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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How is IMViC Biochemical test performed ?

How is IMViC Biochemical test performed ?

In IMViC test there are four biochemical tests and this test are carried out individually. IMViC test is carried out to identify members of Enterobacteriaceae family.

Each letter of IMViC test stand for individual test

  1. I – Indole production test.
  2. M – Methyl red test.
  3. V- Voges- Proskaver test.
  4. C – Citrate utilization.
  5. i – Lower case i is used for ease of pronunciation.

We will go through these tests one by one.

Indole production test.


The aim of this test is to determine the capability of a bacterial culture to produce Indole.


  • Bacterial culture
  • Tryptophan broth
  • Kovac’s reagent or Ehrlich’s reagent.
  • Xylene

Composition of media and reagents

1. Tryptophan broth medium

Tryptone         – 1 gm

Distilled water- 100 ml

2. Kovac’s reagent

p-dimethylaminobenzaldehde -15 gms

Isoamly alcohol                           – 150 ml

HCL                                                – 75 ml

3. Ehrlich’s reagent

p-dimethylaminobenzaldehde  – 1 gm

95 % ethanol                                – 95 ml

Concentrated HCL                        -20 ml



  1. Tryptone broth is prepared, distributed in test tube and sterilized by autoclaving.
  2.  The sterile tryptone broth tubes are inoculated with loopful of suspension and incubated at 37° C for 24 hours.
  3. After incubation add 3-4 drops of xylene in tubes and shake vigorously and keep the tubes still so that two layers get separated.
  4. After separation of two layers add 1 ml of Kovac’s reagent or Ehrlich’s reagent and tubes are observed for formation of pink colour ring.


Indole is aromatic heterocyclic organic compound.Indole can be produce by some members of Enterobacteriaceae family by hydrolysis of tryptophan.Indole production is a protein utilization test. Here we are going to use a protein rich that is tryptophan rich broth. This media contains tryptophan as a source of protein. The bacterial cells that are able to produce trytophanase enzyme hydrolyse and deaminate tryptophan into Indole, pyruvate and ammonia. Further these degraded products are treated with Kovac’s reagent or Ehrlich’s reagent in presence of heat. If the degraded products in the tube contain Indole then this Indole reacts with Kovac’s reagent or Ehrlich’s reagent and form pink colour rose Indole complex. In this way Indole production is detected.

[Note – Indole is soluble in organic compound so it is recommended to add xylene/chloroform/ether by adding this substance Indole get extracted from the whole medium and forms a separate layer on the surface of medium. So that Kovac’s reagent or Ehrlich’s reagent can easily react with the indole and form a pink colour ring.

Result – If there is formation of pink colour ring then the Indole production test is positive and if there is no ring formation then the test is negative.

Methyl Red Test {MR test}


To carry out Methyl-red test to determine fermentation capacity of given bacterial culture.


  • Glucose phosphate broth
  • Methyl-red indicator
  • Test culture.

Composition of media

Glucose phosphate broth

  1. Glucose  – 0.5 gms
  2. K2HPO4 –  0.5 gms
  3. Peptone – 0.5 gms
  4. Distill water – 100 ml
  5. pH – 7


  1. Glucose phosphate broth is prepared and distributed in test tubes this test tubes are further sterilized by autoclaving.
  2. The sterile test tubes are inoculated with test culture and incubated at 37 ° C for 24 hours.
  3. After incubation the five drops of methyl-red indicator is added to the medium and the tubes are observed for development of red colour.


As we know we carry out IMViC test for determination of members of Enterobacteriaceae family. The micro-organisms from Enterobacteriaceae family carry out fermentation pathway they metabolise glucose to pyruvic acid  to formic acid and this formic acid formation is called formic acid fermentation process.

In general there are two types of formic acid fermentation pathway and they are:-

  1. Mixed acid fermentation- This type of fermentation is carried out in E.coli, Salmonella and Proteus.In this fermentation ethanol and mixture of various acid like succinic acid, formic acid, acetic acid and lactic acid are formed.
  2. 2.       Butanediol fermentation – This fermentation is carried out in Bacillus, Enterobacteriacea, Serratia  . Here pyruvate is converted to acetyl methyl carbonyl and further from this Butanediol is formed. This fermentation mainly produces neutral products. 

Now when we compare this two fermentations process, mixed acid fermentation produces more acidic products and it acidifies the incubation media. Here only mixed acid fermentation produces sufficient amount of acid which can be detected by methyl red indicator.

