However, researchers at the University of Arizona in Phoenix suggest that other factors may also be relevant because of their influence on the condition of the cervix.
A paper now published in the journal Scientific Reports describes how they found that women with cancer or precancer of the cervix had different vaginal bacteria to women who did not have cervical tissue abnormalities.
The finding suggests that there might be a direct link between "good" bacteria and a healthy cervix, and "bad" bacteria and raised risk for cervical cancer.
"In cancer and precancer patients," explains senior study author Melissa M. Herbst-Kralovetz, who is an associate professor in the College of Medicine at the University of Arizona, "lactobacilli — good bacteria — are replaced by a mixture of bad bacteria."
Cervical cancer and HPV
Cervical cancer starts when cells in the cervix, or the entrance to the uterus from the vagina, grow abnormally and become a tumor.
The presence of abnormal cells is known as precancer. If the abnormal cells become cancer cells and spread into neighboring tissue, it becomes cervical cancer.
Precancerous tissue should "be removed" to prevent cancer. This can normally be done without harming unaffected tissue.
Estimates for the United States suggest that, "at some point during their lifetime," approximately 0.6 percent of women will be told that they have cervical cancer.
New cases of cervical cancer in the U.S. dropped by at least 50 percent in 1975–2010 and statistics for 2008–2014 show that more than 66 percent of women survive for more than 5 years after diagnosis.
HPV spreads through "intimate skin-to-skin contact," such as during vaginal, anal, or oral sex. There are more than 150 types of HPV, of which only some can give rise to cancer in men and women.
Usually, the immune system can clear the virus without causing any harm. But if the virus persists, it can cause genital warts and cancer.
In both sexes, HPVs can cause cancer of the mouth, throat, anus, and rectum. In men, they can also cause cancer of the penis. In women, HPVs can cause cancers of the cervix, vagina, and vulva.
Impact of vaginal bacteria
Prof. Herbst-Kralovetz and her colleagues studied the relationship between HPV, certain conditions in vaginal tissue, and the severity of growths in the cervix.
The team recruited "100 premenopausal women" with "low- and high-grade" cervical abnormalities, invasive cervical cancer, and "healthy controls." The latter did not have any cervical abnormalities or cancer and included women with and without HPV.
The team found that Lactobacillus bacteria were reduced in line with increasing severity of cervical abnormalities.
This type of bacteria has previously been found to boost vaginal health. It is related to, but not the same as, the bacteria that constitutes "live culture" in yogurt.
Conversely, the results also showed that increases in another type of bacteria called Sneathia was linked to infection with HPV, cervical precancer, and cancer.
Sneathia bacteria are linked with miscarriage, vaginosis, preterm labor, and other gynecological problems. They have also been tied to HPV infection and precancer in the cervix.
The new study is significant because it is the first to show that a high level of Sneathia bacteria in the vagina is linked to all phases of cervical cancer, from initial infection with HPV, through precancer to invasive cancer.
However, what is not clear is whether the high levels of Sneathia bacteria drive the cancer process or are just a byproduct of it. Prof. Herbst-Kralovetz explains that they are actively investigating this question as there is little published research on "how Sneathia functions in the reproductive tract."
The role of ethnicity
Around half of the women in the study were of Hispanic origin and the rest were not. The authors cite evidence of higher rates of cervical cancer among Hispanic women, and they were keen to discover any ethnic factors that might lie outside of "lack of screening" or "unequal access to healthcare."
They did find some evidence to suggest that this may be the case. For example, Hispanic women who took part in the study were more likely to have reduced vaginal populations of Lactobacillus and higher populations of Sneathia bacteria.
This suggests, perhaps, that the different composition of vaginal bacteria in Hispanic women may be a factor in potentially raising the risk of cervical cancer in this population.
Acidity of the vagina
The study was also the first to find that less acid vaginal environments are more likely to be linked with more severe cervical abnormalities.
Previous research has shown that harmful bacteria do not fare well in acid environments — that is, where the pH is 4.5 or lower. As acidity reduces and pH increases, the harmful bacteria have greater opportunity to thrive.
Higher levels of Lactobacillus bacteria, on the other hand, raise acidity because they produce lactic acid.
"If you have high levels of lactobacilli," Prof. Herbst-Kralovetz explains, "you're going to have a lower vaginal pH, and that's associated with health."
She and her colleagues call for larger studies that follow people over time to further explore the cause and effect relationships and the molecular nature of the links that they have uncovered.
"This work serves as the foundation for a lot of other studies."
Prof. Melissa M. Herbst-Kralovetz