Swiss Cheese Niche
- 1 Description of Swiss Cheese
- 2 Inhabitants
- 3 Current Research
- 3.1 Resistance to Freezing and Frozen Storage of Streptococcus thermophilus Is Related to Membrane Fatty Acid Composition
- 3.2 Effects of strains and growth conditions on autolytic activity and survival to freezing and lyophilization of Lactobacillus delbrueckii ssp. lactis isolated from cheese
- 3.3 Interaction between propionibacteria and starter/non-starter lactic acid bacteria in Swiss-type cheeses
- 3.4 Effect of Lactobacillus helveticus and Propionibacterium freudenrichii ssp. shermanii Combinations on Propensity for Split Defect in Swiss Cheese
- 4 Summary
- 5 References
Description of Swiss Cheese
Originally from Switzerland, Swiss cheese is known as Emmental throughout the world. It has made its way into the United States as a very popular dairy product. Swiss cheese is a solid, flavorful cheese that is usually white in color. The term "Swiss cheese" now refers to any cheese with holes in it. Its holes are often known as "eyes" and make it distinguishable from other various cheeses. The larger the eyes, the greater the flavor of the cheese because the bacteria has had more time to act on the cheese. However, if the holes are too big, it becomes difficult to slice and may cause the cheese to crumble. Its flavor and texture make it a desired source of food for many people. As a niche, Swiss cheese is unique due to the interactions between the three major bacteria that occupy it: Lactobacillus casei, Streptococcus thermophilus, Propionibacterium shermanii.
Swiss cheese is found in many different types of environments. Typically, it is made in factories and is then distributed to grocery stores, restaurants, and vendors. From there, it makes its way into our homes for consumption. Eventually, unconsumed Swiss cheese winds up in the trash to undergo decomposition.
But even before this cycle begins, Swiss cheese goes through a series of changes to become the niche that we are familiar with. Cheese is made from pasteurized milk that has gone sour due to the lactic acid bacteria that was introduced to it. The lactic acid bacteria eat the sugar known as lactose in milk, producing lactic acid.  The acidic condition of the milk causes the milk to change into its solid form called curd. An active enzyme complex called rennet, which is found in the inside of cows’ stomachs, help separates the solid curd form of milk from the liquid form of milk, called the whey.  Most of the moisture is removed from the curd to form a solid. At this point, the isolated curd undergoes tremendous pressure and is pressed into its desired shape.
Swiss cheese typically exists in solid form. It is stored in a cooled environment to prolong the spoiling process. At high temperatures, its physical state might change into a softer substance. Neighboring niches do not generally affect Swiss cheese; this niche undergoes most of its changes due to the bacteria that act on it. 
Swiss cheese gets its flavor and texture from the amount of time it is given to ripen. Controlled temperature and change in pH also affect the niche. Bacteria culture that is introduced to the cheese affects the changes in pH, the fermentation process, and the appearance of the holes in Swiss cheese due to escaping carbon dioxide. The nature of the texture of Swiss cheese makes it difficult for the carbon dioxide (produced by bacteria) to escape through the solid. As a result, the carbon dioxide forms holes in the cheese. 
Within the Swiss Cheese environment lives three types of microbes of the genus lactobacillus, streptococcus, and propionibacteria.
[[Image:|thumb|Currently waiting for copyright permissionP. Shermanii is the bacterium that makes the carbon dioxide and propionic acid in the Swiss cheese.]]
Lactobacillus is a homofermentative, thermophilic and gram-positive bacteria that is always found within Swiss cheese due to its ability to provide texture and sharpness of the cheese. Different types of strains of lactobacillus(L. helveticus, L. casei, L. bulgaricus) live in Swiss cheese but the more commonly known bacteria is the strain lactobacillus helveticus. This microbe is part of the lactic acid family of bacteria by converting lactose present in the cheese to lactic acid. 
Streptococcus thermophilus is a homofermentative and gram-positive lactic acid bacterium responsible for the initial lactate fermentation of Swiss cheese. S. Thermophilus is responsible for the acidity and texture in the cheese’s early production and the conversion of casein into nitrogen sources. The initial heating of milk is ideal for the thermophilic lactic acid bacteria. Streptococcus is a common starting culture found in milk products. S. thermophilus is often utilized in Cheddar cheese, Italian cheeses, and yogurt.
