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Yeast is indispensable in human production and life. From the food to the production of biological raw materials, this microbe is full of images. But in the same way, yeast can cause diseases in humans in some cases. Among them, Candida krusei is one of the five most common yeasts. In the United States, there are 46,000 cases of yeast infection each year, and the fatality rate is 30%. But if you tell me now that the yeast used to make wine and bread is incognito, but the body is a pathogen, would you be scared?

In the United States, 46,000 people are infected with a class of pathogenic yeast, Candida (also known as Candida). Due to resistance to antibiotics, once infected, the patient’s mortality rate is as high as 30%. The five most common pathogens, including Candida krusei, account for 98% of yeast infectious diseases. The risk of C. krusei infection is not high, but it is particularly dangerous for people with low immunity: according to a statistical study of cases between 2002 and 2015, C. krusei-induced candidemia can be used by some patients. Mortality increased by more than 20%.

At the same time, many fungi that are harmless to the human body are also widely used in the food industry. Pichia kudriavzevii is one of them. These yeasts are widely distributed in nature and are defined by the US Food and Drug Administration as safe. Over the centuries, the yeast has been used in the processing of a variety of foods, including African cassava and cocoa fermentation, milk fermentation in China’s Tibet and Sudan, Chinese bread fermentation and wheat fermentation to produce vinegar, as well as some kimchi, fine Brewing beer production process. In addition, P. kudriavzevii is increasingly used in biotechnology, including the production of important chemical raw materials such as succinic acid.

Candida krusei strain under the microscope

What is the connection between these two seemingly unrelated yeasts? As early as 1980, taxonomists suspected that the two were the same yeast: the researchers conducted a comparative study of C. krusei and P. kudriavzevii. By means of DNA linkage analysis, proliferation testing, etc., they believe that the former is an incomplete version of the latter. However, due to the lack of more evidence, most scientists at that time did not suspect that the regular customer P. kudriavzevii in food production turned out to be the pathogenic C. krusei.

Gene sequence reveals amazing connections

Previous studies of several Candida and Pichia have independently shown their respective gene sequences. But so far, there is no detailed genetic analysis for C.krusei and P.kudriavzevii. The key reason for this is that the study of the two is completely divided: researchers in the medical field study C. krusei through clinical samples, while scientists in the field of food and biotechnology extract and study P. kudriavzevii from environmental sources. Different research fields and different strain sources make almost no communication between the two.

In response to these questions, Professor Kenneth H. Wolfe from the University College of Dublin selected a total of 32 yeast strains for genome sequencing analysis, of which 20 were from clinically isolated C. krusei and 12 were from environmentally isolated P. kudriavzevii. The genomic DNA of the two yeasts were sequenced separately, and the intron region and ribosomal protein-coding genes were compared and analyzed. The results showed that the genomes of the two showed a highly linear relationship, that is, the genetic information was highly consistent: 99.6% of the genomic DNA sequences were identical. Therefore, the two are actually the same yeast.

Intron and ribosome genomes show high agreement between the two yeasts

In addition, systematic biological analysis also confirmed that C. krusei and P. kudriavzevii belong to the same branch in the evolutionary tree.

The two yeasts on the evolution tree belong to the same branch.

Fluconazole is an anti-fungal antibiotic that is commonly used in infections caused by fungi. It has been previously discovered that the clinically isolated Candida krusei is extremely resistant to this antibiotic. Now, the researchers also see resistance in P. kudriavzevii: sensitive strains die at a drug concentration of 1 mg/L, while P. kudriavzevii still lives very well at a drug concentration of 8 mg/L. exuberant. Even more dangerous is that P. kudriavzevii exhibits greater drug resistance for the antibiotic drug Flucytosine, which is used to treat Candida.

Also tested are amphotericin, micafine and other drugs commonly used in the treatment of fungal infections, the results are also very obvious, except for a few clinically isolated strains appear drug sensitive, most of these strains for these There was no obvious reaction to the drugs. In other words, the resistance of these two yeasts to antibiotics is almost as high. This paper was published in the recent PLOS pathogens journal.

Trade-offs in industrialization

Clinically isolated strains are often highly similar to environmentally isolated strains, and the research team believes that the main reason is that the flora in the environment causes human infection in some way. This reminds us that the application of P.kudriavzevii in the bio-industry poses a potential risk to both workers and consumers.

“If I say that I want to use this drug-resistant pathogen to make food, people will immediately boycott,” said the author of the article, Professor Wolfe. “But when people used a new name in the food industry, it seems that no one I care about this.”

This reminds us that yeast must be used with caution in the food and bio-manufacturing industries. “I think it is better to choose other kinds of yeasts in the Pichia genus as industrial application bacteria,” said Alexander P. Douglass, the first author of the article. “The relevant departments should take more stringent measures to limit this resistant yeast strain. Industrial applications, especially in the food industry.”

P.kudriavzevii has been used in industry for so long, why is there no report of illness? Wolfe said in an interview with Global Science: “In the past, five Candida species were listed as the highest risk pathogens, including Candida krusei, but now we can say that P.kudriavzevii is also one of them. Most of the reported cases use the name Candida krusei, but in fact it is the same name P.kudriavzevii.”

Is it necessary for the public to worry about this issue? Wolfe also gave his opinion: “This yeast infection usually occurs in people with low immunity or defects, such as organ transplants or AIDS patients. People with very healthy immune systems don’t have to worry too much about this disease.”

Since we can find P. kudriavzevii in some craft beers, sour bread or kimchi, Professor Wolfe recommends that people with low immune systems avoid eating these foods. This yeast infection poses additional challenges for the treatment of immunocompromised people because it almost completely ignores common fungal antibiotics.

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