Control Of Microorganisms By Physical Agents

There are microbial flora all around us. Some of them are good while some may cause serious disease in humans. To stay healthy we should regularly clean our body, to reduce the number of microorganisms over our skin surface.

As any other living organism microorganisms also require some optimum physical conditions to grow. These include write temperature, pressure, gases etc. Once we want to limit the population of these microbes it can be achieved by making adverse conditions for their survival.

Fundamentals Of Control 

The term death, as used in microbiology, is defined as the irreversible loss of the ability to reproduce. Viable microorganisms have the capacity to divide. While dead microorganisms do lot divide or grow further.

The determination of death requires laboratory techniques that indicate whether the growth occur  when the sample is inoculated into a suitable medium indicates that the organism is no longer able to reproduce. And failure to reproduce is the criteria of death.

Mode Of Action Of Antimicrobial Agents

The many substance and processes used as antimicrobial agents manifest their activities in one of several ways. For both academic and practical reasons, it is important to know how microorganisms are inhabited or killed.

The manner in which antimicrobial agents inhibit or kill can be attributed to the following kinds of actions:

  • Damage to the cell wall or inhibition of cell wall synthesis.
  • Alteration of the permeability of the cytoplasmic membrane.
  • Alteration of the physical or chemical state of proteins and nucleic acids.
  • Inhibition of protein or nucleic acid synthesis.

Physical Agents

The major physical agents or processes used for the control of microorganisms are temperature (high or low), desiccation, osmotic pressure, radiation, and filtration.

High Temperature 

Microorganisms have found alive on different temperature some may live on very high temperature and other may survive even on freezing temperature. Every type of microorganisms have some optimum, maximum and minimum temperature range on which it can grow better. If we make temperature out of optimum temperature, like temperature raised beyond optimum temperature, it will kill the microorganisms. Other than it if we lower the temperature below optimum, it will freeze the cell cytoplasm and other vital activities inside cell, this type of condition can be termed as preservation for some type of microorganisms. But some other those cannot tolerate such low temperature will die.

Thermal Death Time And Decimal Reduction Time

Thermal death time refers to the shortest period of time to kill a suspension of bacteria (or spores) at a prescribed temperature and under specific conditions.

Another unit of measurement of the destruction of microorganisms by heat is the decimal reduction time. This is the time in minutes to reduce the population by 90%, or stated differently, it is the time in minutes for the thermal death time curve to pass through one log cycle.

Low Temperature

Temperature below the optimum for growth depress the rate of metabolism, and if the temperature is sufficiently low, growth and metabolism cease.

Low temperature is useful for preservation of cultures, since microorganisms have a unique capacity for surviving extreme cold.


Desiccation of microbial cell causes a cessation of metabolic activity, followed by a decline in the total viable population.

In general, the time of survival of microorganisms after desiccation varies, depending on the following factors:

  • The kind of microorganisms
  • The material in or on which the organisms are dried
  • The completeness of drying process
  • The physical conditions to which the dried organisms are exposed, e.g. light, temperature and humidity

In the process of lyophilization, organisms are subjected to extreme dehydration in the frozen state and then sealed in vacuum. In this condition, desiccated (lyophilized) cultures of microorganisms remain viable for many years.

Osmotic Pressure

When two solutions of different concentrations of solute are separated by a semipermeable membrane, there will occur a passage of water, through the membrane towards the direction of the higher concentration.

The solute concentration within microbial cell is 0.95% . Thus if cells are exposed to solutions with higher solute condition, water will drawn out of the cell. The process is called plasmolysis. The reverse process is termed as plasmoptysis. The presser build within the cell because of intake of water is call osmotic pressure.

These phenomena can be observed with animal cells because they do not have a rigid cell wall.


Energy transmitted for space in a variety of forms is generally called radiation.

Radiation effect in many ways to the living cell. By the effect of radiation genetic information of bacterial cell may be changed or may be destroyed completely. Other then it radiation may effect supercoiling of proteins in cell cytoplasm and make them inappropriate to catalyse the reaction. Because of stoppage of metabolic pathways cells die easily.

Beside the fundamental research in radiation microbiology, there have been many development in the application of ionizing radiation to sterilize biological materials. This method is called sterilization.

Filtration – Bacteriological Filters

Since a long time a variety of filters are available to microbiologists, those can filter bacteria from liquid or gas. These filters are mad up of different material.

In recent years a new type of filter termed the membrane or molecular filters have been developed. Whose pore size is of a uniform and specific predetermined size. Membrane or molecular filters are composed of biologically inert cellulose esters. They are prepared as circular membrane of about 150 micro meter thickness and contain millions of microscopic pores of very uniform diameter.

Membrane filters are used extensively in the laboratory and in industry to sterilize fluid material.

Reference: Microbiology by Pelczar

Gaurav Singh

Editor in Chief Medical Microbiology & Recombinant DNA Technology (RDT) Labs - RDT Labs Magazine

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