Introduction

Cancer and bacterial infections are commonly viewed as distinct health issues.¹ Bacterial infections typically bring to mind conditions like sore throats or urinary tract infections, short-lived and effectively treated with antibiotics. Cancer, conversely, is often seen as a more complex disease. Yet, emerging research has uncovered a surprising bridge between these seemingly unrelated conditions. The World Health Organisation (WHO) has identified certain bacteria as Class I carcinogens, agents directly linked to cancer development.² This emerging field of research has uncovered certain bacterial infections to be associated with specific types of cancer, highlighting a novel method of action in the development of cancer.³

Types of bacterial infections linked to cancer

Several different bacterial infections have been identified as potential risk factors for cancer. Importantly, it is not the bacteria that directly leads to cancer development, but the mechanisms of infection and the hosts immune response which play a significant role.¹

Here we look at some common bacterial infections and the cancer they are commonly associated with, however there are others not discussed here.

Helicobacter pylori infection and gastric cancer

Helicobacter pylori (H. pylori) and its connection to gastric cancer is documented well. This bacterium, although often asymptomatic (showing no symptoms), can cause gastritis, an inflammation of the stomach lining, in some patients, due to its pathogenic mechanisms.^1,5 Chronic inflammation caused by H. pylori is a major risk factor for stomach cancer due to the prolonged damage to the stomach lining cells.⁵

Salmonella typhi and gallbladder cancer

Salmonella typhi (S. typhi) is known to cause typhoid fever and releases chemical factors that can transform normal cells into cancerous cells.⁶ Gallbladder cancer development from S. typhi infection typically results from a chronic (long-term) infection with the bacterium.^1,6

Chlamydia trachomatis and cervical cancer

Chlamydia trachomatis (C. trachomatis) is a commonly cured sexually transmitted infection (STI).¹ Persistent infection with C. trachomatis can cause DNA damage and disrupt the normal cell cycle of cervical cells.⁷ Over time, this persistent damage to normal cells can transform them into cancerous cells.^1,7 Coinfection with human papillomavirus (HPV) significantly increases the risk of cervical cancer, with DNA of both pathogens detected in about 99% of cervical cancer cases.⁷

Chlamydia pneumoniae and lung cancer

Chlamydia pneumoniae (C. pneumoniae) causes respiratory infections, including pneumonia.(1) Chronic exposure to C. pneumoniae has been linked to lung cancer development, potentially through chronic inflammation of lung tissue and the production of chemical factors that cause DNA damage.⁸ Factors such as sex, age, smoking, and family history influence the relationship between C. pneumoniae infection and lung cancer prevalence, indicating a need for further research.^1,8

Streptococcus gallolyticus subspecies gallolyticus and golorectal cancer

Streptococcus gallolyticus (S. gallolyticus) is strongly associated with colorectal cancer.¹ The bacterium can contribute to cancer development by causing long-term inflammation, disrupting the balance of the gut’s normal microbes, weakening the body’s immune defence against abnormal cells, and producing harmful substances that damage DNA.^1,9 S. gallolyticus thriving in areas where cancerous cells are growing suggests that this bacterium could be both a contributor to and a marker of colorectal cancer.⁹

Mechanism of cancer development

The development of cancer following a bacterial infection is a complex process that unfolds over an extended period. It is influenced by a variety of factors which include genetics, environmental conditions and the host’s susceptibility to infections.³

After a host is initially infected  symptoms can vary widely; they may be acute (short-term) or chronic, and either symptomatic (showing symptoms) or asymptomatic.^1,3,10 When symptoms arise, antibiotics are often prescribed to eradicate the bacteria. However, our bodies naturally harbour a diverse array of bacteria that form our microbiome, which plays a crucial role in maintaining the health of our cellular environment.³ Antibiotics, while effective in combating harmful bacteria, can sometimes disrupt this balance, unintentionally dysregulating our body’s natural defences.¹¹ This disruption may provide an advantageous environment for the infectious bacteria, reducing the body's ability to fight off these harmful invaders.^1,3 Additionally, bacteria replicate rapidly, which can lead to changes in the genetic material of the bacterium.¹² Occasionally, these changes may enhance the bacteria's resistance to antibiotics, allowing the infection to persist even after treatment.^3,12

