Gut reaction in a complex ecosystem

Richard Collins delves into the poorly understood world of gut bacteria

OUR poor tummies are getting back to normal after the punishing festival ordeal. Digesting so much food is no mean feat, but most of us take the great binge in our strides.

We don’t manage it on our own, however; a huge retinue of devoted servants, the gut bacteria, help us out. These creatures live in a mysterious and poorly-understood world deep inside us — the realm of inner space.

The human body is an ecosystem, as complex and diverse as the Amazon rainforest. Each gut has a unique collection of species, some of which go through a million generations in a human lifetime and evolve into forms which differ from person to person.

Of the six billion people alive today, no two have the same range of bacteria. Cosmologists speculate about parallel universes, inaccessible to each other. Every large creature on Earth supports an isolated internal universe.

So what happens when a juicy morsel takes the fairground ride through the gut? Teeth and lubricating saliva reduce the food to a sticky paste, which slides down into a vat of concentrated acid. A valve keeps the food inside the stomach, although, in an emergency, we can vomit it back up.

Not every creature has this option; horses and rats can’t regurgitate their food. The two litres of acid produced by a human stomach every day reduce the food to a gastric soup, from which nutrients are absorbed in the next section of the canal, the small intestine.

We humans are better digesters than birds, but we’re amateurs compared to the plant-eating hoofed animals.

Our stomachs have a single chamber, whereas deer and cattle have four.

Plants, unlike animals, can’t run from their enemies. They rely on armour and poisons for protection. The tough cellulose walls of plant cells are extremely difficult to breach.

A cow’s first chamber, the rumen, holds about 250 litres of food. Bacteria and single-celled animals in this fermentation tank begin breaking down the cellulose. The mixture is sent back to the mouth to “chew the cud”, then passed to the next two chambers for further processing.

Gastric juices are added in the fourth stomach. Converting plant to animal tissue is a difficult operation and the guts of herbivores must be long to manage it.

Meat-eaters convert one form of animal tissue into another, a relatively straightforward task, so carnivores get by with short food passages.

The human small intestine, about six metres long, is a compromise to suit our omnivorous tastes. Nutrients are absorbed through its walls and carried off in the bloodstream. The next section, the large intestine, is bulky but only a metre long.

Where the two intestines meet there is a pouch, a cul-de-sac known as the caecum. Meat-eating animals don’t need one; stoats and badgers, for example, have none.

The caecum seems to be of limited importance in humans and the appendix attached to it totally superfluous, but this bacteria-filled chamber is vital to horses, zebras and tapirs.

In the caecum, towards the end of the food canal, they do battle with the cellulose. Whereas deer and cows are “front-end digesters”, the caecum-users are “rear-enders”.

The front-end approach is more efficient; dead bacteria from the early stomachs pass into the small intestine where most of the nutrients and energy locked up in their corpses are absorbed. With rear-enders, the bacterial culture is beyond the small intestine and the dead bacteria are, wastefully, expelled from the body.

Some odd-toed rear-end creatures, such as rhinos, may be losing out in the evolutionary struggle to the even-toed front-end deer and antelope.

Hares and rabbits are also rear- enders with large bacteria-filled caecums, but their limited size presents problems. Vegetarian digestion is slow; food takes over three days to pass through a cow.

For hares and rabbits, which have to be light and nimble to escape their enemies, being bloated with half-digested food is not an option. Nor is there room in their bodies for long intestines.

They have a simple, if revolting, solution — eating their own droppings. They pass the food through their bodies twice, not only extracting most of the food’s nutrients, but also re-absorbing the bodies of their gut bacteria.