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From 2012.igem.org

Obesity refers to a health condition that body fat is accumulated to some extent. According to WHO, body mass index (BMI) is an index of weight-for-height that is commonly used to classify obesity in adults.

What obesity would cause obesity is a risk factor for various diseases, such as cardiovascular diseases (mainly heart disease and stroke), type 2 diabetes, musculoskeletal disorders (especially osteoarthritis), some cancers (endometrial, breast, and colon).

Figure 1: Prevalence of obesity in different countries.
(Picture from The Wellington Grey blog)

Obesity is most commonly caused by a combination of excessive food energy intake, lack of physical activity, and genetic susceptibility, although a few cases are caused primarily by genes, endocrine disorders, medications or psychiatric illness.
However, the problem of obesity presented several decades ago, which is unlikely that changes in human genome attributes to prevalence of obesity. For most individuals, controlling food intake and doing physical activity in a proper way are effective strategies to lose weight. But for some people whose health condition or current life pace keeps them away from systemic and regular exercise and dieting, modulate the composition of microorganisms in intestine might act as an alternative.
Reports by Gordon have shown that, apart from human genome, the collective genome of microorganisms (microbiome) in human intestine is associated with our obesity [1]. Furthermore, microbiome is able to be changed through control of food intake [1].
Two groups of beneficial bacteria are dominant in the human gut, the Bacteroidetes and the Firmicutes. The relative proportion of Firmicutes is increased in obese people by comparison with lean people [2].

Figure2: How excess of energy contributes to obesity

Pertinent study by Gordon attested their initial hypothesis that changes in microbial component have a causal relationship with obesity, thus might have potential therapeutic implications [2] [3]. Colonization of germ-free mice with an ‘obese microbiota’ results in a significantly greater increase in total body fat than colonization with a ‘lean microbiota’ [3].
Figure from reference [3]

Previous insights construct our theoretical fundament. Instead of passive alternation of microbiota, we are trying to construct an engineered E.coli----- E.coslim to positively change microbiota in intestine. As Figure 2 shown, we place E.coslim in the role of consuming excessive energy, thus leads to the double effects: lowering the proportion of Firmicutes and increasing that of Bacteroidetes, and decreasing the energy available in one’s intestine.
To achieve these two goals, we designed three devices, consumption of fatty acids, cellulose synthesis, and colonization of E.coslim.

References:
[1] Ruth E. Ley1, Peter J. Turnbaugh1, Samuel Klein1 & Jeffrey I. Gordon1 Microbial ecology: Human gut microbes associated with obesity. Nature 444, 1022-1023 (21 December 2006)
[2] Peter J. Turnbaugh1, Ruth E. Ley1, Michael A. Mahowald1, Vincent Magrini2, Elaine R. Mardis1,2 & Jeffrey I. Gordon1 An obesity-associated gut microbiome with increased capacity for energy harvest. Vol 444|21/28 December 2006| doi: 10.1038/nature05414
[3] Ley RE. Obesity and the human microbiome. Curr Opin Gastroenterol. 2010 Jan; 26(1):5-11.