Sulphur loving bacteria

Next time you are walking along the muddy shore of an estuary, don't just complain about the smell of rotten eggs which is so characteristic of these areas. Think about why it is there. It is caused by sulphate-reducing bacteria which live in oxygen-free (anaerobic) sea sediments. They are able to use the hydrogen which is released by the decay of millions of micro-organisms and the sulphate naturally present in the seawater.

How are these bacteria potentially linked to problems in our guts? The final part of our gut, the distal colon, is almost completely anaerobic and it is here that the final part of digestion and absorption takes place, carried out by bacteria which are normal inhabitants of our bodies and not harmful. However, if some of these bacteria are replaced by sulphate-reducing bacteria this may be where the problems start, according to a recent article in the British magazine, New Scientist.

How do we land up with sulphate-reducing bacteria in our guts? All that is needed is a good supply of sulphur containing compounds, and there are plenty of those around in the modern diet. Large amounts of animal proteins and processed food will do the trick. Most processed foods contain sulphur-based preservatives, as well as most beers and wines. In fact some American salad bars had to reduce the amount of sulphur they used as preservatives for their salads because of complaints by those whose asthma was exacerbated.

Methane producers and non-methane producers

How was this possible link between sulphate-reducing bacteria and gut problems discovered? Strangely enough, by two estuarine ecologists, who realised that there may be a link between the anaerobic conditions in estuaries and those in our distal colons. Working with researchers in Cambridge they set up a series of experiments to measure the types of gases given off by people who belch and fart. Normal gut bacteria produce hydrogen, nitrogen and carbon dioxide, odourless and generally harmless. However, volunteers in the Cambridge experiments produced surprisingly little hydrogen, suggesting that something in the gut was using up the hydrogen, and some people produced a lot of methane while others produced little or none at all.

Methane produced by the gut can only come from methane-producing bacteria, methanogens. These bacteria also use up hydrogen which would account for the low levels of hydrogen found. But when populations in North America and Northern Europe are tested only about half of them have methanogens in their guts. The puzzle is why some people have them and others don't and what is using up the hydrogen if there are no methanogens about?

Sulphate-reducing bacteria were first reported in the human gut in the late 1970s, and it seems that they may be displacing the methanogens and using up hydrogen. High levels of sulphur in Western diets may account for this.

Researchers studied people who were natural methane producers and asked them to eat a diet high in sulphates. Ten days later their breath showed significantly less methane, and what is more, their faeces showed high levels of suphides (the breakdown product of suphates). Further evidence for the role of sulphate-reducing bacteria in methane producion came from studies of rural South Africans, who eat a diet low in suphates, and are virtually all methane producers.

Do sulphur-reducing bacteria affect our health?

Early research on people with ulcerative colitis showed that 96 percent of those with colitis had sulphate-reducing bacteria in their gut compared with 50 percent of healthy people. The bacteria which was most common was that of the genus (type) Desulfovibrio. However, not everyone with this bacteria type in their guts was ill, and ulcerative colitis is a complex disease which affects the whole body via the immune system, so it is difficult to say that the sulphur-reducing bacteria are the sole cause. The gut is host to some 400 species of bacteria, most of which are difficult or impossible to culture in the laboratory and most are harmless. Currently researchers suspect that there is a link between colitis and the presence of Desulfovibrio, but are not convinced that it is the sole cause of the disease.

There is evidence that sulphate-reducing bacteria may damage our gut, however. An Australian surgeon has found that the epithelial cells which line the colon of those with ulcerative colitis are unable to use a fatty acid, butyrate, which is their normal nutrient. This lack of ability to use butyrate seems to be a first step in the development of ulcerative colitis, and is inhibited by exposure to sulphides produced by sulphur-reducing bacteria.

How can we reduce the amount of sulphur in our diets?

A lot more research needs to be done before we fully understand the link between diet, bacteria and disease. Research on rural populations with a low sulphur diet shows that they do not harbour these bacteria, and we know that they are remarkably free of most of the bowel problems which afflict those on a Western diet. So in the meantime it seems sensible to try and leave our bowel flora as free of sulpur-reducing bacteria as possible.

How can we do this?

Meat and other high-protein foods release sulphur-amino acids as they are digested. As meat consumption rises from 60 to 600 grams a day, sulphates in the urine double and sulphides in the faeces increase tenfold. A diet rich in meat has long been implicated in colorectal cancer. It may be that the toxic sulphides released by these bacteria promote cancerous changes in the gut cells by damaging their DNA.

Are vegetarians better off? Vegetable proteins, particularly those in beans and seeds, contain amino-acids with sulphur attached. So why are vegetarians at lower risk of bowel cancer? It may be because plant protein comes in carbohydrate-rich packages and that this combination may make sulphur-amino acids harmless. Carbohydrate speeds up the growth of the bacteria in our guts which use the sulphur-amino acids in making their own proteins, which means that they produce the "bulk" required to speed up the passage of faeces through the gut. Hence the apparent benefits of a high-fibre diet. It is possible that eating lots of vegetables and carbohydrates along with meat will be protective.

Another major source of sulphur in our diet is in food and drink additives. These are collectively called "sulphur dioxide" and consist of a number of different sulphur compounds. They are the major preservative in the West, present in everything from sausages to jam, dried raisins and instant soups. Even fresh foods may be gassed with sulphur dioxide to prolong their shelf life. Soft drinks, wine and beer also contain sulphur compounds. The reason that sulphur dioxide is so widely used is that it is non-toxic, broken down in the liver and kidneys. However, no one has yet measured the amount of sulphur additives which are typically consumed in a Western diet, although there are guidelines for an acceptable daily intake of sulphur-based preservatives. Increasing numbers of people never cook fresh food, relying instead on re-heating convenience foods. So if your diet consists of mainly processed foods washed down with soft drinks, beer or wine, your daily consumption may be well above acceptable levels.

Researchers suspect that inflammatory bowel diseases such as ulcerative colitis, Crohn's disease and the very common irritable bowel disease may be linked to sulphate-reducing bacteria. If a link between gut disease and a dietary source of sulphur is ever scientifically established, it may change the way we think about eating, drinking and cooking.