During this ongoing battle, the bacteria may remain hidden within the host, not causing any obvious symptoms.^1,3 However, they can produce toxins that lead to chronic inflammation, a state where the body is continually fighting against these toxins, damaging healthy cells in the process.¹³ This chronic inflammation prompts the body to release substances known as inflammatory cytokines, which are proteins that can help fight infections but also cause cell damage when produced in excess.¹³ Additionally, this inflammatory state generates reactive oxygen species, a type of unstable molecule that can damage DNA, leading to mutations that may turn normal cells into cancerous ones.^3,13

Furthermore, some bacteria have the ability to subtly alter the host's immune system, making it less efficient at detecting and destroying cells that have begun to turn cancerous.^1,14 At the same time, bacterial toxins can interfere with cellular signalling pathways which are like the communication networks of the body, telling cells when to grow and when to die.^1,3 When these pathways are disrupted, it can lead to uncontrolled cell growth, a hallmark of cancer.¹⁴

Over time, this combination of damaged DNA, impaired immune response, and disrupted cellular communications can result in the development of cancer.¹ Thus, the link between bacterial infections and cancer involves a complex interplay of disruption, defence, and unintended consequences of the body's response to infection.^1,14 This highlights the importance of understanding and managing chronic infections to prevent long-term health issues like cancer.¹

Complications caused by bacterial infections in cancer treatment

Bacterial infections pose significant challenges in cancer treatment, not only as risk factors for developing the disease but also as causes of complications during therapy. Cancer patients, particularly those undergoing treatments like chemotherapy or radiotherapy, often experience weakened immune systems.¹⁵ This reduction in immune defence leaves them more susceptible to infections, which can become serious threats and lead to increased mortality risk.^1,15 These complications can disrupt the treatment process, potentially delaying or interrupting scheduled therapies. Therefore, it is important that cancer patients take extra precautions to avoid infections, emphasising the importance of stringent hygiene practices and vigilant infection control in oncology care settings. Managing these infections effectively is crucial to ensure that cancer treatment is both safe and successful.¹

Prevention and early detection

Prevention and early detection are crucial in managing the risks of bacterial infections 

that can lead to cancer. Following treatment guidelines for bacterial infections and adhering to recommended cancer screening protocols are essential. Early identification and treatment through routine screenings can significantly reduce the progression of these infections to cancer.^1,3

For example, stool and breath tests for H.pylori and adhering to cervical screening testing guidelines help detect infections early before they lead to severe symptoms or cancerous changes.^5,7 Proper compliance with prescribed antibiotics is vital to effectively eradicate infections and prevent recurrence, thus minimising cancer risks.¹² Current UK public health guidelines emphasise the importance maintaining hygiene practices to reduce transmission risks.¹⁶ These measures aim to control infections early and reduce their potential to develop into cancer.

Challenges and research gaps

Research into how bacterial infections may lead to cancer faces several significant challenges. Determining the exact mechanisms that result in cancer from bacterial infections is complex.¹⁷ Proving a direct cause-and-effect relationship between specific bacteria and cancer requires detailed, long-term studies.¹⁷ Additionally, individuals respond differently to infections based on their genetics and lifestyle, making it hard to predict who is at risk of developing cancer.¹⁷ Early detection of infections is also challenging, particularly when they might not immediately show symptoms but still pose long-term cancer risks.^1,3 These challenges highlight the complexity of the research and the need for continued innovation and collaboration.

Summary

Overall, studies have revealed that certain bacterial infections can lead to specific types of cancer, shedding light on how our body's response to infection impacts our long-term health. While the risk of developing cancer from an infection is relatively low in Western Europe and the US due to our stringest hygiene practises and advances in modern medicine, it is much higher in developing countries, where infections contribute to one in five cancer deaths.¹⁸ This highlights a global difference, underscoring the importance of improving hygiene and living conditions. Ongoing research and interdisciplinary collaboration continue to enhance prevention and treatment options, offering hope for breakthroughs that promise to improve health outcomes worldwide.