The invisible killer - Candida albicans

Candida albicans, also known as Candida albicans, is a common pathogenic fungus. Although the word "yeast" is in its name, it is not the same as the brewer's yeast used in daily life to make wine and leaven dough.

As the most common conditional pathogen in humans, Candida albicans usually lives on the mucosal surfaces of the human body, such as the mouth, pharynx, intestines, and urogenital tract, but can also cause visceral and systemic infections.

It is also a commensal bacterium that is non-pathogenic to healthy people and exists in small quantities in the normal body.

However, when the human body's immunity is weakened or the immune system is damaged, or when the normal bacterial flora is imbalanced and the constraints on it are weakened, Candida albicans will multiply in large numbers and change its growth state, transforming from a commensal bacterium to a pathogenic bacterium, infecting human tissues and causing disease.

These diseases include: vaginitis, angular cheilitis, thrush, gastroenteritis, meningitis, bacteremia and sepsis, etc. Candida albicans can be transmitted in various ways, most commonly through sexual intercourse, public bathrooms, bathing pools, swimsuits and medical devices.

Candida albicans is resistant to ultraviolet rays, dryness and chemicals, but is sensitive to heat and will die within one hour at 60°C. The optimal survival pH is 5.5.

It should be noted that the normal pH value of the human vagina is around 4.0, which has a certain self-cleaning function. However, when the value turns to 5.5, Candida albicans will multiply and cause vaginitis.

There are three common forms of Candida albicans: yeast type, pseudohyphae type and hyphae type (as shown in the figure), which vary depending on the growth environment.

Figure: Different forms of Candida albicans photographed by Xu Dayong and others

The yeast type of Candida albicans is round or oval in appearance, with a diameter of 3-6 microns, which is very similar to the brewer's yeast used in brewing wine and making dough.

This type of Candida albicans usually exists on human skin and mucous membranes, in a symbiotic state with the human body. It is not pathogenic, but it can spread in the human blood system.

The hyphae type of Candida albicans is formed by the germination of yeast type Candida albicans, which then grows and extends. The hyphae type of Candida albicans can easily attach to and invade human tissue cells, and can also evade the attack of the human immune system. Therefore, the pathogenicity of this type of Candida albicans is significantly stronger than that of the yeast type.

So, how does Candida albicans infect the human body?

In healthy humans, Candida albicans usually adheres to and colonizes the surface of human epithelial cells in the form of yeast.

When the human immune function is weakened or severely deficient, the Candida albicans cells transform from yeast to hyphae. Subsequently, the hyphae "pierce" the human epithelial cells and vascular endothelial cells and come into contact with the blood.

In the blood vessels, hyphae cells transform into yeast cells, which travel with the blood to new tissue sites and start new infections. Yeast-type Candida albicans can also attach to the inner wall of blood vessels, transform into hyphae cells, penetrate vascular endothelial cells, and infect human tissues.

Currently, the treatment of Candida albicans infection is a major clinical problem. In my country, more than 80% of fungal infections are caused by Candida albicans.

In addition, although there are many antifungal drugs available, the mortality rate of some types of Candida albicans diseases remains high. For example, the mortality rate of bacteremia caused by it is between 40% and 80%.

The above optional drugs mainly include: azole drugs (such as ketoconazole and fluconazole), polyene drugs (such as nystatin and amphotericin B), echinocandins (such as caspofungin and micafungin), and allylamine drugs (such as naftifine and terbinafine).

These drugs should be used in a standardized and reasonable manner, and repeated and large-dose use should be avoided, otherwise the development of drug resistance in pathogenic microorganisms is inevitable.

In addition, compared with taking medicine, it is always the best choice to focus on physical exercise and develop good personal hygiene habits on a daily basis.

Candida albicans painting (Photo provided by Zhu Yongze, Zhejiang Provincial People's Hospital)

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