The Effects of pH and Ultraviolet Light On Microorganisms & Determination of Oxygen Requirements


To learn what the effects of pH (Bacillus subtilis) and ultraviolet light on the bacterial (E.coli) growth are.
To be able to should learn how determine the oxygen requirement of a particular microorganism (Bacillus subtilis).


Effect of pH

Firstly, prepare Glucose-phenol red broth (pH 3, 5, 7, 9) containing 0.5%w/v glucose+basal medium. Then, under aseptic conditions inoculate 4 GPRB tubes with a microorganism obtained from the stock culture. Finally, incubate at 37°C for 24 hr. Following week check bacterial growth by observing turbidity by comparing turbidity observations of 4 tubes relative to each other

Effect of UV

First of all, agar plates should be labeled as in the figure on the bottom before inoculation as shown below:
Study under aseptic conditions inoculate 0.1 ml of culture and open a plate and expose two halves to UV for indicated time periods by shielding the other half with a card. Then, incubate at 37°C for 24 hr. Following week, check bacterial growth by counting colonies via comparing colony numbers of the parts exposed to UV for different time periods and comparing you results with other results of other groups

Oxygen requirements

Study under aseptic conditions, inoculate thioglycollate medium by the culture (B.subtilis). Then, rotate the tube between your palms gently to get good dispersion of inoculation. Continue with incubation at 37°C for 24 hr. Following week, check bacterial growth and decide oxygen requirement

Results & Observations

Table: pH Effect (Bacillus subtilis)

pH 3
No growth
pH 5
pH 7
pH 9
+++(light color change, yellow become darker)

Effect of UV
-Lawn formation due to lack of UV effect in all plates

Oxygen requirements
-No growth is observed, no color change occurred


In this experiment our purpose was that to investigate the effects of irradiation with ultraviolet light on the growth of the bacteria (E.coli), effects of pH of the medium to growth of Bacillus subtilis, and lastly we tried to determine experimentally the oxygen requirements of it.

The results are summarized in the results part. As you can see, Bacillus subtilis growth the best at alkaline conditions especially at pH=9. This indicates that the microorganism is alkaliophile but it also can grow at neutral pH and pH=5 meaning that it can behave like neutrophile and to some extent acidophile. Theoretically it is known that Bacillus spp are classiied as some of them as acidophile and alkaliphile. But we know that B.subtilis is an alkaliphile but not obligatory as we see from our experiment. However, to confirm our results, we should have done some further inoculations into tubes with more refined pH values.

In the following study, we tried to observe the lethal effect of ultraviolet light on the growth of E.coli. However, we could not obtain any results due to not working UV lamb or ineffective UV lamb. Therefore, we obtained lawn on the plate in all plates.

Finally, we tried to determine the oxygen requirements of B. subtilis. We know that it is a aerobic bacteria, therefore, it requires oxygen to live. Thus we expected no change in the color of thioglycollate medium and no growth as we could observe in our experiment.

To sum up, except UV exercise, all of our studies resulted as expected. However, UV experiments should be done with new UV lamb or UV apparatus to prove the effect of UV on the growth of organisms because as scientist candidate I expect to see theoretical events in experiment results to be satisfied.


Information retrieved December 21, 2005 from 

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