Introduction to Industrial Microbiology

Industrial microbiology is a branch of science that deals with the study and uses of micro-organism which have industrial and economic applications. In industrial microbiology, microbes classified, characterized are isolated from their natural environment. Further, these micro-organism are produced on a large-scale by use of a fermentor.

Fermenting tanks with yeast being used to brew beer. Photo by Kafziel

Modern Fermenting Tanks
Photo by Kafziel

The most important applications of industrial microbiology are using microbes to convert inexpensive raw materials into economically important products. Further, these products go through detection, analysis, assay, purification and recovery to make them safe and effective. The end products are valuable, but the microbes creating these products are equally important.

During the process of industrial fermentation, microbial cells produce extra-cellular as well as intra-cellular enzymes. These enzymes play an important role in fermentation, degradation and utilization of media components. Enzymes can be used as a product by itself and have many end use applications. For example, enzymes like amylase and proteases are used in a variety of products.
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Growth of a Bacterial Cell

Growth of a Bacterial Cell

It can be defined as orderly increase in  quantity of all cell components of a cell.The cell increases in its size,shape and weight and further undergoes cell division.The bacterial cell undergoes different growth phases.This growth phases of bacterial cell can be studied by plotting a growth curve.

As we know studying growth pattern of an individual bacterial cell is impossible due to its small size so here we use population of a bacterial cell for determining the growth curve. The population of a bacterial cell is obtained by innoculating  a small number of innoculum from old culture to a suitable  fresh culture media and growth of a cell is monitored at regular interval of time.The growth curve is plotted by using logarithm of cell number against time.

A bacterial cell undergoes four different growth phases and they are as follows:-

  1. Lag phase
  2. Log phase
  3. Stationary phase
  4. Death phase

Graphical representation of a growth phases of bacterial cell.

Growth Phases of Bacterial population

Growth Phases of Bacterial population

Where,On X axis L is equal to logarithm of number of cell.

On Y Axis T is equal to time in hours

Growth phases of a bacterial cell

1.Lag phase

Initially when we innoculate a old bacterial cell to a new fresh medium the bacterial cell does not carry out immediate cell division  and the inoculated population remains steady.As the bacterial cell get exposed to a new fresh medium these cells try to adjust in the new environment like present nutrients,pH,temperature as well as on other hand this cell synthesize amino-acids,protein,RNA,enzyme and intermediate metabolic products which are further necessary for growth and cell division .

The time of lag phase may vary from species to species due to various environmental factor.At the end of lag phase the bacterial cells starts to multiply and cells enter in logarithmic phase of growth.

2. Logarithmic phase

This log phase is also called as exponential growth phase.In this growth phase of bacterial all cell multiply and carry out cell division.Cell division in this growth phase is constant and maximum.Here the number of cells increases exponentially with time.In this growth phase all bacterial cells are small in size and biologically active as they are constantly dividing so in most of the biochemical and biological studies bacterial cells from this exponential growth phase are used.

If we plot a graph of logarithm of cell number against time on X axis it result in a straight line with positive slope.The multiplication rate and generation time of bacterial cell varies from species to species under different environmental condition.At the end of log phase the growth rate decrease that is generation time of cell increases .After the end of log phase the bacterial cell enters in stationary phase.

3.Stationary phase

As the cells enter in stationary phase the number of cells remain constant because generation time of cell is very low.It occurs due to various factors like depletion of nutrients, accumulation of toxic waste products,change in pH etc. Due to all this factors in this phase the growth rate is equal to death rate and the total number of cells till the end of stationary phase remains constant.

If we plot a graph of log of cell number against time a straight line parallel to X axis is formed with the end of this phase the death phase starts.

4. Death phase

In death phase of growth the bacterial cells starts to die.Here death rate is higher than the growth rate and the death rate increases to maximum at the end of death phase. There are various factors responsible for this phase like exhaustion of nutrients,accumulation of toxic waste products.drastic changes in pH etc.

If we plot a graph of log of number of cells against time on X axis a straight line with negative slope will be observed.Here with the death phase the bacterial growth curve end.

These are the growth phases of a bacterial cell.

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Nitrate Reduction Test

Nitrate is a poly-atomic inorganic molecule and its molecular formula is NO3–  

Nitrate is reduced by micro-organism in two ways.

1. Assimilatory Nitrate reduction.

Assimilatory nitrate reduction mainly occurs during aerobic condition in absence of nitrogen.In this reduction process nitrate and ammonia are used for synthesis of organic nitro-compounds and amino-acids.In assimilatory nitrate reduction the nitrate do not play any role in energy production.

2. Dissimilatory nitrate reduction.

Dissimilatory nitrate reduction is carried out under anaerobic condition where bacteria use nitrate as a electron acceptor.

These assimilatory and dissimilatory nitrate reduction process is carried out nitrate reductase enzyme.This nitrate reductase enzyme converts nitrate to nitrite .The reaction is as follows

NO3 + NADPH + H+       Nitrate reductase           NO2–  + NADP+ + H2O           

Further this nitrite is converted to ammonia by nitrite reductase and the ammonia formed is assimilated.The nitrate reductase test is performed by using two indicators and the reduction of nitrate to nitrite is detected by using two indicators and that are sulphanilic acid and α- napthylamine.


