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A Microbial Biorealm page on the genus Yogurt


Higher order taxa:

1. Lactobacillus:

        Bacteria; Firmicutes; Bacilli; Lactobacillales; Lactobacillaceae; Lactobacillus

2. Streptococcus thermophilus:

        Bacteria; Firmicutes; Cocci; Lactobacillales; Streptococcaceae; Streptococcus; S. salivarius; S. salivarius subsp. thermophilus

NCBI: [1]

NCBI: [2]

Introduction of Yogurt Niche

Description of Niche

Yogurt is a dairy product that is made by blending fermented milk with different ingredients that gives flavor and color. Yogurt was discovered accidentally thousands of years ago. It has been said that the origin of yogurt is from Mesopotamia because evidence shows that these people reared goats and sheep at around 5000 B.C. The milk from the animals was stored in gourds, which formed a curd naturally in warm climate. The early form of yogurt was the curd. Yogurt is made with a variety of ingredients including milk, sugars, stabilizers fruits and flavors and a bacterial culture called Lactobacillus bulgaricus.In the process of fermentation, the organism interacts with the milk and converts it to a curd. In addition, it changes the flavor of milk to a yogurt flavor and acetaldehyde contributes to this process.Yogurt is a good source of vitamin B, protein and calcium, it is more digestible than fresh milk and it keeps the intestinal system populated with good bacteria and in healthy condition.


Yogurt is a commercial product that is available in grocery stores, supermarkets, and restaurants.

Physical Conditions

In making of the modern day yogurt, bacterias such as L.acidophilus or a lactose-fermenting yeast is introduced into concentrated sterilized milk and milk solids. The product is kept for 4 to 5 hours at 110 – 112 °F (43 – 44 °C) until it forms a cord. Yogurt is mostly stored in the fridge and if frozen it destroys most of the beneficial bacteria.

Microbes Specific to Yogurt

Yogurt Microbial Community

The most common microbial ingredients inside some of your favorite yogurts includes the species Streptococcus Salivarius subsp. thermophilis (http://microbewiki.kenyon.edu/index.php/Streptococcus_salivarius) and Lactobacillus delbrueckii subsp. bulgaris (http://microbewiki.kenyon.edu/index.php/Lactobacillus_delbrueckii). Often these two are both co-cultured with other lactic acid bacteria for health and taste effects. Some of these probiotics include but are not limited to L. acidophilus (http://microbewiki.kenyon.edu/index.php/Lactobacillus_acidophilus), L. casei(http://microbewiki.kenyon.edu/index.php/Lactobacillus_casei) and Bifidobactium(http://microbewiki.kenyon.edu/index.php/Bifidobacterium_adolescentis) species.

Prescence of Non-microbe

Initially none, but when yogurt is exposed to high temperatures and when theres lack of refrigeration, yeast will grow. (8)

Microbial Interactions

Protocoorperation and antibiosis are the most important interactions in the growth of the yogurt bacteria Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (Lb. bulgaricus). Protocoorperation can be understood as a mutual beneficial relationship while antibiosis is remarkingly quite the opposite. (11) The rod shaped L. bulgaricus are more proteolytic(metabolically more efficient in the breakdown of proteins) than their symboint counterpart. The S. thermophilus forms acid much slower, especially in milk, which lacks some amino acids. Hence L. bulgaricus liberates small peptides and amino acids, mainly valine, of which serves to enhance the growth of S. thermophilus. In return the cocci enhances the growth of L. bulgaricus by the formation of formic acid from pyruvic acid under anaerobic conditions. Under these protocooperative stimulations during combined growth of the yogurt bacteria, lactic acid is produced at a must faster rate than by that of individual pure cultures. Antibiosis is observed after a certain acidity has been reached, after this point, growth of S. thermophilus is halted. However, L. bulgaricus are less susceptible to acid and continues to grow. (6)

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. S Streptococcus liquifiecence

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

7. Bacillus substallis

    Purpose: Sweet curdling	
    Method: Curd formation

8. Streptococcus paracitrovorus

    Purpose: Attacks citric acid	
    Method: Flavors curd.

