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  Microbial Growth-Reproduction and Control, Biology tutorial

Introduction:

Microbial growth is stated as an increase in the number of cells. A microbial cell consists of a life-span and a species is maintained only as an outcome of continued growth of its population. Growth is the final process in the life of a cell - one cell becoming two and afterward leading to a rise in the number in a population of microorganisms. In microbiology, growth is identical to reproduction.

Definition of Growth:

Growth is stated as a rise in the number of cells in a population of microorganisms. It is a raise in cellular constituents leading to the rise in cell number if microorganisms reproduce by processes such as binary fission or budding.

The Prokaryotic Cell Cycle:

A prokaryotic cell cycle is the complete series of events from the formation of a new cell via the next division. Most prokaryotes reproduce through binary fission, budding and fragmentation.

Binary Fission:

Binary fission is a kind of asexual reproduction method. In which a single cell splits into two cells after building up a transverse septum (that is, cross-wall).  

Binary fission is a simple kind of cell division and the methods comprised are: the cell elongates, replicates its chromosomes and separates the recently made DNA molecules in such a way that there is a chromosome in each half of the cell. A septum is made at mid cell; split the parent cell into two progeny cells and each having its own chromosome and a copy or complement of other cellular components.

Chromosome Replication and Partitioning:

Most of the prokaryotic chromosomes are circular. Each and every circular chromosome consists of a single site at which replication begins termed as the origin of replication or simply the origin. The replication is completed at the terminus that is positioned directly opposite the origin. Early in the cell cycle, the origin and terminus move to the mid-cell and a group of proteins required for DNA replication carry on in both the directions from the origin and the parent DNA synthesis accumulate to form the replisome at the origin. DNA replication continues in both directions from the origin and the parent DNA is thought to spool via the replisome, which remains relatively stationary. As progeny chromosomes are synthesized, the two recently formed origins move toward opposite ends of the cell, and the rest of the chromosome follows in an arranged fashion.

Cytokinesis:

Septation is the procedure of making a crosswall among two daughter cells. Cytokinesis, a word that consists of traditionally been employed to explain the formation of two eukaryotic daughter cells, is employed to explain the procedure in prokaryotes as well. Septation is splitted into a number of steps:

a) Choosing of site where the septum will be made.

b) Assembly of a specialized structure termed as the Z ray that splits the cell in two by construction.

c) Linkages of the Z ring to the plasma membrane and possibly components of the cell walls.

d) Assembly of the cell wall-synthesizing machinery.

e) Construction of the Z ring and septum arrangement.

The Z protein, a tubulin homologue found in most of the bacteria and most Archaea makes the Z ray. Fts Z, such as tubulin, polymerizes to form filaments.  Once the Z-rings form, the rest of the division machinery is made. First one or more anchoring protein connects the Z ring to the cell membrane. Then the cell wall - synthesizing machinery is accumulated.

The ultimate steps in division comprise construction of the Z ring, accompanied through invagination of the cell membrane and synthesis of the septal wall.

Measurement of Microbial Growth:

Population growth is evaluated or measured by following modifications in the number of cells or changes in the level of several cellular components. This could be completed by measuring the total cell number or by evaluating or measuring the cell mass. 

Measurement of total Cell number:

The total number of microbial cells can be accomplished by employing direct count methods, Viable Counting Methods and Most Probable Number (MPN) method.

1) Direct Count Methods: 

Bacteria, microorganisms can be specified by direct counting method by utilizing:

a) Special counting chambers like hemocytometer and Petroffitausser chamber can be used to find out the number of bacteria.

b) Big microorganisms like protists and yeast can be directly counted by utilizing electronic chambers like the coulter counter and the flow cytometer.

c) The Membrane Filter method: The number of bacterial in aquatic section can be find out from direct counts after the bacteria have been trapped on particular membrane filters like a nitrocellulose 0.2 µM pore size filter or a black polycarbonate membrane filter.

2) Viable Counting Methods:

These techniques comprise plating serial dilutions of a suspension of microorganisms unto a appropriate solid growth medium and after a period of incubation (that is, in which single cells multiply to form the visible colonies) the number of colonies are counted or specified.

3) Most Probable Number (MPN) method:

Most probable number is a statistical process based on the probability theory. In this technique, multiple serial dilutions are functioned to reach a point of extinction. The point of extinction is the dilution level at which no single cell is deposited to one or more multiple tubes.

It is a criterion by which growth of cloudiness or turbidity or gas production is established for determining whether a specific dilution includes bacteria.

Measurement of Cell Mass:

Methods for measuring changes in cell mass can be employed to measure the growth of microorganisms. They comprise:

1) Finding of Microbial dry weight:

Cells growing in the liquid medium are collected through centrifugation, washed, dried in an oven and weighed. This is a helpful method for evaluating growth of filamentous fungi; though, it is time taking process and not very responsive.

2) Spectrophotometry:

This process based on the fact that microbial cells scatter light which strikes them as microbial cells in a population are of roughly constant size; the amount of dispersion is directly proportioned to the biomass of cell present and indirectly associated to cell number.

Factors affecting Microbial growth:

1) The rate of microbial growth and death are highly affected by ecological factors or parameters.

2) Some of the environmental conditions favor quick microbial growth whereas others don't permit bacterial reproduction.

3) Comprehending the influence of ecological factors on microorganisms assists in the control of microbial growth and the study of ecological distribution of the micro-organisms.

These factors comprise: solute, temperature and water activity, pH, pressure, oxygen level and radiation.

The growth and actions of microorganisms are greatly influenced through physical and chemical state of the environment. The four main factors are pH, temperature, water and availability.

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