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Factors effecting microbial growth


 

Physical and chemical factors affecting microbial growth

 The growth of microorganism is effected to a large extent by physical and chemical factors. The physical and chemical condition include environment, temperature ,ratio of various gases, pH , presence or absence of lights and salt concentration etc.

Effect of moisture on the growth of microorganism

Microorganisms very vividly in their requirements with regard to water content. To express the moisture content ,the term water activity or relative humidity is used. These parameters are relative to the vapour phase which is in equilibrium with a solid material or an aqueous solution. Thus water activity of a medium is the ratio of the water concentration in the vapour phase over that medium and the water concentration in the gaseous state over pure water at a given temperature.

Microorganisms can grow over a range of water activities of 0.998 to 0.6 . The lowest water activity tolerated for growth has been reported in case of yeast i.e Saccharomyces Which grow at water activity of 0.6 . Most of the fungi Aspergillus can grow at Water activity of 0.8 but the most of the bacteria need water activities of more than 0.98 The only exception are halophiles with water activity requirement of 0.75.

 It has been seen that greater the moisture content in the medium, lower will be the temperature required to kill the bacteria . The reason is moist heat is more effective as a sterilising agent than dry heat.  Scientists have shown that Micrococci when heated under moist conditions ,were low in resistance as compared to those which were heated in dry conditions.

If moisture is completely absent fungi will note grow, it needs at least some moisture to be grow.

Effects of bactericidal agent

Bactericidal is an agent that kills both pathogenic and non pathogenic bacteria, but not necessarily there spores.  In practice, the term is synonyms with germicide the bactericidal agent including oxidising agents,acids ,alkali ,salts reducing agents, phenols ,alcohols mercury compounds ,silver compounds and soaps

Effect of acids on growth

The bactericidal efficiency of acid is proportional to the hydrogen ion concentration of their solutions Strong acids like hydrochloric acid, sulphuric acids etc. are more bactericidal than weak acids like acetic acid and lactic acid

Effect of alkali

The action of alkalise on bactericidal activity  depend upon the release of hydroxyl ion in solution . The greater the degree of dissociation , the more is the effectiveness of germicide .Alkalise that are specially toxic to bacteria include potassium hydroxide , sodium hydroxide ,lithium hydroxide, ammonium hydroxide . Out of all, potassium hydroxide is more toxic by virtue of its greater degree of dissociation and ammonium hydroxide is the lowest in bactericidal action. Because ammonium hydroxide is the least ionise.  There are some exceptions to the above rule, a weak base such as barium hydroxide is less dissociated than potassium hydroxide yet it is more toxic to bacteria this is due to high toxicity of barium ions in salts.

Effect of salts

Some salts ,especially in high concentration ,are toxic to bacteria example :chloride of various elements in very high concentration.  On the other hand, salts of heavy metals example mercury and silver are toxic even at very low concentration. Toxicities of cations for various bacteria increases with valency . Gram positive bacteria are usually more sensitive to the action of salts than gram negative foforms Exceptions are helophiles which grow in presence of very high salt concentrations . Example pathogenic Staphylococcus

Effect of reducing agents

These are bactericidal by virtue of their power of reduction. Example sulphites (sulphur dioxide )and ferrous compounds are reducing agents and so are bactericidal.

Effect of oxidising agent

Some compounds which give up oxygen freely or are capable of releasing oxygen from other compounds acts as bactericidal. These destroys bacteria by the process of oxidation . Sodium hypo chloride and Calcium hypo chloride are important oxidising compounds. The agents that caused oxidation is hypochlorous acid which ultimately gives the nascent oxygen.

  Effect of Phenols

Phenols or carbolic acids were among the first chemical identified as the sterilising agent . These are strong bactericidal at a very high concentration . Phenol act as protoplasmic poisons, destroying the cell wall by penetrating the cell and precipitating the cellular protein.  In low concentration ,they inhibit protein and DNA synthesis.  In some experiments , for the isolation of microbes,  phenol solution is used for surface sterilization. Example- root nodules for isolation of Rhizobium

Effect of alcohols

Ethyl alcohol is widely used as bactericidal . The maximum bactericidal efficiency of alcohol is exhibited at a concentration of 70% by weight . It was found to be an effective germicidal in the early days. So the presence of alcohol in the medium reduces growth rate of bacteria . Alcohols are believed to be act by denaturing protein

Effect of mercury compounds

All the heavy metals are toxic to the microbes and their effect on the growth of microorganism has been investigated in detail by the scientists.  Mercury among of the first to be used as bactericidal . It was later shown that mercury compound did not necessarily kills bacteria but exerted a strong bacteriostatic effect.

Effect of silver compounds and other heavy metals

Silver, like mercury is a strong bacteriostatic metal. Other heavy metals like cadmium and lead act as bacteriostatic and effect growth by attacking various systems inside the microbes . Example -cadmium and lead reduces the heterocyst formation frequency of bacteria  (cyanobacteria.)

Effect of soaps

It has been known since the beginning of bacteriology that both soft and hard soaps are mildly antiseptic i.e they either reduces the growth rate or kill the microbes.

Effect of time and temperature

In general bactericidal action is increased with time . This means that a high dilution may be employed with an increase in the period of action similarly increase in temperature increases the ineffectiveness of bactericidal effect. However, no general rules can be made . An exception to the rule that Germicidal action is increased with time in the case of iodine. This bactericides is the vigorous oxidising agent and acts almost immediately when placed in contact with bacteria.

Effect of organic matters

Probably all bactericides are reduced in activity in presence of organic matter . This is especially due to presence of protein i.e amino acids and compounds of similar nature.


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