The understanding of cancer’s origins now includes a microbial dimension, as certain bacteria are recognized agents of the disease. These microorganisms can establish long-term infections that contribute to specific cancers. The study of these carcinogenic bacteria is opening new pathways for understanding, preventing, and treating cancer.
How Bacteria Promote Cancer Development
Bacteria contribute to cancer through several processes, often involving long-term interaction with host tissues. A primary mechanism is chronic inflammation. A persistent bacterial presence triggers a prolonged immune response, creating an inflammatory environment rich in molecules like reactive oxygen species that can damage cellular DNA and lead to cancer-initiating mutations.
Some bacteria produce carcinogenic substances directly. These toxins or metabolites can damage host cell DNA or interfere with signaling pathways that control cell growth and death. This disruption can prevent cells from dividing uncontrollably, and over time, the accumulated damage can lead to a malignant state.
Bacteria can also manipulate a host cell’s internal machinery. Some pathogens inject proteins that hijack cell functions, disrupting the cell cycle and forcing continuous proliferation. These proteins may also inhibit apoptosis, the programmed cell death that eliminates damaged cells, allowing abnormal cells to survive and multiply.
Key Bacteria and Associated Cancers
The most well-documented link between a bacterium and cancer involves Helicobacter pylori. This bacterium colonizes the stomach lining, is a primary cause of peptic ulcers, and has been classified as a Group 1 carcinogen. Its presence is strongly associated with gastric adenocarcinoma and a rarer cancer called mucosa-associated lymphoid tissue (MALT) lymphoma. The bacterium’s CagA protein can be injected into stomach cells, where it disrupts cell signaling pathways that control growth, promoting cancerous transformation.
Research has identified other bacteria with potential links to cancer. Chronic infection with Salmonella Typhi, the cause of typhoid fever, is associated with an increased risk of gallbladder cancer. This risk is attributed to the long-term inflammation of the gallbladder in carriers of the bacterium.
Imbalances in the gut microbiome have been linked to colorectal cancer. Certain strains of Escherichia coli and Streptococcus bovis are found more frequently in colon tumors. These species may contribute to cancer by producing toxins or promoting inflammation in the colon.
The bacterium Chlamydia trachomatis, a common sexually transmitted infection, is also under investigation. While human papillomavirus (HPV) is the primary cause of cervical cancer, co-infection with C. trachomatis may increase the risk. The bacterium causes chronic inflammation in the female reproductive tract, which might create a more favorable environment for the persistence of HPV and the progression of cervical lesions.
Diagnosis and Screening
Diagnosing an H. pylori infection is a key step in assessing gastric cancer risk. Several reliable and non-invasive tests are available to detect its presence.
- The urea breath test detects carbon dioxide released by the bacteria after a patient consumes a special urea solution.
- The stool antigen test identifies H. pylori proteins in a fecal sample.
- An upper endoscopy allows a doctor to view the stomach lining with a camera and take tissue samples (biopsies) to test for the bacteria and check for cancerous changes.
Screening for H. pylori is a preventative strategy, particularly in populations with a high incidence of stomach cancer. Identifying and treating the infection reduces the long-term risk of the disease. A positive diagnosis prompts further evaluation of the stomach lining to screen for any pre-cancerous or cancerous conditions.
Targeting Bacteria in Cancer Treatment and Prevention
The link between bacteria and cancer has led to new prevention and treatment strategies. For H. pylori-associated cancers, the most direct preventative measure is eradicating the bacterium. A course of therapy, typically involving a combination of at least two antibiotics with a proton pump inhibitor to reduce stomach acid, is highly effective at eliminating the infection and reducing the risk of developing gastric cancer.
This strategy also applies to active treatment. For MALT lymphoma of the stomach, the cancer’s growth is dependent on inflammation from H. pylori. In many early-stage cases, eradicating the infection with antibiotics can lead to a complete regression of the lymphoma, curing the cancer without chemotherapy or radiation.
Broader preventative measures also reduce the risk of these bacterial infections. Proper hygiene, including handwashing, and access to clean water and safe food handling limit exposure to bacteria like H. pylori and Salmonella. These public health measures lower the incidence of infections that can cause cancer.