Yogurt

Other Niches Affecting Microbes in Yogurt

Influence by Adjacent Communities

The yogurt niche and its environment overlap its similarity between some of the other dairy niches, specifically being the milk niche which is the root of all dairy niches. The making of yogurt is first through the transition of the milk niche. Firstly, yogurt is made from fermented milk. Milk is rich in sugars, more specifically the sugar being lactose. An environment rich in sugars is an environment that microbes love to thrive in; thus, milk is a great feast for microbes. The following are a list of microbes that thrive in milk. Of all the microbes that live in milk, the Bacillus family and the Streptococcus family is the one that overlaps into the yogurt niche. However, there are only two particular microbes that feast in yogurt which are Lactobacillus bulgaricus and Streptococcus thermophilus. So, as we can see there is a similarity between the microbes that thrive in milk to the ones that thrive in yogurt.

Related Microbes in Adjacent Communities

1. Streptococcus lactis

    Purpose: Souring	
    Method: Lactose-lactic acid precipitation

2. Streptococcus bulgaricus

    Purpose: Souring	
    Method: Lactose-lactic acid precipitation

3. Lactobacillus casei

    Purpose: Cheese ripening	
    Method: Controls altermentation.

4. E coil

    Purpose: Souring & gassiness	
    Method: Lactic acid & gases and affects cheese ripening.

5. Bacillus substallis

    Purpose: Protecolysis	
    Method: flavors change.

6. Alkaligenes viscus

    Purpose: Ropiness	
    Method: Ropi milk

7. S Streptococcus liquifiecence

    Purpose: Bitter Flavour	
    Method:Bitter flavour to cream &butter.

8. Bacillus substallis

    Purpose: Sweet curdling	
    Method: Curd formation

9. Streptococcus paracitrovorus

    Purpose: Attacks citric acid	
    Method: Flavors curd.

Conditions under which the environment changes

In the process of using the lactose sugar from the milk, Lactobacillus produces acid which makes the yogurt sour and a less suitable place for other microbes. This is why there is a dramatic decrease in the amount of microbes that live in the yogurt niche when compared to the milk niche. Thus the essential conversion between milk to yogurt is the acidic levels. The increase in acidic levels is the sourness that is tasted in yogurt which lacks in milk, another characteristic difference. Lactic acid also known as lactate is not good for bacteria. So, as a response to this change in environment, they excrete lactate into their environment. This again is what causes the pH to fall to become more acidic. Another affect of excreting lactate is that the protein molecules in the milk become denatured. What this means is that the protiens unfold from their normal structures and become disordered. After becoming distroted, the protein molecules begin to stick to each other forming a semi-solid matrix. Thus, this is what gives the yogurt a semi-solid state, another characteristic different from the liquid-milk.

Microbe Metabolism, Ecology

Lactobacillus

Lactobacillus is found to be living in highly acidic environments of pH 4-5 or lower, thereby altering the pH and suppressing pathogens by producing lactic acid. Under the optimal temperature of 37°C, it derives the energy, such as ATP, by converting the glucose to lactic acid through homolactic fermentation. Nevertheless, it is unable to breakdown complex sugars, like ribose, under the optimal temperature. In addition, Lactobacillus secretes nonbacteriocin antibacterial substances. In humans, Lactobacillus is found in the gut and vagina. In the vagina, it plays an important role by keeping the pH low to deter infection.

Streptococcus thermophilus

Under the optimal temperature of 42°C, Streptococcus thermophilus, can generate ATP through fermentation. In contrast to Lactobacillus, it is also able to produce ATP through aerobic respiration in the presence of oxygen. Through fermentation, it converts lactose to lactic acid at the optimal pH of 4.6. In humans, Streptococcus thermophilus is found in the upper part of the intestine and can help people with lactase-deficiency to digest lactose due to the low level of lactase they produce.

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by [Chung Abbott, Ibukun Osindele, Anusha Sridharan, Jerry Wang], students of Rachel Larsen