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Factors affecting growth of bacteria.

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Introduction:

The growth of bacteria is affected by various factors, Some of them are given below:

  • Nutrients:

If cultivating medium is rich in growth promoting nutrients, growth occurs faster. Decrease in nutrient concentration decreases the growth rate of bacteria. Different bacteria requires different types of nutrients and growth faster for their growth. Growth rate of microorganims with increase in concentration of substrate uo to certain level, then growth reain constant. The relationshio between substrate concentration and growth rate of organisms is given in figure;

fig; Growth rate vs nutrient concentration graph
fig; Growth rate vs nutrient concentration graph

  • Temperature:

It affects the growth of microorganismsby various ways. The lowest temperature that allows the growth in minimum temperature and the highest temperature that allows the growth is called maximum temperature for that bacteria. there is no growth of bacteria below minimum and above maximum temperature.Below the minimum temperature, cell membrane becomes stiff so growth cannot transport nutrient and cannot develop proton gradient so growth cannot occour. At high temperature denaturation of protein and enzyme and thermal lysis of cell occurs, so growth does not occour above maximum temperature.When temperature increases, growth of microorganisms increases slowly and growth becomes maximum at certain temperatureand on furthur increasing the temperature, growth rate decreases abruptly. The temperature at which growth is maximum is called optimum temperature for growth. With increase in the temperature, rate of various cellular and metabolic activity ncreases so growth increases with temperature. But above certain temperatures, growth rate decrease because at higher temperature denaturation of protein occurs.

Classification of bacteria on the basics of temperature.

  1. Psychrophiles:Organisms that can grow at 0ocbelow the optimum temperature for the growth is 15oc or lower and maximum temperature is 20oc are called psychrophiles. They have poly unsaturated fatty acids in their cell membrane which gives fluid nature to the cell membrane even at low temperature. e.g Psychroflexus, Vibrio marinus, Polaromonas etc.
  2. Facultative Psychrophiles:Organism that can grow even at 0oc or below but optimum temperaturefor the growth is (20-30)oc are called psyychrophiles.
  3. Mesophiles: These organisms that can grow best above (25-40)oc but optimum temperature for the growth is 37oc. Most bacteria that are pathogen are mesophiles. eg. E. coli, Salmonella typhi etc.
  4. Thermophiles: These organisms grow best above 45oc. Thermophiles capable of growth is mesophilic range are called facultative thermophiles and cannot grow in mesophiles range are called obligate/ true thermophiles. They contain saturated fatty acid in their cell membrane so that their cell membrane does not become too fluid even at higher temperature. eg. Streptococcus thermophilus, Bacillus steara-thermophilus Thermus aquaticus etc.
  5. Hyper-Thermophiles: These organism have optimum temperature for the growth above 50oc. Mostly member of archae-bacteria are hyper-thermophiles. Their cell membrane is more resistant to heat so these organism are adapted too high temperature. eg, Aquifex, Thermocrinis, Thermatogoa etc.
  • pH( Hydrogen ion concentration) :

pH effect the ionic propert of cell constituent of bacteria. So growth is affected by pH. Most bacteria grow in neutral pH. Optimum pHat which most bacteria grow is 5-9. Classification of bacteria based on pH requirement is:

  1. Acidophiles: These organisms grow best at acidic pH.. Cytoplasm of these organisms in acidic in nature. Some acidophiles arealso thermophiles in nature. Such organisms are called thermo-acidophiles. eg, Thiobacillus thioxidane, ferroplasma, Pocrophilus etc.
  2. Neutrophiles: Most baceria grow in neutral pH . Optimum pHat which most bacteria grow is 6.5-7.5 eg, E.coli, Salmonella typhi. etc.
  3. Alkaliphiles: These organisms grow best at alkaline pH . Optimum at which most bacteria grow is 9.2.
  • Salt:

All bacteria requires meals as k+, Ca++, Mg++, and Fe++, to synthesizevarious enzymes and proteins.These four elements are required in higher concentration and hence aare called macro elements. Other metals sucs as Zn++, Cu++, Mn++, Ni++, and Co++ are required in lower concentration and hence are called trace elemnts. they are also required to synthesize cell constituents.

Most bacteria do not require Na+ ion so most bacteria grow at very low Nacl concentration. But some archae-bacteria require high nacl concentration for growth such organisms are called halophiles. Such halophiles have celll wall composed of glucoprotein with high content of glutamic acid and aspartic acid. On the surface of cell wall, so high concetration is required to shield the negative charges of COOH group. If Na+ is insufficient negative charge of aspartic acid and glutamis acid repeal each other and cell wall break. Hence high Na+is required to stabilize the cell wall of such bacteria and these bacterua cannot grow at low salt concentration.

  • Gaseous requirement:

Oxygen and carbondioxide are important gasthat affects the growth of microorganisms. Oxygen is required to carry out various oxidation reactions in the body. Carbon dioxide is also required by some microbes. It is required as sole source of carbon by some microbes. However requirement of these gases is different for different bacteria;

Classification of bacteria on the basics of Oxygen requirement:

  1. Obligate aerobes: These organismsrequire oxygen for growth and cannot grow in absence of oxygen. These organisms can tolerate atmospheric level of oxygen. They carry out oxidation type of metabolism. Eg, Bacillus, Mycobacterium. etc.
  2. Facultative anaerobes: These organisms do not require ocygen for growth and can use it if available. Growth of organisms becomes better in the presence of oxygen. They carry both oxidative and fermentative type of metabolism eg, E.coli, etc.
  3. Aerotolerant or non stringest anaerobes: These organisms do not require oxygen for growth but can tolerate atmospheric level of oxygen. Growth of microorganism is not affected by the presence of oxygen. They carry out only fermentative type of metabolism.
  4. Microaerophiles: These prganisms do not require oxygen for growth but can tolerate only low concentration of oxygen. Growth of organisms is inhibited by the presence of oxygen. They carry out only the oxidative type ofmetabolism eg, Campylobacter.
  5. Obligate anaerobes: These organisms can grow in absence of oxgen. Oxygen is harmful or lethal for them. They carry out only fermentative type of metabolism eg, Peptococcus, Clostridium etc.
  6. Capnophiles: These organisms require carbondioxide for growth and cannotgrow in presence of oxygen eg, Campylobacter, Helicobacter, Brucella abortus etc.

Refrences:

C.H.Collins. (1995). Microbiological Methods. Oxford Univ Press.

D.Greemwood, R. S. (2002). Medical Microbiology. London: Churchill Livingstone.

M.J.Pelezar. (1993). Microbiology. Tata McGraw hill.

M.T.Madigan, J. a. (1997). Biology of microorganisms. London: Prentice hall International Inc.

R.N.Atlas. (1984). Fundamentals and Applicatins. NewYork: MacMiHiam PUblishing Co.



Nutrion is requird for the faster growth of microorganisms.

Decrease in nutrition directly decreases number of microorganisms.

The temperature a which growth of microoorganisms is favoured is called optimum temperature.

Beyod optimum temperature the growth stops as high temperature favours denaturation of protein that results is death of microorganism.

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