classsification on the basis of pH and gaseous required and water activity
In most cases , the cytoplasm of the cell has a higher solute concentration than the environment . so water tends to diffuse into the cell . however , when the cell is in an environment of low water activity .there is a tendency for water to flow out of the cell. If the water availability is too low , it affects metabolism reducing growth pattern. Some microorganism can growth best at reduced water potential and even required the high level of salt in order to grow.
Summary
In most cases , the cytoplasm of the cell has a higher solute concentration than the environment . so water tends to diffuse into the cell . however , when the cell is in an environment of low water activity .there is a tendency for water to flow out of the cell. If the water availability is too low , it affects metabolism reducing growth pattern. Some microorganism can growth best at reduced water potential and even required the high level of salt in order to grow.
Things to Remember
- Most microorganism prefers environment within a narrow range of osmotic pressure solution with high concentration of salt or sugar tend to allow water out of the cell resulting in dehydration and elevation of metabolism.
- It does not only depend on the water content of an environment but is also a function of the concentration of solutes such as salt, sugars, or others substances dissolved in water .
- All bacteria require metals as k+,Ca++,Mg++ and Fe++ to synthesize various enzyme and protein.These four elements are required in higher concentration and hence are called as macro elements.
- Each microorganism has a PH range within which growth is possible and usually has a well defined optimum PH.Most natural environment have PH value between 5 and 9 and organism with optimum growth in this range are most common.
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<p>Eliminate</p>
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<p>Remove, eradicate</p>
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<p>Proactive</p>
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<p>Taking action by causing change</p>
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<p>Career</p>
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<p>The job</p>
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<p>Communicate</p>
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<p>To share with others, to talk with others</p>
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<p>Measurement, proportions</p>
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classsification on the basis of pH and gaseous required and water activity
PH

Each microorganism has a PH range within which growth is possible and usually has a well-defined optimum PH.Most natural environments have PH value between 5 and 9 and organism with optimum growth in this range are most common. PH effect the ionic property of cell constituent of bacteria.So growth is affected by PH .Most bacteria grow in the neutral PH. on the basis of PH bacteria are of three types:
Acidophiles: Organisms that grow best at low PH are called acidiophiles.The cytoplasm of these organisms is acidic in nature.Some Acidophiles are also thermophiles in nature.Many fungi grow optimum PH 5 or below and few grow well at PH values as level as 2.However a few bacteria prefer to grow at more extreme PH values for eg Thiobacillus,Thioxydents has an optimum PH of 2-3.5.
Nutrophils: Most microorganism whose optimum PH for growth is between PH 6 and 8 are refer to as neutrophils.Most bacteria grow best at the neutral PH (7) with a normal range between 6 and 8.for eg;E. Coli klebsiella,salmonella etc.
Alkaliphiles: Some organisms which have very optimum PH for growth.Sometimes as high as PH 10-12 are called alkaliphiles.Alkaliphilic microorganisms are usually found in highly basic habited such as soda lakes and high carbonate soil.Some alkaliphiles have been found to be industrial importance because they produce hydrolytic enzymes such as proteases and lipases which function well at alkaline and are used as supplements for household detergents for eg:bacillus Alkaliphile
Salt
All bacteria require metals as k+,Ca++,Mg++ and Fe++ to synthesize various enzyme and protein.These four elements are required in higher concentration and hence are called as macro elements.Other metals such as Zn++,Cu++,Mn++,Ni++ and Co++ are required in lower concentration and hence are called trace element.They are also required to synthesize cell constituents.Most bacteria do not require Na+ ion so most bacteria grow at very low naCl concentration for growth such organisms are called halophiles.Such halophiles have the cell wall composed of glycoprotein with the high content of glutamic acid and aspartic acid on the surface of the cell wall,so high concentration is required to shield the negative charge of COOH group.If Na+ is sufficient negative charge of aspartic acid and glutamic acid repeal each other and cell wall break.Hence high Na+ is required to stabilize cell wall of such bacteria and these bacteria cannot grow at low salt concentration.
Gaseous Requirement: Oxygen and carbon dioxide are important gas that affects the growth of microorganisms.Oxygen is required to carry out various oxidation reaction in the body.Carbon dioxide is also required by some microbes.It is required as the sole source of carbon by some microbes.However requirement of these gases is different for different bacteria.On the basis of their need for tolerance of oxygen.Microorganisms are divided into following types:
Aerobic bacteria
Microaerophilic bacteria:
They require the low level of oxygen for growth but cannot tolerate the level of oxygen present in an air atmosphere. This is because of their limited capacity to respire or because they contain some oxygen sensitive molecules like enzymes. For eg: vibrio fetus.
iii> anaerobic bacteria:
they do not utilize oxygen to obtain energy and may even be killed by the presence of oxygen. This organism lack of the respiratory system which cannot use oxygen as terminal electron acceptor. These are 2 types.
Aerotolerant anaerobes and
Obligate (strict anaerobes)
Aerotolerant anaerobes can tolerate molecular oxygen and grow in its presence even though they can’t use it . eg. Streptococcus pyrogens. Obligate anaerobes cannot tolerate oxygen and are killed upon brief exposure to air. For eg: clostridium Titania , clostridium botulinum. The death of strict anaerobes in the presence of oxygen is probably due to inability to detoxify some of the products of oxygen metabolism such as hydrogen peroxide (H2O2).
Water availability:
It is an important factor affecting the microbial growth in nature. It does not only depend on the water content of an environment but is also a function of the concentration of solutes such as salt, sugars, or others substances dissolved in water . In physical term, it is generally expressed as water activity(aw). It is the ratio of the vapour pressure of the solution to the vapour pressure of the pure water. Thus, values of water activity vary between 0 and 1.
In most cases , the cytoplasm of the cell has a higher solute concentration than the environment . so water tends to diffuse into the cell . however , when the cell is in an environment of low water activity .there is a tendency for water to flow out of the cell. If the water availability is too low , it affects metabolism reducing growth pattern. Some microorganism can growth best at reduced water potential and even required the high level of salt in order to grow.
Most microorganism prefers environment within a narrow range of osmotic pressure solution with a high concentration of salt or sugar tend to allow water out of the cell resulting in dehydration and elevation of metabolism. Osmotic dehydration explains antimicrobial properties of honey , sweet contained milk, etc. treatment with salt or sugar has been used since ancient times to pressure foods.
Halophiles:
Microorganism found in sea usually required sodium ion to grow optimally organisms that require high concentrate of salts are called halophiles. For eg: halobacterium sps, halococcus sps. Halophilis may be mild(which can grow at 1-6% NaCl concentration), moderate (6-15%) and extreme (15-30%). Halophiles, bacteria found in the nose I.e. staphylococcus aureus can also tolerate in high salt concentration (7.5-10%). That is why it is known as the halotolerate organism . as it does not required high salt concentration for their growth.
References
Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.
Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.
Powar. and Daginawala. General Microbiology.
Rangaswami and Bagyaraj D.J. Agricultural Microbiology.
Lesson
Growth and physiology of bacteria
Subject
Microbiology
Grade
Bachelor of Science
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