Bacteria as causes of cancer
Bacteria have been implicated as potential contributors to certain types of cancer, primarily through chronic infections that can alter cellular function and promote tumor development. Notably, Helicobacter pylori is linked to gastric cancer, while Salmonella enterica serovar Typhi is associated with gallbladder cancer. These bacteria can cause long-term infections, allowing their toxic metabolic byproducts to damage the host's cells, much like traditional carcinogens. Other bacterial species, such as Streptococcus bovis and Chlamydophila pneumoniae, have also been linked to colorectal and lung cancers, respectively.
Exposure to these bacteria typically occurs through ingestion or inhalation, with contaminated food, water, and surfaces serving as common sources. Factors like genetic susceptibility, chronic infections, and dietary habits can increase the risk of developing cancer associated with these bacteria. For instance, a diet high in salted and pickled foods may elevate the risk of gastric cancer in infected individuals. The relationship between bacteria and cancer is complex, and while research has established correlations, the precise mechanisms and risk factors continue to be an area of active investigation. Understanding these connections is crucial for early detection and potential preventive measures against these cancers.
Subject Terms
Bacteria as causes of cancer
ALSO KNOWN AS: Helicobacter pylori, Salmonella enterica subsp. enterica serovar Typhi, Streptococcus bovis, Chlamydophila (previously Chlamydia) pneumoniae
RELATED CANCERS: Gastrointestinal cancers, lung cancer
DEFINITION: Although no bacterium is responsible for causing cancer, solid evidence implicates two types of bacteria in the process of cancer formation, and less convincing evidence implicates others. Each of these types of bacteria has been shown to cause a persistent (chronic) infection in which the organisms are in contact with the tissue for a long time. During that time, genes for cell division and other cell maintenance functions, the same ones that are damaged by other known chemical and physical carcinogens, are damaged by the metabolic and toxic products of these bacteria. Equally important are immune system defenses and additional genetic and environmental factors that make only specific individuals susceptible to these organisms. The two types of bacteria are Helicobacter pylori, associated with gastric cancer, and Salmonella enterica subsp. enterica serovar Typhi, associated with gallbladder cancer. Additional bacteria probably associated with different cancers include Streptococcus bovis (colorectal cancer), Chlamydophila pneumoniae (lung cancer), certain strains of Escherichia coli (colorectal cancer associated with Crohn's disease and ulcerative colitis), and different types of oral streptococci (oral cancers).
Exposure routes: Exposure routes to these bacteria primarily include ingestion and inhalation, which are important factors to consider in understanding the potential sources of infection and the associated risks.
Where found: Infected and uninfected persons carrying certain bacteria; contaminated water, air, and surfaces
At risk: Genetically susceptible individuals specific to each type of bacteria
Etiology and symptoms of associated cancers:
The etiology and symptoms of cancers associated with H. pylori and Salmonella Typhi are important to understand for early detection and prevention. For instance, cancer caused by H. pylori is characterized by tumors in the protective layer of mucus in the stomach, including the immune tissue. The continued high incidence of stomach cancer in some regions of Asia suggests that a diet high in salted, smoked, and pickled foods is a contributory factor. Transmission of H. pylori is thought to be by the fecal-oral route.
The second strongest case for bacterial causation of cancer involves Salmonella Typhi and its role in causing cancer of the gallbladder. Since the famous case of Typhoid Mary, it has been known that survivors (or carriers without symptoms) of typhoid fever carry Salmonella Typhi for at least a year and can transmit the pathogen in their feces. The organism resides in the gallbladder during that time, thus fulfilling one of the conditions necessary to cause cancer: long exposure. Cancer formation in the gallbladder is frequently associated with the formation of gallstones, solid deposits of cholesterol, or calcium salts. Symptoms that may indicate a diseased gallbladder include chronic indigestion, upper abdominal pain, nausea and vomiting, and fever. All of these symptoms may also be associated with additional gastrointestinal problems, so they are not definitive for gallbladder problems, including gallbladder cancer. Associated risk factors include:
- Cholelithiasis (especially untreated symptomatic chronic gallstones).
- Obesity.
- Reproductive factors.
- Environmental exposure to toxic chemicals.
- Congenital malformation of bile ducts.
- Chronic infections affecting the gallbladder.
The most significant risk factor for those cancers associated with Salmonella Typhi appears to be chronic gallbladder infection. Data show that patients with chronic infections with this organism are eight times more likely to have cancer of the gallbladder than those who had acute infections after which the microorganisms were cleared from their bodies. Genetic susceptibility of North American Indians, Mexican Americans, and women of all populations to gallbladder cancer suggests that these groups are predisposed to develop it primarily or secondarily due to an increased incidence of cholesterol gallstones. A secondary pathway of cancer development, which involves congenital malformation of the bile duct, is seen primarily in people from Japan, Korea, and possibly China.
History: The first indication that a relationship existed between bacteria and cancer came in 1951 when researchers found that patients frequently had both infective endocarditis (infection of the heart) and colonic carcinoma. It took until 1974 to identify S. bovis as the bacterium associated with colonic cancer and bacteremia (bacteria in blood). It is now known that S. bovis is a natural inhabitant of the human gastrointestinal tract and causes both endocarditis and bacteremia. Patients with colonic carcinoma frequently also harbor S. bovis, and these cancerous lesions may develop years after having endocarditis or bacteremia.
Further research has focused on the bacteria naturally found in the gastrointestinal tract and their ability to metabolize nutrients into carcinogens. Experiments done using gastrointestinal flora from rats determined that their metabolic products are mutagenic, meaning that they cause mistakes in deoxyribonucleic acid (DNA). The fact that germ-free rats did not develop tumors as frequently as normal animals was also significant in defining the relationship between diet and gastrointestinal flora.
Data from later experiments have suggested that the involvement of microorganisms as a causative agent for cancer formation may be more complicated than first thought. Reports indicate that while the presence of H. pylori is associated with the development of certain gastric cancers, specifically non-cardia gastric cancers (in the lower part of the stomach), it may actually lower the risk of esophageal cancer as well as cardia gastric cancer (in the top inch or so of the stomach, where it meets the esophagus).
The ability to use the presence of microorganisms to indicate that cancer may also be present would be a significant medical advance in situations where the tumor is not easily diagnosed at an early stage. There is increasing evidence to link the presence of certain oral bacteria, such as Porphyromonas gingivalis and Fusobacterium nucleatum, with the development of oral tumors, most commonly oral squamous cell carcinomas. However, the precise relationship between microorganisms and different types of cancer is still unclear and remains to be determined.
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