As here we use glucose phosphate broth as incubation medium this medium is highly buffered and small amount of acid is produced the media can resist change in pH. So here we use Methyl red the pH range of methyl red is at 4.4 ph it show red colour and where as at 6.2 pH it shows yellow colour. So after incubation the pH indicator methyl red and medium is observed for change in colour of medium.

If acid is produced in medium and after addition of methyl red the medium shows red colour then the Methyl red test is positive.

The Methyl red test is shown positive in case of mixed acid fermentation as complex mixture of acid is produced.


If there a stable red colour is developed in the media after addition of Methyl red indicator then the M-R Test is positive.

Voges-Proskauer (V-P) Test


To carry out Voges-Proskauer test determine the capacity of bacterial culture to carry out fermentation.


  • Glucose phosphate broth
  • 5 % alcoholic alpha-naphthol
  • 40 % KOH solution
  • Test culture


  1. Glucose phosphate broth is prepared and distributed in test tubes these test tubes are further sterilized by autoclaving.
  2. The sterile test tubes are inoculated with test culture and incubated at 37 ° C for 24 hours.
  3. After incubation 0.6 ml of alpha naphthol and 0.2 ml of KOH solution per ml of culture broth media is added.
  4. Further after addition of these two reagents the culture tubes are shaken properly and kept in slanting position to increase aeration. Keep these tubes in slanting position for about one hour and then results are noted down.


As we know there are two types of fermentation carried out by members of Enterobacteriaceae family( Details are mention above in M-R test) In methyl red test we detected the mixed acid fermentation and here in Voges-Proskauer test we are going to determine butanediol fermentation by detecting product acetoin.

This acetoin is a precursor of butanediol and this acetoin is produced during butanediol fermentation. Here in V-P test after fermentation the medium is treated with alpha-naphthol and KOH solution and due to addition of this acetoin is oxidised to diacetyl .Now this diacetyl react with the peptone present in the broth and produce pink colour.

[ Peptone is present in a culture broth and this peptone contains guanidine nucleus of arginine and diacetyl reacts with guanidine nucleus of arginin and gives pink colour to the medium.]

Thus formation of pink colour indicates presence of acetoin and conforms Butanediol fermentation and indicates V-P test as positive.

Citrate Utilization Test


To carry out citrate utilization test to determine the ability of bacterial cell to utilize citrate.


  • Simmons’s citrate agar
  • Test culture.

Composition of Simmons’s citrate agar

  1. Sodium citrate                                    –0.2 gm
  2. Magnesium sulphate                         –0.02 gm
  3. Sodium chloride                                  –0.5 gm
  4. Ammonium dihydrogen phosphate – 0.1 gm
  5. Dipotassium phosphate                      – 0.1 gm
  6. Bromothymol blue                               – 0.008 gm
  7. Agar-agar                                               – 3 gm
  8. Distill water                                            – 100 ml
  9. pH                                                             – 7


  1. The Simmons’s agar is prepared according to the composition given above, sterilized and after sterilization the slants of this media are prepared.
  2. Further this slant are streaked heavily on the slant and incubated for 24 hours at about 37°C
  3. After incubation the slant are observed for change in colour and the results are recorded.


The members of Enterobacteriaceae family have the ability to obtain energy and carbon by utilization of citrate. This is an important characteristic to identify the members of Enterobacteriaceae family. For carrying out citrate utilization test the test culture should be exposed to medium which contains citrate as a sole source of carbon and energy .If the bacterial cells are able to produce enzyme citrate permase can utilize citrate because this enzyme can facilitate transport of citrate into bacteria then a bacterial cell can utilize citrate. For carrying out this test we need citrate agar that contains, ammonium phosphate as a source of nitrogen, Sodium citrate as source of carbon and bromothymol blue as a pH indicator .Bromothymol blue is yellow at acidic pH and blue at alkaline  pH  .

In citrate utilization test we use agar slant because for this test oxygen is required and after oxidation of citrate CO2 is liberated. Now this CO2 reacts with the sodium and water present in the medium and form sodium carbonate and this sodium carbonate is a alkaline products other hand the bacteria utilize ammonium citrate release nitrogen with production of ammonia and convert it to ammonium hydro-oxide and this is the other alkaline product. Now this alkaline products increases the pH of medium to alkaline side and thus colour of the medium changes to blue colour. Hence change in colour of medium to blue indicates positive citrate utilization test.


Formation of a deep blue colour indicates positive citrate utilization test and the test culture has a ability to utilize citrate.





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