Objective:
- To find the ability of
microorganisms to ferment the given Carbohydrate.
- To determine the ability of
microorganism to produce gaseous end products in fermentation.
Principle:
A metabolic process performed by almost all types of bacteria is known as
fermentation. This will result in the production of ATP, the ultimate energy
source of the organism. This will happen either in the presence or absence of
atmospheric oxygen. Bacteria utilize the nutrients in their environment to
produce ATP for their biological processes such as growth and reproduction. The
enzyme systems in bacteria allow them to oxidize environmental nutrient
sources. Bacteria will use different energy sources in the medium depends on
the specific enzymes of each bacteria. Many bacteria possess the enzymes system
required for the oxidation and utilization of the simple sugar, glucose. Some
bacteria have the ability to degrade complex carbohydrates like lactose,
sucrose or even polysaccharides. Such bacterium should possess the enzymes that
should cleave the glycosidic bonds between the sugar units and the resulting
simple carbohydrate can be transported into the cell. Lactose is a disaccharide
consisting of the glucose and galactose connected by glycosidic bond. The
bacteria which produce the enzyme lactase will break this bond and thus release
free glucose that can be easily utilized by the organism. The characteristics
feature of the enzyme production in the bacteria enables them to use diverse
carbohydrates and this will aid in the identification of unknown bacteria.
Phenol
red broth is a general purpose fermentation media comprising of trypticase,
sodium chloride, phenol red and a carbohydrate. The trypticase provides amino
acids, vitamins, minerals and other nitrogenous substances making it a
nutritious medium for a variety of organisms. Sodium chloride helps in
maintaining the osmotic balance and provides the essential electrolytes for the
transport into the cell while the carbohydrate acts as the energy source. The
phenol red is the pH indicator and is initially neutral (pH 7). It supports the
growth of most organisms whether they are able to ferment sugar or not.
When the bacterium is inoculated into the tube, the bacterium which ferments
the sugar will result in the production of acid that will change the color of
phenol red. Fermentation reactions often begin with glycolysis. Glucose acts as
an electron donor in the fermentation reaction, pyruvate, and metabolic product
of glucose act as an electron acceptor. The other disaccharides and
polysaccharides are hydrolyzed into glucose or converted into glucose and then
the fermentation reaction will occur. Finally the reaction will result in the
end products such as acid, ethanol, Hydrogen and Carbon dioxide and other
compounds. This depends on the species of bacteria. Phenol red broth is a test
is differential for gram negative bacteria.
When the organism ferments carbohydrates, acidic organic by products (Lactic
acid, formic acid or acetic acid) is accumulated which turns the medium into
yellow color with reduction in the pH (acidic). The inverted Durham tubes will
detect the presence of gas. The degradation of peptones in the broth may result
in the production of alkaline end products, which will change the broth color
to pink often at the top of the tube.
Materials Required:
- Phenol Red
Carbohydrate Fermentation Broth(Phenol red dextrose broth, Phenol red
sucrose broth, Phenol red glucose b).
- Bacterial
culture (Staphylococcus aureus,
Escherichia coli and Alcaligenes faecalis).
- Inoculation
loop.
- Incubator(370
C).
- Boiling
test tubes.
- Durham’s
tubes.
Procedure:
I.
Preparation of Carbohydrate Fermentation Broth
- Prepare
desired phenol red carbohydrate broth (Trypticase/peptone:
10 g/L, Carbohydrate;Dextrose, Sucrose, lactose : 5 g/L, Sodium Chloride: 5
g/L, Phenol red : 0.0189 g/L) in a conical flash.
- Transfer
40-50 ml of desired phenol red carbohydrate broth(s) to boiling test tubes
and label these with respect to the sugar added.
- Insert
inverted Durham tubes into all tubes, the Durham tubes should be fully
filled with broth.
- Autoclave
at 1150 C for 15 minutes.
Important: Do not overheat the Phenol red
Carbohydrate fermentation broth. The overheating will result in breaking down
of the molecules and form compounds with a characteristic color and flavour. The
process is known as caramelisation of sugar (the browning of sugar).
II. Inoculation of Bacterial Culture into the Phenol Red Carbohydrate Broth
- Aseptically
inoculate each labeled carbohydrate broth with bacterial culture(s) in
laminar air flow hood.(keep uninoculated tubes as control tubes).
- Incubate
the tubes at 24 hours at 37oC.
- Observe the
changes in colour of the medium and accumulation of gas in the durham’s
tube with respect to the control fermentation tube.
Precautions:
- During
aseptic inoculation of bacterial culture, never shake the inoculation loop
in the sugar tubes in order to eliminate the chance of gas entrapment in
the durham’s tube
- After
inoculation into a particular sugar, sterilize the loop in order to avoid
cross contamination of the tube with other sugars.
- Keep
uninoculated sugar tubes as control tubes.
- Do not use
the tubes with Durham tubes that are partially filled or with bubbles.
- Over
incubation will help the bacteria to degrade proteins and will result give
false positive results.
Expected
Results:
- Acid production: Changes
the medium into yellow color- organism ferments the given carbohydrate and
produce organic acids there by reducing the ph of the medium into acidic.
- Acid and Gas production: Changes
the medium into yellow color-organism ferments the given Carbohydrate and
produce organic acids and gas. Gas production can be etected by the
presence of small bubbles in the inverted durham tubes.
- Absence of fermentation: The broth
retains the red color. The organism cannot utilize the carbohydrate but
the organism continues to grow in the medium using other energy sources in
the medium.
BACTERIAL
SPECIES
|
CARBOHYDRATES
|
GLUCOSE
|
SUCROSE
|
LACTOSE
|
Escherichia coli
|
AG
|
A
|
AG
|
Staphylococcus aureus
|
A
|
A
|
A
|
Alcaligenes aureus
|
|
|
|
Control
|
|
|
|
NOTE:
A: Acid only
i.e Sugar broth turns yellow.
AG: Acid and gas
production i.e sugar broth turns yellow and gas/air bubble accumulation in
the Durham’s tube.
: No change
:
Variation reaction
|