The Role Of Enterobacter Cloacea In The Development Of Human Metabolic Syndrome

Obesity and related metabolic diseases have become a worldwidedevastating epidemic in recent decades. This epidemic has probably beeninduced by exposure to factors introduced through changes in the diet andlifestyle, triggering aberrant host responses that lead to obesity. Severallines of evidence suggested a role for the gut microbiota in obesity andrelated conditions such as non-alcoholic fatty liver disease (NAFLD).Among the potential mediators of this association, lipopolysaccharide (acomponent of Gram-negative bacterial cell walls) could induce obesitywhen subcutaneously infused into mice and might play a key role in thepathogenesis of obesity and NAFLD. However, a causative role of theLPS-producing bacteria in human obesity remains to be demonstrated.In our present study, we found an overgrowth of a LPS-producingbacterium genus Enterobacter in the gut of a morbidly obese human.After a23-week nutritional intervention with a designed diet composed ofwhole grains, traditional Chinese medicinal foods, and prebiotics, the endotoxin-producing genus Enterobacter reduced from35%of thevolunteer’s gut bacteria to non-detectable, when he lost51.4kg of174.8kg initial weight and recovered from hyperglycemia and hypertension. Adiminution of endotoxin biosynthetic genes in his gut was correlated witha decreased circulating endotoxin load and an alleviated inflammation.We hypothesized that this endotoxin-producing bacteria Enterobacterpopulation may play a causative role in the metabolic deteriorations of itshuman host. To confirm this, via sequence-guided isolation, we obtainedone LPS-producing bacteria B29, which was identified as Enterobactercloacae and used in the subsequent germfree animal trial.Firstly, we showed that B29induced obese and insulin-resistantphenotypes in germfree mice. Colonization of germfree C57BL/6J micewith strain Enterobacter cloacae B29isolated from the volunteer’s gutinduced fully developed obesity and insulin resistance on high fat diet(HFD) but not on normal chow diet. The Enterobacter-induced obesemice showed increased serum endotoxin load and aggravatedinflammatory conditions. It was also found that B29increasedproinflammatory gene expression in the liver and fat pat, suppressed Fiafgene expression in the gut, and promoted expression of Acc1and Fas inthe liver of mice fed on high fat diet but not in those fed on normal chowdiet. This is a direct piece of evidence to show that some LPS-producingbacteria in the gut may work as a contributing factor to obesity development in its human host.Secondly, we demonstrated that the Enterobacter-induced obesemice showed significant NAFLD symptomes. NAFLD was observed inHFD+B29mice, but not in germfree control mice on either high fat ornormal chow diet by liver histology. The NAFLD activity score showedthat HFD+B29mice had more advanced state, nonalcoholicsteatohepatitis. And the results of liver funtional analysis in serum wereagreed with that of liver histology. The concentration of serum lipids wassignificantly increased in Enterobacter-associated gnotobiotic obese mice.Genome-wide expression analysis based on Affymetrix expression chipsshowed that24genes significantly increased their expression and10significantly decreased in the liver of Enterobacter cloacaeB29-associated obese mice compared with HFD germfree controls. Thegene cidea was the most upregulated expression gene, which has beenreported to have closely relationship with lipid metabolism and insulinresistance.We established a gnotobiotic mouse model for obesity and relatedmetabolic conditions such as NAFLD with a human-originated endotoxinproducer in this study. This work demonstrates that theendotoxin-producing bacteria Enterobacter population may play acausative role in the obesity and NAFLD development of its human host.The protocol we established here can be used to identify more such disease-inducing bacteria from various human populations, which mayhelp to gain a better understanding of the molecular mechanisms anddevelop new strategies for reducing the devastating epidemic of obesityand related metabolic diseases.