Microbes and obesity--interrelationship between infection, adipose tissue and the immune system.

The world is currently experiencing an obesity epidemic as declared by the World Health Organization. The traditional view is that behaviour leading to overeating and under-activity is the major contributing factor for this worldwide epidemic. However, several microbes are linked to obesity in animals and humans. On the one hand, various microbes, including animal and human viruses, bacteria, parasites and scrapie agents, increase adiposity in several animal models. Some of these microbes show an association with human obesity, but conclusive evidence for a causative role of microbes in human obesity is lacking. On the other hand, obese individuals show an altered response to infections. Obesity is often associated with impaired immune function, which may lead to increased susceptibility to infection with a number of different pathogens. Hence, certain microbes appear to induce obesity, whereas, obesity itself may exacerbate certain other infections. Linking the two phenomenon is the immunological property of adipocytes and their progenitors. For instance, proliferating pre-adipocytes share embryonic origin with immune cells and exhibit phagocytic activity. Taken together it appears that there is a close interrelationship between adipose tissue, inflammatory response, immune system and infections. Hence, it is conceivable that in response to certain infections, adipose tissue expands similar to the expansion of cells of the immune system. The impaired immune function of adipose tissue in obesity may exacerbate infections. Considering the global obesity epidemic, it is necessary to further investigate both phenomena.