3. Factors affecting Bacterial growth
• Moisture : Like all living things, micro-organisms are
sensitive to their environment. Water is necessary
for the growth of microorganisms because they
cannot absorb food unless it is in solution.
• Nutrition: Micro-organisms obtain their
nourishment from organic and inorganic matter for
growth and multiplication. The minimum nutritional
requirements are a source of water, carbon,
nitrogen, and some inorganic salts.
4. • Optimum temperature: Every species of micro-
organism contain certain temperature range for its
growth which is termed as optimum temperature. For
most pathogenic micro-organisms the optimum
temperature is 37 degree centigrade. Most are killed
in 30 mins at 56 degree centigrade.
– Psychrophilic: Bacteria growth at an optimum temperature
at 100c to 200c.
– Mesophilic: Bacteria growth at an optimum temperature at
200c to 400c.
– Thermophilic: Bacteria lives best at 500c to 600c.
5. • Acidity and alkanility: Most pathogenic micro-
organisms grow best in a neutral or slightly
alkaline medium ( pH 7.2 to 7.6). An acid
medium prevents the growth of many micro-
organisms. Eg: preservation of fish, meat in
vinegar.
6. • Gases:
– Oxygen:
• Aerobes: Organisms that grow in the presence of free oxygen.
e.g. – Pseudomonas.
• Facultative aerobes: Organisms that live with or without
oxygen. E.g. – Vibrios, E.coli
• Obligate anaerobes: The microorganisms that grow only in
absence of oxygen. e.g. – Clostridium.
– Carbon dioxide: The metabolic activities of some
organisms are improved in presence of CO2. e.g. –
Neisseria gonorrhoeae.
• Osmotic pressure: Many micro-organisms are
sensitive to a concentrated solution of salt or
sugar because of their high osmotic pressure.
Preservation of food in concentrated salt and
sugar solution is based on this principle.
7. • Light and radiation: Most bacteria grow well
in the dark. Direct sunlight destroys many
bacteria within a few minutes or hours. It is
therefore possible to disinfect articles by
exposing them to sunlight.
8. Bacterial Growth Curve
• When a bacteria put into a suitable liquid
medium and incubated, the growth follows a
definite path. The growth of the bacteria with
time follows a curve on a graph. This curve is
called the bacterial growth curve.
9. Phases of Bacterial growth
1. Lag phase
2. Log phase
3. Stationary phase
4. Phase of decline.
10. • Lag phase: During this phase the bacteria takes time
to adapt to the environment. The necessary enzymes
and other growth factors are made up in proper
quantity for multiplication to continue. The duration
of lag phase depends on the species, nature of
culture media temperature etc.
• Log phase: In this phase the cells start dividing. Their
number increase with time.
11. • Stationary phase: At the period of high growth cell
division comes to a decrease because nutrients are
finished and toxic products are formed. Number of
new cells equal to number of dying cells.
• Phase of decline: Population decreases due to cell
death. The number of bacteria dying is more than
dividing. So the total number of bacteria decreases.
