Routine examinations of the colon including colonoscopy and barium enema widely carried out in clinical practice require evacuation of colonic contents. This is most conveniently carried out using osmotic laxatives the most widely used preparation being based on polyethylene glycol such as Moviprep. When fully made up as 1 litre of Moviprep , it provides 100 grams of polyethlene glycol '3350' with a measured osmolarity of around 530 mosmol/l. Being nonnutrient, this would be expected to rapidly leave the stomach and generate a substantial inflow of fluid in the upper small intestine as water flows down the osmotic gradient markedly increasing the small intestinal water content compared with fasting.
Previous MRI studies using a Mannitol solution of 300 mosmols/l resulted in a small bowel secretion of fluid increasing total small bowel water to nearly 400 mls after a 300 ml meal. The investigators would anticipate a much larger increase after the hypertonic Moviprep. The flush of small bowel fluid induced by mannitol produces a radical change in the heterogeneous structure of the ascending colon as shown in our recent MRI images of the colon before and after mannitol.
Contrary to expectation colonic contents are far from homogenous with a spatially organised bacterial flora which others have described at a microscopic level which the investigators are just beginning to demonstrate using their novel MRI techniques. The colon should be regarded as a complex bioreactor which is spatially highly organised with a mucous layer overlying the enterocytes, the outer layer of mucus containing a germinal layer with bacteria which reseed the bioreactor when it is purged. The rate of recovery after purgation with polyethylene glycol is said to be normally rapid as the intact germinal layer rapidly recolonises but slower in certain patient groups though data is very limited as yet. The anecdotal yet persuasive patient accounts of improvement in bowel function after colonic lavage suggest that the reconstituted microbiota may be different, though this has yet to be studied using modern techniques.
Until recently the investigators have had no way of noninvasive imaging this complex structure but recent developments of high resolution MRI in Nottingham show that this is now feasible. Pilot data demonstrates that purging removes the heterogeneous three dimensional structure leaving homogeneous fluid contents with an intense proton signal. Just how this impacts on the colonic microbiota is as yet unknown as there have been no studies to date combining these novel approaches with new techniques now available for assessing the microbiota.
Early culture-based assessments identify only a minority of all the faecal organisms present and it is only in the last decade that methods based on assessment of microbial DNA have evolved to make it possible to describe the full complexity of the faecal microbiota. PCR of the highly conserved 16s ribosomal RNA gene shows the microbiota of individuals to be highly complex and individualised. Similarity indices show similarities of around two thirds over a 7 week period in healthy controls while those developing acute diarrhoea due to radiation enteritis show a marked reduction to just 26%. Similarly, HITChip analysis showed that subjects with IBS had an unstable microbiota that was stabilised following a probiotic intervention trial that improved the IBS symptom score. These techniques have yet to be applied to subjects undergoing bowel purgation. Moreover, there are novel approaches to identify the viability of the microbiota such as the use of specific probes that allow discrimination between intact, damaged and dead cells in fecal samples.
