that the microorganism does show life signs.

The situation in natural environments with mixed populations is much more complex. Often only 1 to 10% of observable cells are able to form colonies. The microbiologist is attempting to grow microorganisms that perhaps never have been cultured or characterized. At present, molecular techniques involving PCR amplification and small subunit ribosomal RNA analysis are increasingly used to analyze diversity of uncultured microbial population.

Quorum sensing:

Bacteria can communicate with one another and behave cooperatively. Quorum sensing or autoinduction is a phenomenon in which bacteria monitor their own population density through sensing the levels of signal molecules, sometimes called autoinducers because they can stimulate the cell that releases them. The concentration of these signal molecules increases along with the bacterial population until it rises to a specific threshold and signals the bacteria that the population density has reached a critical level or quorum. The bacteria then begin expressing sets of quorum-dependent genes. Quorum sensing has been found among both gram-negative and gram-positive bacteria. Quorum sensing was first discovered in gram-negative bacteria and is best understood in these microorganisms. The most common signals in gram-negative bacteria are acyl-homoserine lactones (HSLs). These diffuse into the target cell and once they reach a sufficiently high level, acyl HSLs bind to specific receptor proteins and trigger conformational change, they bind to target sites on the DNA and stimulate transcription of quorum sensitive genes. The genes needed to synthesize acyl HSLs is also produced frequently, thus amplifying the effect by the production and release of more auto inducer molecules (Fig. 8).