Propionibacterium Shermanii is a slow growing and gram positive bacterium that grows in an anaerobic glucose medium. P. shermanii is responsible for producing the holes and the distinct flavor of Swiss cheese. The growth rate of the bacterium is dependent on the surrounding temperature, pH, and bacteria. The optimal growth for P. shemanii is in warm temperature and at a pH of 5.3. The bacterium’s growth is also dependent of the availability of lactic acid which is produced by L. helveticus and S. thermophilus. 
L. helveticus and S. thermophilus
Lactobacillus helveticus is mainly involved with Streptococcus thermophilus in controlling the pH level of the Swiss cheese environment through their fermentation of lactose. The Swiss cheese enivronment changes temperatures due to the pastuerization process and in its intial stages Streptococcus thermophilus is the more dominant bacteria in the cheese. However since makers of Swiss cheese allow it to sit at room temperature for the ripening process allows for the growth of Lactobacillus helveticus. Since room temperature is slightly off from the optimum growth temperature which is around 45 degrees Celcius for Lactobacillus helveticus, the microbe is slowly able to grow. When L. helveticus reaches its maximum growth or reaches the stationary phase it begins to autolyse and starts to affect the viability of Streptococcus thermophilus. There is correlation between the survivability of Lactobacillus and Streptococcus but there are no specifics as to why the correlations occur. 
The specifics of the symbiotic relationship between S. thermophilus and L. helveticus rely mainly on Lactobacilus’s strong proteolytic system. Lactobacilus’ proteolytic enzymes liberate milk protein casein into smaller peptides and amino acids that serve as peptide substrates for S. thermophilus’ aminopeptidases. The growth of L. helveticus is stimulated by formic acid and carbon dioxide production of S. thermophilus.
The growth of P. Shermanii is heavily dependent on L. helveticus and S. thermophilus. Since low pH level does not favor growth for P. Shermanii, the ability of L. helveticus and S. thermophilus to change the pH of the niche determines the population size of P. Shermanii. At very low temperature and low pH, the bacterium's growth is retarded. Additionally, the bacterium relies on L. helveticus and S. thermophilus to produce lactic acid for it to consume. If lactic acid is not present, P. Shermanii can still survive by consuming lactose but the growth rate of the bacterium will be slower. 
Microbes and their environment
S. thermophilus and Lactobacillus are responsible for the increase in pH and the coagulation of casein during lactic acid fermentation. The increase in acidity as a consequence of the proteolytic microbes contribute to the cheese's distinct flavor, texture, and composition. 
The decrease in pH is attributed to the utilization of lactose and galactose during early stages of Swiss cheese manufacture. S. thermophilus and Lactobacillus alter the pH by fermenting the lactose that surrounds them and converting it into lactic acid. In addition to changing their environment Lactobacillus can also produce formation of lactate calcium crystals and toxic amines. The lactate calcium crystals form on the surfaces on the cheese and has no adherent effects on the Lactobacillus and is just a defect within the cheese environment. No information or study has been found yet on how the production of toxic amines would affect the surrounding microbes of Streptococcus and Propionibacteria but was more commonly known as an outbreak of food poisoning when ingested by humans. 
Along with Lactobacillus, S. thermophilus is responsible for the proteolytic activity of lactose in lactic acid fermentation. Since S. thermophilus is only weakly proteolytic compared to Lactobacillus, it aids the aminopeptidase producing bacteria through their symbiotic relationship. Both of these primary microbes produce other metabolites such as acetaldehyde, diacetyl, and ethanol released into the Swiss cheese. Lactobacillus' autolytic activity and release of intracellular enzymes such as peptidases or lipases or enzymes from amino acid catabolism has effected the viability of other bacterias in Swiss cheese.
P. Shermanii consumes the lactic acid that is excreted by L. helveticus and S. thermophilus in the Swiss cheese and release carbon dioxide gas and propionic acid via fermentation. The carbon dioxide gases stay in the cheese in forms of bubbles which make the holes of the Swiss cheese. The propionic acid contributes to the Swiss cheese’s characteristic flavor. 
