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Lactic Acid Bacteria


Bacteria performing lactic acid fermentation convert pyruvate to lactic acid with the help of the lactate dehydrogenase enzyme. This process requires specific conditions of temperature and pH. Two species, which can be present in yogurt and perform lactic acid fermentation, include Streptococcus thermophilus and Lactobacillus bulgaricus. The experiment tested for the presence of these two bacterial species in yogurt and identified the specific conditions required for the existence of these organisms. The results indicated that pH optima for Streptococcus thermophilus and Lactobacillus bulgaricus were 4.4 and 6.8 respectively. Bacteria varied in their shape, which was bead-like for Streptococcus and filamentous for Lactobacillus. The results indicated that various sugars are utilized in the fermentation process, as well as identified the optimal conditions for lactic acid fermentation.
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Streptococcus thermophilus and Lactobacillus bulgaricus Identification in Yoghurt


Lactic acid fermentation occurs under anaerobic conditions after glycolysis has provided two pyruvate molecules. These end products of glycolysis can either be converted to lactic acid in case of anaerobic fermentation or enter the Krebs’s cycle and undergo oxidative phosphorylation. Several different organisms are can perform lactic acid fermentation under anaerobic conditions, and they include yeast, lactobacillus species, and others. These species have been exploited for the manufacture of several different products, including yogurt. Two organisms contributing to the production of yogurt include Streptococcus thermophilus and Lactobacillus bulgaricus. They play a critical role in determining taste, maintaining pH, and improving the quality of yogurt. These organisms have been exploited for the manufacture of other products as well, and their use has been widely accepted. Streptococcus thermophilus and Lactobacillus bulgaricus survive under specific conditions of pH and temperature (Curic-Bawden, Junge, Janzen, & Johansen, 2016a, 2016b). Extreme temperatures may inhibit the growth and activity of these species. This experiment tests for the specific temperature and pH, which are optimum for Streptococcus thermophilus and Lactobacillus bulgaricus from a yogurt sample.

Materials and Methods

The methods used were adapted from Brosseau-Demore and Mallory (2014).

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Before the incubation step, the pH value of the contents in the first tube was 5.8. The solution was colorless. After the addition of water in the tube, the state did not change and remained liquid. Nonetheless, the pH value increased from 5.8 to 6.1 during cultivation. After incubation was carried out, the texture of the products in the first tube changed to hard and solid. The pH value also changed to 4.4. The consistency of the sample within the second tube changed from clumpy to solid. In addition, the pH decreased from 5.0 to 4.0. The changes in the state of the tubes’ contents indicate the effect of different conditions on pH and temperature. Bacteria present in the first tube are more likely to be Streptococcus thermophiles, while the second one is more likely to contain Lactobacillus bulgaricus.

The bacteria indicated on the left are forming a bead-like structure and therefore, are more likely to be Streptococcus thermophilus. The other bacteria are filamentous and thus, can be identified as Lactobacillus bulgaricus.


Streptococcus thermophiles is an example of Gram-positive bacteria, that utilize lactic acid fermentation for the obtainment of energy. It has a bead-shaped morphology. The organism survives under extreme conditions of temperature and thus, is classified as a thermophile (Elsanhoty, Salam, Ramadan, & Badr, 2014). Lactobacillus bulgaricus is classified as a Gram-positive bacterium with filamentous morphology. The organism is immotile due to the absence of the flagellum. Lactobacillus bulgaricus does not form any spores and survives under acidic conditions. Bacteria are classified based on several dimensions, including the presence of specific molecules in their outer cell layer, optimal conditions of pH and temperature, as well as metabolism, which is the ability to carry out specific processes, such as lactic acid fermentation. In the studied case, both bacterial species undergo lactic acid fermentation. This process produces specific energy conversion molecules and is facilitated by the enzyme lactate dehydrogenase. This enzyme makes it possible for lactic acid to be produced.

The different heating and cooling temperatures used in the current experiment have been used to reduce contamination, as well as provide optimum conditions for the functioning of specific bacteria (Xu et al., 2015). For example, the temperature of 88(C was used to provide adequate conditions for thermophiles and remove the other organisms, that cannot survive under high temperatures. The value of 45(C is used to provide optimum conditions for the bacteria Lactobacillus bulgaricus. Cooling allows for the appropriate incubation steps to be undertaken. Bacteria are ubiquitous in nature, meaning that they are more likely to be found everywhere in the environment. However, their survival is dependent on specific temperatures and pH conditions. Extreme heating, as shown by the described case, is more likely to kill other bacteria present in the environment and which are not required for the occurrence of fermentation. Therefore, only bacteria of interest remained within a sample after heating, and they could be detected through wet staining.

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The main primary sugar, which is used in lactic acid fermentation, is glucose. It undergoes glycolysis to provide two molecules of pyruvate, which are then, converted to lactic acid under the presence of lactose dehydrogenase that is identified in lactic acid fermenting bacteria. NADH and NAD+ are interchanged during the entire conversion of pyruvate to lactic acid and the latter goes back to refuel the glycolytic pathway (Settachaimongkon et al., 2014). Extra amounts of glucose would result in an increased rate of pyruvate production, ultimately resulting in a higher concentration of lactic acid. These sugars were largely present in the milk in yogurt. Galactose, which is considered the major sugar in milk, has to be converted to glucose before it can be used in any metabolic process. In the absence of glucose, the formation of pyruvate is likely to be inhibited, since glycolysis will not occur. Therefore, glucose can be considered as the precursor molecule that gives rise to pyruvate, which is then converted to lactate by lactate dehydrogenase. Bacteria survive under specific conditions of pH and temperature. Lactobacillus bulgaricus is considered a bacterium that survives under acidic conditions, meaning it is largely acidophilic. The obtained pH value of 4.4 value indicates the likelihood of this bacterium is Lactobacillus bulgaricus. Streptococcus thermophiles survive under slightly acidic conditions and therefore, which is consistent with the identified pH value of 6.5. The two bacteria have different morphologies and structures. Lactobacillus bulgaricus is a filamentous bacterium, while Streptococcus thermophiles form bead-like structures. In addition, different textures can be attributed to their morphologies, and they change based on optimal conditions of temperature and pH.


Unique differences exist between Lactobacillus bulgaricus and Streptococcus thermophiles with respect to the optimum conditions of pH and temperature. These bacteria allow for the preservation of various unique nutrients in yogurt. The presence of these bacteria improves the taste properties of yogurt, as well as preserves all the other unique attributes. Various bacterial organisms utilize lactic acid fermentation as their major pathway converting sugars present into lactic acid. The final conversion largely involves changing pyruvate to lactic acid with the help of the enzyme lactate dehydrogenase. The process has to be regulated to ensure that there is no release of any product that could affect the conversion of pyruvate to lactic acid.

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