**5. Impact of different environmental conditions on gut microbiome**

The most important environmental factors that may lead to dysbiosis include (i) Physical or psychological stress, (ii) use of antibiotics, and (iii) diet (**Figure 2**).

#### **5.1 Physical or psychological stress**

Stress is usually defined as homeostasis disruption due to physical, psychological or environmental stimuli known as stressors leading to adaptive behavioral and physiological response in order to restore homeostasis [121]. The effect of both psychological and physical stress on gut microbiome is widely recognized and has been observed in both humans as well as animals [122]. Some research conducted in mice has shown that the microbial composition in the cecum was altered in response to the exposure of a social stressor by placing an aggressive male mouse into the cages of non-aggressive mice. Furthermore, the plasma concentration of stress hormones such as adrenocorticotropic hormone (ACTH) and corticosterone was found to be significantly higher in germ-free mice as compared to specific pathogen-free mice. In addition, several stressors including acoustic stress, self-control conditions and food deprivation have a negative impact on the gut microbiome resulting in the impairment of the immune system [123, 124].

#### **5.2 Use of antibiotics**

It has been observed in both humans and animals that the treatment with antibiotics can result in a decreased population of beneficial bacteria including *Lactobacilli* and *Bifidobacteria* along with the increased population of potential pathogenic bacteria like *Clostridium difficile* and the pathogenic yeast *Candida albicans*. The GI symptoms for example diarrhea, abdominal pain, bloating as well as yeast infections may occur in response to microbial shifts or dysbiosis. However,

**Figure 2.** *Environmental factors influencing gut microbiota.*

more serious and long-lasting consequences have been suggested. For example, it was reported that at the end of a 5-day treatment with the antibiotic ciprofloxacin, most of the gut bacteria was restored to the pre-treatment levels in 4 weeks, but some intestinal bacteria failed to recover even after 6-months. Moreover, a 7-day treatment with clindamycin, a drug of choice for treatment of *Bacteroides* infections, resulted in disrupted gut microbiome for up to 2 years [125].

## **5.3 Diet and obesity**

Food is metabolized by the gut microbial species to extract nutrients, but some microbial species are more efficient in extracting nutrients from food as compared to other species. As different individuals have slightly different microbial populations, it is probable that more nutrients are harvested by some people's gut microbes making them perhaps more prone to become overweight. A high percentage of *Firmicutes* was found in the gut microbiome of genetically obese mice while a high percentage of *Bacteriodetes* were observed in lean mice. Similar observation was reported in lean and obese human volunteers. Moreover, it was also seen that the obese people who used a low-caloric diet to lose weight, their gut microbiota shifted to a similar bacterial population as observed in lean people [125].