Resistance to Freezing and Frozen Storage of Streptococcus thermophilus Is Related to Membrane Fatty Acid Composition
S. thermophilus’s ability to resist freezing and frozen storage was examined under four experimental factors and quantified by the ability to regain acidification activity of the lactic acid bacteria. The four experimental factors observed include: concentration of fatty acid with storage time, addition of glycerol as a cyroprotective agent, addition of oleic acid, and fermentation pH. The lactic acid bacteria was grown at 42C until the beginning of stationary phase at which experimental conditions were implemented. The acidification activity was measured before, after and during storage with the Cinac System. Experimental results show recovery of acidification activity of S. thermophilus with observable changes in membrane fatty acid composition with the experimental factors: increase in unsaturated fatty acid concentration with storage time, increase in unsaturated:saturated fatty acid ratio with addition of oleic acid and pH fermentation, but no effect in the addition of glycerol. The adaptation in unfavorable pH triggered a homeostatic mechanism that affected transbilayer movement of phospholipids and maintained permeability of membrane. The dependence of fermentation pH and oleic acid in unsaturated:saturated ratio still remains unclear 
Effects of strains and growth conditions on autolytic activity and survival to freezing and lyophilization of Lactobacillus delbrueckii ssp. lactis isolated from cheese
There has been a correlation between the autolytic activity of Lactobacillus and the survivability of other organisms within cheese, specifically the viability of Streptococcus thermophilus. This study further examines the autolysis of Lactobacillus when growth conditions are varied between room temperature and freezing. Certain strains of Lactobacillus, specifically L. delbrueckii, were used due to their nature to have a higher sensitivity in harsher treatment conditions to be able to determine if certain factors will change their survival rates. The study goes on to control the pH levels and aeration in addition to temperature. Lactobacillus' ability to adapt and change to survive at a range of temperatures and how effective it is at each of those steps is what this study tries to address. The conclusion gained from this study was that while survivability was good at almost freezing temperatures, viability after lyophilization showed significant losses. However, adding acidic pH will induce a stress-like state for the Lactobacillus which increased the viability after lyophilization. 
Interaction between propionibacteria and starter/non-starter lactic acid bacteria in Swiss-type cheeses
Studies showed that Swiss-type cheeses shared three fundamental bacteria: thermophilic lactic acid bacteria, propionibacteria, and heterofermentative lactobacilli. In 2002 the Swiss Dairy Research Station inspected the bacteria’s interactions and their influence on the quality and late fermentation of the cheese. Multiple emmental cheeses were used as models for this project. These models were separated in four factors. The aspartase activity strength in different cultures of propinoibacteria, the presence of Lactobacillus casei strains, the presence of Lactobacillus helveticus strains, and the season, winter and summer. The research discovered that the strength of aspartate metabolism in propionibacteria is correlated to their growth rate and propinoic acid fermentation. The propinoibacteri’s growth is also greater in winter than in the summer. In addition, it was found that the defect of late fermentation can be stopped by the presence of L. casei and weak aspartase activity in propionibacteria and the absence of L. helveticus. However, the study was not able to determine whether the aspartase metabolism is the activator or indicator of propionibacteria’s growth and fermentation. Future studies will focus on this particular area in order to further understand the effect of propionibacteria in Swiss-like cheeses. 
Effect of Lactobacillus helveticus and Propionibacterium freudenrichii ssp. shermanii Combinations on Propensity for Split Defect in Swiss Cheese
Split defect, which are cracks in the cheese, is one of the most popular problems occurring in Swiss cheese. This issue is difficult to resolve because knowledge about controlling the problem is limited. In this research, the Western Dairy Center Department of Nutrition and Food Sciences studied the effect of Lactobacillus helveticus and Propionibacterium shermanii on split defect in Swiss cheese in attempt the find a solution to the issue. The study was designed in a “2X2X2 factorial experiment” in which different cultures of L. helveticus and P. shermanii were tested in twenty-four 4-kg blocks of Swiss cheese in a 90 days process. The research concluded that the split formations in the cheeses were magnified by 100% when there were presence of both L. helveticus and P. shermanii. In addition, the season was also a factor of the split defect in cheese. During the summer, split defect occurred in 14 to 90% of the cheese, while only 1 to 6% occurred during the winter. This study had only grazed the tip in finding the solution to split defect. There is still no exact answer to how the bacteria influence the split defect. In order to develop an effective solution to controlling the split defect in Swiss cheese, further studies must be done. Without a doubt that if Swiss cheese fanatics desires to preserve their cheese for a long time, this would be the exact topic for future research. 
Swiss cheese is usually known for its holes and unique texture. It is not usually known for the three major bacteria that are involved in making the cheese the way it is. Recent studies show that these bacteria are also very important in other aspects of microbiology. Experiments have been done on Swiss cheese to determine the effects of freezing the microbes. The results obtained from these studies suggest that these bacteria are not only limited to the cheese-making process. On the contrary, further studies may result in the next big discovery that may even rival Swiss cheese itself!
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Edited by Heidi Cung, Derrick Low, Binh Phung, and Danny Tran, students of Rachel Larsen