(15) (16)

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 and Its effect on environment and human body


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 (1). 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 (2). 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 (3). 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 (3). 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.


Probiotics that are specific to genus Lactobascillus are found in foods and also food supplements (13). It is the most common bacteria that is classified as a probiotic as it is considered to be a "friendly" bacteria. While inhabiting in the intestine and the vagina, the purpose of this friendly microbe is to prevent other "bad" microbes which cause diseases. This is done by proliferating at the intesntine. Thus, the Lactrobascillus microbe acts as a defense system. This is accomplished through a variety of mechanisms. For example, the breakdown of food by Lactobascillus acidophilus leads to production of lactic acid, hydrogen peroxide, and other byproducts that make the environment hostile for undesired organisms. Other probiotics responses include improving lactose absorption digestion in people who are lactose intolerant, enhancing the immune response, and alleviating symptoms of the bowel syndrome (14).

Current Research

Milk fermented with yogurt cultures and Lactobacillus casei compared with yogurt and gelled milk: influence on intestinal microflora in healthy infants (4)

Consuming fermented dairy products have profound health effects, such as providing the minerals and vitamins to humans. They regulate the equilibrium and metabolism of microflora in healthy infants. The experiment was based on comparing the effects of consumption of regular yogurt, milk fermented with yogurt cultures and Lactobacillus casei (YC), and nonfermented gelled milk on the fecal microflora of healthy infants who were divided in three groups, and each group received one of three products. The results show indexes, such as anaerobes, bifidobacteria, bacteroides were not modified during supplementation period. However, in the yogurt group, the number of enterococci in fecal samples had increased. In the YC group, the number of fecal lactobacillus had increased in the colon where lactobacillus may provide physiological benefits. This may prove that Lactobacillus can be used for preventing infectious diseases and stimulating the immune system.

Lactobacillus species properties and Lactose intolerance (5)

This paper talks about the experiments done to test the isolated Lactobacilli from the stomachs of piglets that were related to the properties of fermented milk. Lactobacillus was extracted from piglets because it is likely that there is a high concentration of Lactobacilli. This was specifically directed to the lactose- intolerant humans. There were several properties that were tested for. It was tested for the pH of the fermented milk over a span of 30 days, the reduction of the concentration of the lactose in milk, the viability, and the b- galactosidase activity. Optimal pH levels and ability to grow in a environment of bile salts were determined. Finally, commercial yogurt was also test to products varied in characteristics. The results were found that the Lactobacillus sp. Isolated from pigs had lower b-galactosidase activity than did Lactobacillus delbrueckii. Also, the optimal temperature was found to be 4 degrees Celsius for the b- galactosidase activity of the fermented milk to decrease with a lactose concentration of 4% and an active pH of 5.5. Lastly, when comparing the Lactobacilllus bacteria found in the piglet’s stomach to that found in yogurt, it has been proposed that for lactose-intolerant subjects, improved fermented milk products could result by using strains of this bacterium that possess high b galactosidase activity at acidic pH. And thus, these microbial cells that are more resistant to gastric digestion.

Contribution of Streptococcus thermophilus to Growth-Stimulating Effect of Yogurt on Rats (6)

Yogurt is produced by lactic acid formentation of milk by L. bulgaricus and S. thermophilus. Both L. bulgaricus and S. thermophilus exhibited a protocoorperation association to produce lactic acid at a greater rate. Symbiotically growing, L. bulgaricus provides S. thermophilus with formic acid, which provides better growth, while S. thermophilus releases amino acids, mainly valine to accerlater L. bulgaricus's growth. A study was done in rats to identify the source of contribution to growth-stimulating effect in yougurt containing both active lactic acid bacteria L. bulgaricus and S. thermophilus. The tests variables were consists of milk, yogurt, milk fermented individually by L. bulgaricus and S. thermophilus, and milk which both cultures has been added. Rats also fed on diets containing cell fractions, cell supernatant and sonicated cell. The results, 6 of 7 experiments showed significant weight gain in yogurt, and milk fermented by S. thermophilus and milk plus S. thermophilus cells. Because rats fed diets of L. bulgaricus alone did not stimulate any growth, this is a hard evidence supporting the sole responsibility and role in stimulating growth in rats


