How does our Western diet influence Crohn's?

"Dietary-fat-induced taurocholic acid promotes pathobiont expansion and colitis in Il10−/− mice", Devkota et al. in Nature (2012) Preview"The composite human microbiome of Western populations has probably changed over the past century, brought on by new environmental triggers that often have a negative impact on human health1. Here we show that consumption of a diet high in saturated (milk-derived) fat, but not polyunsaturated (safflower oil) fat, changes the…"

Milk fat found in junk food may be to blame for dramatic rise in IBS cases

"Concentrated milk fat is a powdery substance that is abundant in processed foods and confectionery.
...
Prof Chang's team found that milk fat, but not polyunsaturated fats, caused an explosion in the numbers of a normally rare gut microbe called Bilophila wadsworthia.

The bugs were almost undetectable in mice on low-fat or unsaturated fat diets, but made up about 6% of all bacteria in the guts of mice fed milk fat.  Bilophila wadsworthia has an affinity for bile, which is released into the intestines to help break down fats.
...
Milk fats are unusually difficult to digest, requiring the liver to produce bile rich in sulphur.

Since B. wadsworthia thrives on sulphur, this fuels its population growth.

'Unfortunately, these can be harmful bacteria,' Prof Chang said.

'Presented with a rich source of sulphur, they bloom, and when they do, they are capable of activating the immune system of genetically prone individuals.'

By-products of the bugs' interaction with bile enhance the effect by making the bowel more permeable, the scientists said.

Immune cells then find it easier to infiltrate the bowel wall and damage tissue.
...
Prof Chang added: 'Right now we can't do much about correcting genes that predispose individuals to increased risk for these diseases, and while we could encourage people to change their diets, this is seldom effective and always difficult." [Emphasis mine.]

Western diet changes gut bacteria and triggers colitis in those at risk

"Bacteria isolated from surgical infections and its susceptibilities to antimicrobial agents--special references to bacteria isolated between April 2009 and March 2010" in The Japanese Journal of Antibiotics (2011)
"We should carefully follow up B. wadsworthia which was resistant to various antibiotics...."

"Diversity and distribution of sulphate-reducing bacteria in human faeces from healthy subjects and patients with inflammatory bowel disease" in FEMS Immunology and Medical Microbiology (2012)
"The relative abundance of different groups of sulphate-reducing bacteria (SRB) in faecal DNA collected before and after therapy from patients suffering from Crohn's disease (CD), irritable bowel syndrome (IBS) or ulcerative colitis (UC) has been compared with that from healthy controls. Growth tests revealed that SRB were not more abundant in samples from patients with CD before treatment than in the healthy control group. ... However, some samples from patients with CD before treatment contained a growth inhibitor that was absent from IBS or UC samples. In-depth sequencing of PCR-generated dsrB fragments revealed that the diversity detected was surprisingly low, with only eight strains of SRB and the sulphite-reducing bacterium, Bilophila wadsworthia, detected above the 0.1% threshold. The proportion of the two major species detected,B. wadsworthia and Desulfovibrio piger, was as high as 93.5% of the total SRB population in the healthy control group and lower in all patient groups."