Author(s): Do-Young Park 1 , Young-Tae Ahn 1 , Se-Hoon Park 1 , Chul-Sung Huh 1 , * , Sae-Rom Yoo 3 , Rina Yu 2 , 4 , Mi-Kyung Sung 2 , 5 , Robin A. McGregor 2 , Myung-Sook Choi 2 , 3 , *
The gastrointestinal tract in an adult human contains approximately 10 12 microorganisms per milliliter of luminal content and harbors approximately 500 to 1000 distinct bacteria species  collectively termed the microbiota. The gut microbiota plays an important role in the innate immune system and host metabolism-. There exists conflicting evidence whether the gut microbiota plays a role in obesity. Bäckhed et al.  observed that Germ-free (GF) B6 mice fed a chow diet appeared to be protected from excessive fat accumulation compared to conventionalized mice fed the same diet in both males and females. When GF animals were fed a Western-style, high-fat and sugar-rich diet, they appeared to be protected from diet-induced obesity . However, another study reported that the absence of gut microbiota did not provide general protection from diet-induced obesity . Mestdagh et al.  also reported that the total body fat content of GF C3H/Orl female mice fed a standard chow diet was not significantly different from that of conventional female mice fed the same diet. Therefore, the protection of GF mice from obesity appears to be dependent on diet and animal strain. Nevertheless, diet-induced obesity is reported to be associated with marked but reversible alterations in the mouse gut microbiota . Hence the gut microbiota represents a therapeutic target with the potential to reverse existing obesity.
Probiotics consist of individual or multiple live bacteria species, which directly alter the gut microbiota, such as lactobacilli and bifidobacteria , . Multiple in-vivo studies provide evidence that some probiotics can reduce diet-induced obesity in rodents-, although there are reports of probiotics with no effect on body weight gain  or in some cases probiotics that actually cause weight-gain in rodents . While many studies indicate probiotics intake causes functional changes, such as lower blood lipids in hyperlipidemic animals , evidence is lacking for the impact of probiotics containing individual or multiple bacterial species on gut microbiota diversity and composition of obese animals.
Importantly, different probiotic strains may have varying functional effects on the gut microbiota and obesity . The apparent lack of microbiota changes in response to probiotics in some studies may be partly due to inter-individual variability in microbiota composition caused by genetic background, age, diet, or other environmental related factors. Some studies suggest a subset of microbial species appear to more widely spread colonizers of the human gastrointestinal tract, although no species appear to be universally present in all individuals , , while other studies suggest a common microbiome at the gene level may be shared between individuals , . Pre-clinical models of diet-induced obesity in mice provide a useful way to assess physiologically relevant gut microbiota changes associated with both obesity and probiotic treatment, while controlling for the...