(1.)Virginia Vadillo-Rodríguez, Henk J. Busscher, Willem Norde, Joop de Vries, and Henny C. van der Mei1. "Dynamic Cell Surface Hydrophobicity of Lactobacillus Strains with and without Surface Layer Proteins". Bacteriol. 2004 October; 186(19): 6647–6650.doi: 10.1128/JB.186.19.6647-6650.2004.

(2.)Jessica J.Kious. "Lactobacillus and Lactic Acid Production". LeTourneau UniversityApplied Biological Sciences Branch, Alternative Fuels Division, August 8, 2000

(3.)LYN C. RADKE-MITCHELL and W. E. SANDINE. "Influence of Temperature on Associative Growth of Streptococcus therrnophilus and Lactobacillus bulgaricus". Journal of Dairy Science Vol. 69, No. 10, 1986

(4.)Corinne Guerin-Danan, Claire Chabanet, Christophe Pedone, Françoise Popot, Pierre Vaissade, Christine Bouley, Odette Szylit, and Claude Andrieux. "Milk fermented with yogurt cultures and Lactobacillus casei compared with yogurt and gelled milk: influence on intestinal microflora in healthy infants". Am J Clin Nutr 1998;67:111–7. Printed in USA. © 1998 American Society for Clinical Nutrition

(5.)[http://jds.fass.org/cgi/reprint/80/10/2318 J. P. BURTON and G. W. TANNOCK. "Properties of Porcine and Yogurt Lactobacilli in Relation to Lactose Intolerance". Journal of Dairy Science Vol. 80, No. 10, 1997]

(6.)N. P. WONG, F. E. McDONOUGH, and A. D. HITCHINS. "Contribution of Streptococcus thermophilus to Growth-Stimulating Effect of Yogurt on Rats". Journal of Dairy Science VoI. 66, No. 3, 1983

(7.)TOMMASO SOZZI AND MARTIN B. SMILEY. "Antibiotic Resistances of Yogurt Starter Cultures Streptococcus thermophilus and Lactobacillus bulgaricus". APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Nov. 1980, p. 862-865

(8.)Bennie C. Viljoen, Analie Lourens-Hattingha, Bridget Ikalafenga and Gabor Peterb. "Temperature abuse initiating yeast growth in yoghurt". Food Research International, Volume 36, Issue 2, 2003, pages 193-197

(9.)Amber Kahl. "Lactobacillus delbrueckii subsp. bulgaricus". 2007

(10.)Bennie C. Viljoen. "The interaction between yeasts and bacteria in dairy foods , Food Research International". Volume 69, Issue 1-2, September 2001, pages 37-44

(11.)P. Courtin, F. Rul. "Interactions between microorganisms in a simple ecosystem: yogurt bacteria as a study model". 13 November 2003

(12.)David B. Fankhauser. "YOGURT MAKING ILLUSTRATED". December 1980

(13.)Dick Crill. "Growing Microbes". NutriNexusTM 2005

(14.)Ruth DeBusk, Jacqueline A. Hart, Steven Ottariono, Joseph Lamb, Enrico Liva. "Lactobacillus acidophilus". 1 April 2002

(15.)Thusitha S. Gunasekera, Anders Sørensen, Paul V. Attfield,1 Søren J. Sørensen, and Duncan A. Veal. "Inducible Gene Expression by Nonculturable Bacteria in Milk after Pasteurization". Applied and Environmental Microbiology, April 2002, p. 1988-1993, Vol. 68, No. 4

(16.)Douglas Goff. "Dairy Science and Technology". University of Guelph 1995

(17.)Robert Roberts. "History of Yogurt". Associate Professor of Food Science, March 14 2001

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