Several lines of evidence pointed to sulphur metabolism, being implicated in the aetiology of mucosal injury in IBD. A prime example is the use of sulphated polysaccharides, to induce features of colitis in animal models (451).
First Roediger and colleagues found that fatty acid oxidation at the epithelium lining could be hampered by a range of sulphur containing compounds. Among those tested, sodium hydrogen sulphide selectively reduced fatty acid beta oxidation at the level of short chain acyl dehydrogenation (407;452) and caused greatest mucosal injury. Based on these observations a hypothesis was proposed that cellular inhibition of butyrate by sulphides can induce energy deficient state in colonocytes resulting in mucosal inflammation and injury.
An extensive literature review was carried out to identify studies linking IBD with sulphide metabolism. The retrieved studies in IBD were categorized into the following four categories.
1. Prevalence of Sulphate Reducing Bacteria (SRB) in UC and comparison with healthy subjects.
2. Luminal concentrations and production rate of sulphide in UC and comparison with healthy controls.
3. Functionality of sulphide detoxification pathways in IBD. 4. Pharmaceutical intervention and sulphide metabolism.
Prevalence of SRB in UC and comparison with healthy subjects
Increased numbers of SRB bacteria in faecal samples of UC patients have been reported in some studies. Loubinoux et al (453) using cultivation techniques on selective growth media linked with multiplex PCR found that SRB were more frequently observed in IBD patients (68%) than in healthy controls (24%) or patients with other GI disorders (37%). Moreover different SRB strains were observed between healthy and IBD subjects suggesting strain specific differences between IBD and healthy controls. In particular the prevalence of
Desulfovibrio Piger was significantly higher in IBD (55%) compared with healthy individuals
(12%) or patients with other GI disorders (25%). Association of SBR counts with disease activity was reported by one study (454), which found that total viable counts of SRB were increased in patients with clinical and endoscopic active disease activity compared with patients with quiescent disease (454).
On the other hand recent studies using robust molecular biology techniques of bacterial community analysis failed to confirm the results from bacteriological studies of increased growth or number of SRB counts. For example Fite et al detected SRB in all
105
mucosal samples of IBD patients and healthy individuals using molecular microbiology techniques (455).
Sulphide concentration and production
There is inadequate evidence to suggest that luminal sulphide concentrations are different in IBD patients. A group from the University of Dundee, Scotland, showed that in untreated UC patients faecal sulphide excretion was higher than in healthy controls and treatment with 5- ASAs lowered both concentration and production of sulphide (454;456). On the contrary comparable free and total faecal sulphide concentrations in IBD and healthy controls were presented by others (406;457). No association between luminal sulphide concentration and disease activity or location have been found (457).
Compared with healthy subjects, a two to threefold increase in sulphide production rate has been found in faecal and mucosal samples of UC patients (458;459). Addition of various organic sulphur containing substrates (mucin and taurocholate) increased hydrogen sulphide release much more readily than sulphate intake implicating the action of other protein fermenting bacteria in the production of sulphide over and above the SRB. Increased production of sulphide has been reported also in pouchitis, an inflammatory condition of the ileal-anal pouch of UC patients (460).
Different form of sulphide, different effect?
Only a few studies have tried to account for the origin of the measured concentrations of faecal sulphide in samples from IBD patients. Sulphide occurs in two forms in the intestinal tract, free as hydrogen sulphide and bound to divalent metal ions. Differences in the toxicity of the two forms and failure to distinguish between them in the studies might explain some of the discrepancies reported above. Jorgensen & Mortensen (406) hypothesized that the binding ability of faecal material to sulphide was low in UC compared with healthy controls but they failed to prove this.
Functionality of sulphide detoxification pathways in IBD
Faecal sulphide concentrations represent a balance between production and removal of this toxic metabolite. Epithelial enzymes react with this and produce other sulphur containing compounds which are less harmful for the colonic mucosa (461;462). Thus elevated concentrations of sulphide could be a secondary result of defective detoxification mechanisms in the large bowel (463) although the evidence so far is circumstantial and not supportive (462).
Pharmaceutical intervention and sulphide metabolism
In view of a possible causative role of sulphide in IBD mucosal injury, a few studies investigated whether the action of mainstream IBD drugs is mediated through changes in the metabolism of sulphide in the gut. Among them, 5-ASAs such as sulphasalazine reduced the production of sulphide but not the growth of SRB in in-vitro experiments (454;456) and subsequently differences in the concentration of faecal sulphide were observed between patients on treatment with 5-ASAs and those not. Moore et al (457) however did not confirm these results and indeed found comparable sulphide concentrations between patients on 5- ASAs and others on different medical regimes. Difference in disease activity between these studies may explain this discrepancy.
The results of the effect of metronidazole and ciprofloxacin, two commonly used antibiotics, on the production of sulphide and SRB counts have been contradictory (460;464). Recently prebiotic ingestion reduced hydrogen sulphide concentrations but not SRB counts in healthy volunteers and may thus have a putative place in the treatment of patients with UC (464). Other new agents, like bismouth based derivatives, have also been proposed for use in IBD to inhibit the growth of SRB and suppress production of hydrogen sulphide (460).
Hydrogen Sulphide in IBD. History Revised?
Despite the evidence that hydrogen sulphide can be a toxic mediator in gut health, recent studies found paradoxical actions depending on its concentration and the circumstances in which it is generated. For example, hydrogen sulphide may play a role in protecting gastric mucosal tissue from injury and exerts anti-inflammatory actions, and potentially inhibits leukocyte adherence to the vascular endothelium (465). Fiorucci and his colleagues (466) in a recent study compared the regular form of mesalamine with a new hydrogen sulphide releasing derivative and found that the latter was more effective than mesalamine in reducing the severity of animal colitis. In particular the new derivative was more effective in reducing granulocyte infiltration into the colonic tissue, and furthermore reduced the expression of mRNA for several key proinflammatory cytokines and chemokines.
Effect of dietary sulphate and sulphur containing nutrients on sulphide
production
Diet is a strong determinant of faecal sulphide concentration (467). A diet rich in sulphur containing amino acids increases the production and excretion of sulphide in a dose dependent manner (404) by the action of protein fermenting bacteria. Similarly ingestion of sulphate products increased generation of sulphide by the action of SRB in the colon in one study (468) but not in a rat model given sulphate supplemented drinking water for one year
107
(469). An association with dietary sources of sulphur may justify discrepancies found between studies, such as the elevated levels of sulphide previously reported by Pitcher (454) in active UC but not by Moore. The high sulphide concentrations observed in the first study could actually be an epiphenomenon, related to sulphur amino acid fermentation of the mucus and excessive sloughed cells in active disease or increased dietary intake of sulphur containing food that has been shown to precede disease exacerbation (470).
Effect of transit time on sulphide production
Gut transit time can also affect sulphide production and comprise a confounding factor in the interpretation of sulphide’s role in health and disease. Concentration of faecal sulphide and reduction rates of faecal sulphate are increased in faster intestinal transit times and are decreased in slower transit (468) and may explain any differences between active and inactive disease.