Relative carriage rates of nuclear dehydrogenating clostridia in two populations of different colorectal cancer risk

Role of bacteria in oncogenesis

Although scientific knowledge in viral oncology has exploded in the 20th century, the role of bacteria as mediators of oncogenesis has been less well elucidated. Understanding bacterial carcinogenesis has become increasingly important as a possible means of cancer prevention. This review summarizes clinical, epidemiological, and experimental evidence as well as possible mechanisms of bacterial induction of or protection from malignancy.

A prospective study of faecal bile acids and colorectal cancer

A prospective study of 7079 people aged 45-74 recruited through general practices in South Wales, Herefordshire and Edinburgh, Scotland was undertaken to test the hypothesis that faecal bile acids are implicated in the causation of large bowel cancer. The population was recruited between 1974 and 1980 and the response rate for stool collection was 67%. Bile acid analyses were performed on those cases that presented by 1990. It was decided in advance to examine the hypothesis separately for left- and right-sided bowel cancer because of known epidemiological differences between the two sites and to exclude the cases presenting within 2 years of the stool sample from the analyses because the cancer could have been present at recruitment and might have possibly affected faecal bile acid concentrations. Each case (n = 51 left-sided and 8 right-sided) was matched with three controls by age (within 5 years), sex, place of residence and time of providing the stool sample (within 3 months). Statistical analyses using conditional logistic regression showed no significant differences between the left-sided cases and controls for any of the concentrations of individual bile acids, total bile acid concentrations, faecal neutral steroids, percentage bacterial conversion and the ratio of lithocholic acid to deoxycholic acid concentrations. There was a statistically significant (P = 0.021) association of the presence of chenodeoxycholic acid (5/8 samples) in the right-sided cases compared with the controls (3/23), odds ratio 6.26 (95% confidence interval 1.19, 32.84). A high proportion of primary bile acids has also been found in other studies of patients with a genetic predisposition to proximal bowel cancer, however this pattern may also occur in low risk groups, such as Indian vegetarians, suggesting that they may predispose to right-sided bowel cancer only in the presence of other, as yet unknown factors. If bile acids are involved in the causation of large bowel cancer, they may be part of a complex set of interacting factors.

Bacterial infection as a cause of cancer

Bacterial infections traditionally have not been considered major causes of cancer. Recently, however, bacteria have been linked to cancer by two mechanisms: induction of chronic inflammation and production of carcinogenic bacterial metabolites. The most specific example of the inflammatory mechanism of carcinogenesis is Helicobacter pylori infection. H. pylori has been epidemiologically linked to adenocarcinoma of the distal stomach by its propensity to cause lifelong inflammation. This inflammation is in turn thought to cause cancer by inducing cell proliferation and production of mutagenic free radicals and N-nitroso compounds. H. pylori is the first bacterium to be termed a definite cause of cancer in humans by the International Agency for Research on Cancer. Mutagenic bacterial metabolites are also suspected to increase risk for cancer. This model is best exemplified in colon cancer. Bile salt metabolites increase colonic cell proliferation. Exogenous compounds such as rutin may be metabolized into mutagens by resident colonic flora. Moreover, Bacteroides species can produce fecapentaenes, potent in vitro mutagens, in relatively high concentrations. In vivo data on human carcinogenesis by bacterial metabolites, however, are inconsistent. Local bacterial infections may also predispose to nonnodal lymphomas, although the mechanisms for this are unknown. Gastric lymphomas and immunoproliferative small intestinal disease have been most strongly linked to underlying bacterial infection. Because bacterial infections can be cured with antibiotics, identification of bacterial causes of malignancy could have important implications for cancer prevention.

Breath methane excretion and intestinal methanogenesis in children and adults in rural Nigeria

Breath methane excretion was measured in 274 healthy subjects from 2 rural communities in northern Nigeria. Studies in 24 adults showed a normal faecal flora with no enteric pathogens. Breath methane was detected in 122 (77%) of 159 adults, 19 (40%) of 47 older children (2-6 years), and 4 (8%) of 68 young children (< 2 years). Women were slightly more commonly breath methane-positive than men (82% versus 75%, respectively). Hyperventilation did not influence the specificity of the breath methane assay, although levels were circa 30% lower after deliberate hyperventilation. Methanogens were estimated by enrichment culture of faeces from 49 subjects. Of the subjects 76% had faecal methanogens estimated at > or = 10(2)/g, 45% at > or = 10(4)/g, and 16% at > or = 10(6)/g. There was no significant difference in distribution of methanogenic cultures between different age or tribal groups, and there were no obvious correlations between breath methane excretion and either the faecal carriage of methanogens per se or numbers present.

Characterization of cytotoxic steroids in human faeces and their putative role in the etiology of human colonic cancer

It has been shown previously that chemically induced nuclear abeerrations in the murine colon are correlated with the carcinogenicity of the respective chemicals. Consequently, the nuclear aberration assay was utilized for the identification of putatife carcinogens in human faeces. Human fecal samples were fractionated by several chromatographic methods, and the assay led to the isolation of two substances. A combination of spectroscopic (mass, nuclear magnetic resonance, ultraviolet, and infrared) and chromatographic (HPLC and GLC) methods showed that they are 5-alpha- cholestan-3-one (I) and cholest-4-en-3-one (II). A number of C-27-C-30 steroids isolated from closely related fractions of feces were inactive in this assay. Thus I and II could play a role in etiology of large bowel cancer in humans.

Long-term management of patients who have had urinary diversions into colon

Thirty-four patients with urinary-colonic diversions have been followed up for 13 to 41 years (mean 20.3). The commonest long-term complication was hyperchloraemic acidosis (50%). The most serious was neoplasm at the anastomotic site: benign lesions occurred in three patients and carcinomas in two (15%). Staining for sialomucins in colonic biopsies adjacent to the anastomoses was positive in 17 of 19 patients. It has been suggested that this represents a pre-malignant change. Analysis of faecal flora in 17 diverted patients and 27 controls revealed a significant difference in the carriage rate and viable count of Peptostreptococcus species. This finding has not been reported previously and the species could have a role in the aetiology of the neoplasms.