Growth and Reproduction of Microorganisms
The growth of microorganisms means the increase of all the components and total mass of microbial cell resulted from the enhanced synthesis of a new cellular material.
Cellular growth in bacteria and fungi is stimulated by active microbial metabolism with prevalence of anabolic reactions over the cellular catabolism. This is supported by favorable conditions for microbial propagation. The continuous growth of the cells ultimately leads to their division and microbial reproduction.
Reproduction of microorganisms means their ability of self-multiplication that results in the rise of the number of microbial cells in the same volume of the medium.
Bacterial cells reproduce by simple non-mitotic transverse division known as binary fission. It happens in different planes and generates the great variety of morphological forms of bacteria.
The process of reproduction starts from cellular DNA replication and formation of at least two nucleoids migrating to the opposite poles of bacterial cell. Then they are separated by deep cytoplasmic insertions of the cell wall and cytoplasmic membrane resulting in cell separation. The new daughter cells appeared after binary fission are genetically identical if the mutations are absent in the process of replication.
The rate of DNA replication and bacterial fission is an intrinsic characteristic of each bacterial genus or species.
The rate of the cell multiplication differs strongly among the bacteria. It depends not only on the species origin, but also at the nutrient medium contents, the age of the culture, and the state of the environmental conditions (temperature, humidity, concentration of oxygen and carbon dioxide, etc.).
The growth and reproduction of bacteria in artificial nutrient media can be efficiently managed and controlled. When cultured in standard conditions, the bacterial populations demonstrate the uniform or similar behavior that can be predicted and regulated depending on various external or internal stimuli.
Bacterial reproduction in liquid media demonstrates characteristic growth curves that include a number of basic reproduction phases.
Primary lag phase is the initial cultivation stage that proceeds from the moment of bacterial inoculation into nutrient medium. The adaptation of the bacteria to growth conditions occurs. Reproduction is absent or low intensive, albeit the growth rate is accelerated. This phase may last about 3-4 hours.
Acceleration phase demonstrates the initial increase of growth rate.
Exponential (or logarithmic) phase is characterized by a maximal and constant division rate. Usually the duration of this period is about 5-6 hours. Due to the intensive mictrobial reproduction, two growth-limiting factors begin to prevail – the exhausting of nutrient medium and accumulation of toxic metabolites.
The next retardation phase renders the gradual inhibition of microbial propagation, where the reproduction rate becomes slower. It lasts about 2 hours.
In the maximum stationary phase the quantity of newly produced cells is equivalent to the number of dead bacteria. A total concentration of bacterial cells remains the highest through this stage. This phase usually covers about 2-3 hours. As the result, the insufficiency of nutrients or the increased concentration of toxic products causes the termination of culture growth. However, a certain cell turnover is observed in the stationary phase: a slow loss of microbial cells is still balanced by generation of new bacteria.
And finally, the decline or death phase comes, where the equilibrium between the microbial accumulation and their death rate is interrupted, and the cells progressively die. In many cases, after the death of the majority of cells, the death rate falls down, and some number of survived microbial cells may stay alive for a long time.
Actually, the duration of these phases is greatly arbitrary, depending on microbial species and the conditions for growth.
As above indicated, the generation time (or the doubling time) is an intrinsic characteristics of certain bacterial species. It differs strongly between various bacteria.
Thus, for instance, E. coli duplicates every 15-17 minutes, salmonellas – about 23 minutes, streptococci – near to 30 minutes, pathogenic C. diphtheriae – 34 minutes, and the most slowly growing M. tuberculosis – as long as 18 hours.