Carcinogenesis is the process of transforming normal cells into cancer cells. And if the role of genetics has been studied for a long time, the role of microbiota has been studied since 2008.
Microorganisms perform various functions. Some help the tumor survive, while others, on the contrary, expose it to immune cells.

What bacteria live in a tumor
A tumor is not isolated in the body, it actively interacts with surrounding cells and structures. Around the tumor are normal tissue cells, immune system cells and blood vessels. This affects the growth and development of the tumor itself.
A study conducted at the Weizmann Institute of Science in Israel showed that bacteria can also create a tumor microenvironment. They belong to the internal microbiota of the tumor. The microbiota is the totality of all microorganisms that live in the human body. They are found in the intestines, skin and other organs. Presumably, microorganisms colonize an already existing tumor. The conditions created by tumor cells turned out to be attractive for microbes. For example, microorganisms have been found in lung tumors of smokers that can break down chemicals contained in tobacco smoke (nicotine, anthranilate, toluene and phenol). Bacteria capable of this are types of Proteobacteria, Actinobacteria and Cyanobacteria. It is assumed that the presence of bacteria in the tumor has a positive effect on the activation of special immune cells that can destroy tumor cells. Thanks to the bacteria, the number of immune cells around the tumor increases. This improves the effectiveness of the body's fight against cancer.

How the intestinal microbiota counteracts carcinogenesis
The microbiota is a colony of bacteria throughout our body. But most of the bacteria are in the intestines. The gut microbiota can influence tumors that are located both in the intestine and in other organs. Bacteria process plant cellulose that enters the intestines. They convert them into fatty acids. Short-chain fatty acids, which include butyrate, acetate, and propionate, play an important role. Butyrate can protect the intestinal mucosa by affecting the secretion of cytokines. Cytokines are special molecules that allow cells to communicate with each other. They are divided into pro-inflammatory (those that increase inflammation) and anti-inflammatory (those that suppress it). They are responsible for increasing the activity of immune cells and protect the body from tumors. There are many bacteria that can produce butyrate. Most of them belong to the Faecalibacterium and Roseburia genera.
Japanese scientists have made an important discovery. They have described specific microorganisms that enhance the activity of anti-tumor immune cells. The number of these cells increased not only in the large intestine, but also in distant organs. Thus, the anti-tumor immunity of the whole organism may depend on the composition of the microbiota in the intestine. And special tests helped to clarify this composition.
How intestinal bacteria affect breast cancer
Intestinal estrobolome is a set of intestinal bacterial genes, the products of which are able to metabolize estrogen. Bacteria with these genes can affect the level of estrogen in the blood. And high levels of estrogen are a risk factor for breast cancer.
Estrogen is excreted from the body in an inactive state through the intestines. Bacteria of the types Collinsella, Edwardsiella, Alistipes and Bacterioides can convert estrogen to an active state and increase its reabsorption. A diet that is high in sugar and fat but low in cellulose can increase the number of these bacteria in the intestine.
A healthy diet rich in plant foods, cellulose and whole grains, on the other hand, promotes the growth of the "right" microbiota. Bacteria of the types Firmicutes, Bacteroidetes and Actinobacteria can increase the metabolism of estrogen, reducing its content in the body.

When gut bacteria contribute to carcinogenesis
There must be a balance between the different organisms in the intestines. If this is disturbed (due to diet, lifestyle, illness or certain medications), dysbiosis can occur. This causes chronic inflammation, which increases the risk of developing cancer.
The main reason why normal cells turn into cancer cells is damage to the DNA structure. Some inhabitants of the intestine can do this. For example, certain strains of Escherichia coli and Klebsiella pneumonia produce a substance called colibactin, which can cause damage to the DNA of intestinal cells. If the cell is unable to repair the damage, this can be the beginning of the development of cancer.
Finally, some bacteria in the mouth can convert alcohol into a carcinogen. Bacteria such as S. gordonii, S. mitis, S. oralis, S. salivarius, S. sanguinis, fungi of the Candida genus, and bacteria of the Neisseria genus are capable of doing this.

What is important to remember
Bacteria