At a physical level, "the human body" is not a single organism. A human being operates a body which is a co-operative enterprise consisting of human cells plus bacteria, viruses, even parasites, each part of which contributes to the survival of all. Science has discovered that disruptions in the ecosystem have major effects on "mental health".

In addition to that physical aspect of a person–the various molecules and cells–there is the mind, which consists of a form of organized energy, and which interacts with the body in many ways. Body imbalances can influence mental states, causing anxiety, depression and other issues labeled as "mental illness", for which the usual treatments are drugs (which carry their own risks).

Exposure to antibiotics, chlorine and other chemicals in air, water and food can suppress the probiotics (friendly bacteria) in the gut, disrupting gut balance and health. The result is mental stress of various kinds.

Have you ever had to take antibiotics for a while and felt sort of "off" or "strange" or depressed or anxious? I know many who have experienced that. Some people feel depressed or unhappy all the time because they have never really enjoyed good gut health in the first place!

Recent research has revealed a definite connection between gut health and mental health. Multiple strains of probiotics work together to improve mental states, and deficiencies of these probiotics worsen mental states. 

Many of our great grandparents fermented and consumed their own foods, such as sauerkraut. The probiotics in such foods were actually maintaining their gut health. The toxic environment is quite literally driving the world mad!  If you are looking for a good probiotic here is a great one called Prosymbiotic from Standard Process.

How Lactic Acid Bacteria (LAB) Support the Gut Microbial Environment

The LAB in ProSynbiotic include: Bifidobacterium lactis, Lactobacillus acidophilus, and Lactobacillus paracasei. While lactic acid bacteria (LAB) make up a relatively small fraction of the native human gut microbiota, they are widely held as excellent probiotics because they can benefit the gut microbial environment through multiple mechanisms. [9, 10]

B. lactis is a bacterium found in the colon. In a study that looked at 4 weeks of supplementation with 1 billion CFU/d of Bifidobacterium lactis, it was shown to support normal bowel regularity [14].

L. acidophilus is a bacterium found in the intestine where it ferments glucose into lactic and acetic acids[15]. These substances make the gut environment more acidic and help balance the microbiome. 

L. paracasei is a bacterium found in the gut where it produces lactic and acetic acid. Some strains have been shown to break down prebiotics such as inulin to produce SCFAs.

How Probiotic Yeast Supports Healthy Immune System Response

S. boulardii is a probiotic yeast that can support proper intestinal permeability. In recent studies, support for proper intestinal permeability was observed in patients given Saccharomyces boulardii at a dose of 1.6 × 109 CFU/d for 4 months[16, 17]. Additionally, oral intake of S. boulardii has also been associated with positive microbiome/immune system benefits based on microbiome composition[18]. 

ProSynbiotic includes two billion CFU of S. boulardii per serving, consistent with evidence that supports a potentially beneficial effect on immune system response function and supporting normal bowel regularity. [14, 16-18].