Carbohydrate Fermentation Test

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Voger-Proskaeur test is done to detect acetoin in a bacterial broth culture. A positive result is obtained if the medium change to red colour after Barritt’s solution A (contain alpha-naphthol) and B (contain KOH) is added to the medium. Referred to Figure 4, both bacteria showed a negative result. In contrary, only E. coli should have negative result as it does not produce acetoin to react with alpha-naphthol and KOH. As for B. subtilis, it produce acetoin to react with both solution and produce red colour. We thought, it did not produce red colour because we may not shake it vigorous enough after adding the Barritt’s solution so that the acetoin produced did not able to react. It may also due to low number of bacteria that have been inoculated, so less acetoin is produced to react with the solution.

In addition, in order to find out the ability of an organism to use citrate as the sole source of carbon and energy, the citrate test is conducted. The changes of the medium from green to blue indicates a positive result. This is because when citrate is been used, enzyme citritase will break down citrate to oxaloacetate and acetate. Then, oxaloacetate is broken down to pyruvate and carbon dioxide. This reaction will produce sodium bicarbonate and ammonia as a result from the use of sodium citrate and ammonium salts, caused alkaline in pH. Figure 5 showed that B. subtilis that supposed to have change in colour from green to blue, remain unchanged. In our opinions, the bacteria B. subtilis may need longer time of incubation due to their limited rate of growth in citrate medium.

The final test which is catalase test was done to test whether the organism produce enzyme catalase. This enzyme is very important in protecting the cell from oxidative damage as it breaks down hydrogen peroxide to water and oxygen. The test is positive if there is gas bubbles (oxygen) produce. Based on Figure 6, the presence of gas bubbles can only be seen in E. coli culture plate. This showed that E. coli is aerobes and may be facultative anaerobes too while for B. subtilis no gas bubbles were produced as they are anaerobes which do not respire using oxygen as electron acceptor.

Last but not least, several improvements may be done while doing this experiment in order to get a better result. One of it is we need to make sure all the inoculating process should be done using aseptic technique to prevent any unwanted microbes to contaminate our bacterial culture.

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