Aim of the experiment is to carry out nitrate reduction test .


  1. Peptone nitrate broth
  2. Test culture.
  3. Reagent A-Sulphanilic acid.
  4. Reagent B- α- napthylamine.
  5. Zinc dust.

Composition of Peptone nitrate broth

  • Peptone            -0.5 gm
  • KNO3                 -0.2 gm
  • Distill water    -100 ml
  • pH                       -7


In nature many micro-organism posses nitrate reductase enzyme  when this micro-organisms are exposed to nitrate as a sole source of energy have a ability to reduce nitrate to nitrite.Here in nitrate reductase test we expose these micro-organism to a media containing nitrate as a sole source of nitrogen and energy and further this nitrite is detected by using two reagents Sulphanilic acid and α- napthylamine.


  1.  Take sterile peptone nitrate broth and  innoculated it with loopful of  test culture  and this innoculated broth is incubated for about 24 hours at about 37 ° C .
  2. After incubation 0.5 ml of reagent A that is  Sulphanilic acid is added and then reagent B α- napthylamine is added observe the test tube for development of red colour.
  3. Further if no red colour develops a pinch of zinc dust is added and mix properly and test tube is observed for development of red colour.

Flow chart of Nitrate reduction test procedure


Reagent A is Sulphanilic acid and Reagent B alpha napthylamine

Reagent A is Sulphanilic acid and Reagent B alpha napthylamine



  • If development of red colour is observed after addition of reagents then it indicates positive nitrate reduction test.
  • If after addition of both reagents there is no red colour formation then it may indicate negative nitrate reduction test or false result  because there is a possibility that the microflora present in the tube may reduce the nitrate to other byproducts like ammonia,molecular nitrogen,nitrous oxide,nitric oxide and as the test reagents detects only nitrates so it can give a false negative result so to clear this doubt a pinch of zinc dust is added and tube is observed for red colour development .Here zinc ions reduce nitrates to nitrites and we can observe development of red colour as zinc ions confirms presence of residual nitrates and this development of red colour indicates negative nitrate reduction test.

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Catalase Biochemical Test

Catalase Biochemical Test


Catalase is a name of a enzyme.This enzyme is nearly present in all living plants and animals which are always exposed to oxygen.Catalase enzyme is also present in aerobic and facultative anaerobic micro-organisms.Generally this micro-organisms utilise oxygen and produce hydrogen peroxide and this hydrogen peroxide is toxic to the cells and it effects the cells enzyme system so to avoid this toxic effect micro-organism produces catalase enzyme. The role of catalase enzyme is to convert hydrogen peroxide molecule to water and oxygen and this reaction is a irreversible reaction.

2H2O2   is converted to     2H2O + O2

It is also one of the reason that anaerobic micro-organisms do not survive in presence of oxygen as hydrogen peroxide is produced and due to lack of catalase enzyme this hydrogen peroxide destroy cells enzyme system and the cell dies.

Why is it important to carry out Catalase test ?

The purpose of carrying out a catalase test is to detect production of catalase enzyme by bacteria. Secondly catalase test help us to determine whether the micro-organisms are aerobic,anaerobic or obligate anaerobic in nature and due to this we can differentiate the organisms in catalase positive or catalase negative group. This differentiation helps us in identification and classification of bacteria.

How is the Catalase test performed ?

The Catalase test can be performed by using two methods.

  1.  Test tube method.
  2. Slide method.

1.Test tube method.

In test tube method the catalase test is performed in sterile test tube.


The aim of the experiment is to carry out catalase test by tube method.


  1. Bacterial culture
  2. Sterile test tube.
  3. Dropper.
  4. Wireloop.
  5. Hydrogen peroxide.


  1. A sterile test tube is taken and with the help of a dropper 5 to 6 drops of hydrogen peroxide are added in the all this procedure is carried out under sterile conditions.
  2. Further with the help of a sterile nichrome wireloop a colony of a test culture is picked up and inserted in hydrogen peroxide containing test tube.
  3. The test tube is observed for production of effervescence that is bubble formation this reaction occurs immediately after addition of culture.


 Positive catalase-test

Positive catalase-test


Negative catalase test


If effervescence is observed, test culture is catalase positive and if their are no effervescence test culture shows negative catalase test.

2. Slide method


The aim of the experiment is to carry out Catalase test by Slide method.


  1. Bacterial culture.
  2. Clean grease free slide.
  3. Wireloop
  4. Hydrogen peroxide.


  1. Take a clean grease free slide and on it take a drop of sterile water and with the help of a sterile nichrome wireloop pick a colony of a test culture.
  2. Further with the with the help of a dropper add 3 to 4 drops of hydrogen peroxide all this procedure should be carried in sterile condition.
  3. After addition of hydrogen peroxide the slide is observed for formation of effervescence that is bubbles.


If effervescence are observed within 20 second then catalase test is positive and if effervescence are not observed then catalase test is negative.

List of some Catalase positive micro-organism.

  • Pseudomonas aeruginos
  • Mycobacterium tubercule
  • Nisseria gonorrhea
  • Helicobacter pylori
  • Candida
  • Staphylococcus aureus
  • E.coli


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