2. vds
Bacterial Growth
Bacteria divide by the process of Binary
Fission;
It feeds on the nutrients & metabolise;
When it reaches its maximum size, it divides to
form two daughter cells;
Nuclear division precedes cell division –
ingrowth of division septum – “Pinching”
Process – may or may not separate from the
Parent cells.
2
3. vds
Bacterial Growth
Generation Time: The interval of time between
two cell divisions, or the time required by a
bacterium to give rise to two daughter cells under
optimum conditions.
Generation time of medically imp. bacteria:
Coliform bacilli & alike – 20 min;
Tubercle bacilli & alike – 20 hrs;
Lepra bacilli - 20 days.
A single bacterium theoretically, may give rise to
1021 progeny in 24 hrs.
Practically, in a liquid medium, growth is arrested
due to depletion of nutrients & accumulation of
toxic metabolites;
In the host tissue, it has to contend with the
defence mechanisms of the body.
3
4. vds
Bacterial Growth
On solid medium, bacteria grow in the form of
Colonies;
“A Colony represents a clone of cells derived from
a single parent cell.”
An isolated colony represents pure growth of a
single bacterium;
Such numerous colonies developed from a single
type of bacteria is called Pure Culture.
4
5. vds
Bacterial Growth Curve
When a bacterium is seeded in a suitable liquid
medium & incubated, its growth follows a definite
course.
If bacterial count is made at regular intervals & a
graph is plotted in relation to time, a growth curve
is obtained (Bacterial Growth Curve).
The curve shows the following phases:
Lag Phase,
Log Phase
Stationary Phase
Decline Phase
5
Lag
Phase
Log
Phase
Stationary
Phase
Decline
Phase
Total
Count
Time
Number
of
Cells
6. vds
Bacterial Nutrition
Water – Principle constituent of living cells,
including bacterial cell;
Accounts for 80 % of the total wt. of cell – rest 20
% comprises of Proteins, Polysaccharides, Lipids,
Nucleic acids, Mucopeptides & low mol. wt.
compounds.
For bacterial growth & multiplication the
minimum nutritional requirements are –
Water, a source of Carbon, Nitrogen, energy &
inorganic salts;
Water acts as the vehicle for intake of nutrients &
outlet of all waste products; participates in
metabolic reactions & forms the integral part of
bacterial Protoplasm.
6
7. vds
Bacterial Nutrition
Contd……
Energy requirement & ability to synthesise
essential metabolites:
1. Phototrophs – derive energy from Sunlight;
2. Chemotrophs – obtain energy from
chemical reactions;
3. Autotrophs – synthesise all their organic
compounds;
(utilise atm. CO2 & N2 ; exist in soil – Agri.
Importance)
4. Heterotrophs – depend on preformed
organic compounds.
(some use only G, while others require Amino acids,
lipids, CHO & co-enzymes.)
7
8. vds
Bacterial Nutrition
Contd……
Oxygen requirement & metabolism:
1. Aerobes – Obligate aerobes – strictly require O2 ,
Facultative aerobes – are aerobes but
can grow in the absence of O2 ,
2. Anaerobes – Obligate anaerobes – strictly grow
in the absence of O2 ,
Microaerophilic – grow in the presence
of trace amm. of O2 ,
Carbon dioxide requirement:
Capnophilic – all bacteria require small amm. of
CO2 for growth but some require much higher
levels (5-10 %) – these bacteria are called
Capnophilic bacteria.
8
9. vds
Bacterial Nutrition
Contd……
Temperature requirement:
1. Psychrophilic – grow below 20 °C;
(some may even grow at -7 °C)
2. Mesophilic – grow between 21-40 °C;
(most of the medically imp. mos. are Mesophilic)
3. Thermophilic – grow above 41 °C;
Optimum Temp – Temp. at which growth occurs Best,
Temp. range – growth does not occur above the
maximum & below the minimum temp.
9
10. vds
Bacterial Nutrition
Contd……
Temperature requirement:
Bacterial Spores are Thermoresistant;
Most heat resistant spore, produced by
bacteria,
Bacillus stearothermophilus,
destroyed at 121.5 °C;
by Autoclaving,
Used as an Indicator of successful Sterilisation.
Extremely thermophilic bacteria grow at 250 °C;
10
11. vds
Bacterial Nutrition
Contd……
Moisture & Drying:
Water is an essential ingredient of bacterial
protoplasm, hence drying is lethal to bacterial
cell.
Very delicate bacteria, like Treponema pallidum, -
highly sensitive;
Others like Staphylococci withstand drying for
months;
Spores resist desiccation & survive in dry state for
decades;
Drying in vacuum in the cold, Freeze Drying or
Lyophilisation, is a method of preservation of
bacteria, viruses etc.
11
12. vds
Bacterial Nutrition
Contd……
H-ion (pH) Concentration:
Bacteria are very sensitive to pH variation;
pH range – above or below which growth does not
occur;
Optimum pH – at which growth occurs Best;
Acidophilic bacteria – ex. Lactobacillus & Brucella,
grow in Acidic pH;
Basophilic bacteria – ex. Vibrio cholerae, grow in
Alkaline pH;
Most pathogenic mos. grow at slightly alkaline pH
(7.2-7.6) ;
12
13. vds
Bacterial Nutrition
Contd……
Light:
Bacteria grow well in the dark;
They are sensitive to UV light & other radiations &
Sunlight;
Photochromogens – bacteria that produce
pigment in the light;
Scotochromogens - bacteria that produce
pigment in the dark;
13
14. vds
Bacterial Nutrition
Contd……
Osmotic Effect:
Flow of liquid from lower conc. to higher conc.
Bacteria are more tolerant to osmotic variations due
to rigid cell wall.
But sudden exposure to hypertonic solution – leads to
Plasmolysis; (Ghost Cell) ; Usually occurs in GNB;
Sudden transfer from conc. solution to distilled water
– leads to Plasmoptysis; (Protoplast or
Spheroplast).
14
15. vds
Bacterial Nutrition
Contd……
Mechanical & Sonic Stress:
Bacterial cell ruptures by mechanical stress such
as grinding or vigorous shaking with glass
beads.
They may also be disintegrated by exposure to
ultrasonic vibrations.
15
