Aim
To study the effect of pH on growth
of bacteria
Principle
pH is a scale used
to specify how acidic or basic a water-based solution is. Acidic solutions have
a lower pH, while basic solutions have a higher pH. It is the logarithm of the
reciprocal of hydrogen ion concentration.
It is expressed as a value from 0-14.
pH change in the environment has a profound influence on growth and
survival of microorganisms. Enzyme
system of organisms has a particular pH range at which it can function at
optimum rate. So each organism has a maximum pH (alkaline) and a minimum pH
(acidic) above/below which it cannot grow and an optimum pH at which it can
grow at maximum growth rate.
Most bacteria are
neutrophiles, grow optimally at a pH 7 ± 1- 2 units. Most
familiar bacteria, like Escherichia coli,
Staphylococci, and Salmonella spp. are neutrophiles. Microorganisms
that grow optimally at pH less than 5.5 are called acidophiles. For example,
the sulfur-oxidizing Sulfolobus spp. Alkaliphiles
are microorganisms that grow best at pH between 8.0 and 10.5. Vibrio cholera is an example.
Many
bacteria produce organic acids from carbohydrate metabolism and alkalis from
protein breakdown and these cause pH change of media resulting in lower rate of
growth. Buffers are frequently added to
culture media to neutralize these acid or alkali shift. Peptones, proteins, and amino acids act as
natural buffers in media due to their amphoteric nature. Phosphate salts are
used in chemically defined media as a buffer.
Materials required
1. Culture
of E coli and Bacillus sp
2. Sterile
Nutrient broth tubes having pH adjusted to 3, 5, 7, and 9
3. Equipments
- bunsen burner, inoculating needle, Incubator
Procedure
1. Each
nutrient broth tube was labelled with pH and organism to be inoculated.
2. The
tubes were inoculated with E coli and
Bacillus sp.
3. Tubes
were incubated at 370C for 24 hours.
4. The
tubes were observed and recorded the optical density at 600 nm.
Observation and result
The
result was tabulated. The bacterial species were found to have different growth
patterns at different pH. The optimum pH
for growth was found to be 7. There was no growth at pH 3 for E coli and Bacillus sp.
Effect
of pH on bacterial growth (left hand side)
|
Organism |
OD
at 600 nm |
|||
|
3 |
5 |
7 |
9 |
|
|
E coli |
|
|
0.36 |
|
|
Bacillus
sp |
|
|
|
|
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