The team from the Medical College of Georgia have identified a transporter in the colon, called SLC5A8, which plays a role in the final stage of the digestion process, absorbing the nutrients produced by bacteria living in the organ.
In an online accelerated communication in the Journal of Biological Chemistry, they explain how good bacteria in the colon produce an enzyme that breaks down glucose found in the cell walls of vegetables, fruits and cereals, and which cannot be digested in the small intestine. In the oxygen-less environment of the bacteria-packed colon, bacteria ferment this glucose to use for energy which also results in the production of short-chain fatty acids, the preferred nutrients for colon cells.
The researchers have found in both animal and human cells that SLC5A8 is a final piece of the chain, a transporter expressed by colonic cells to absorb the energy-packed, short-chain fatty acids.
"We used to teach that bacteria produced short-chain fatty acids which are used by colonic cells but it was not known that these cells possessed an efficient active transport system to absorb these fatty acids," said Dr Vadivel Ganapathy, the study's principal author.
The finding that SLC5A8 is the transporter helps clarify why fruits and vegetables are beneficial and why antibiotics, which wipe out good bacteria along with bad, should only be taken when absolutely necessary as they upset the model and colonic cells.
"We do not make the enzyme to digest cellulose; bacteria make the enzyme in the colon," said Dr Ganapathy. "Therefore, you need to eat dietary fibre to provide the food for bacteria. Otherwise, they are not going to survive there. Antibiotics can wipe out good bacteria as well, leaving a void where disease-causing bacteria can grow."
Researchers at Case Western Reserve University in Ohio reported in 2003 in the Proceedings of the National Academy of Sciences that they had cloned the SLC5A8 transporter from the human colon. While they knew it was a transporter, they did not know what it transported, but reported instead its function as a tumour suppressor.
"When colon cells become cancerous, this particular transport system gets silenced," Dr Ganapathy says.
Where the transport system is expressed in the body provides clues of what it transports, according to the researcher. The Case Western Reserve research told him the transporter was heavily expressed in the colon, a seemingly odd place because nutrient transport systems are not typically expressed in the colon since digestion and absorption take place almost exclusively in the small intestine.
But he also knew that colon cells need short-chain fatty acids to stay healthy. "Normal colon cells express this transport system so they can make use of the products made by the bacteria. If these essential nutrients do not come in, cells become sick,"he said.
Colleague Dr Robert G. Martindale, a gastrointestinal surgeon with a special interest in probiotics, added: "The gut is a huge immune organ; there are more immune cells in our gut than there are in the rest of the body put together. This [new] work is showing very nicely that if, in fact, we keep this short-chain fatty acid transporter healthy, we then can keep the whole immune system healthy."
Immune cells also have a specific receptor for short-chain fatty acids on the cell surface, and the Georgia team is pursuing the idea that the SLC5A8 transporter is delivering these fatty acids to immune cells to interact with the receptors and keep the immune cells vigilant as well.
They also want to know what happens to SLC5A8 and the receptor when inflammation, such as inflammatory bowel disease, occurs. Dr. Ganapathy hypothesizes that inflammation occurs when something goes awry in the symbiotic relationship between good bacteria and colon cells.