You are not alone

Be assured, not alone at all, indifferent what you wish for. Between you and me and the bedpost, you are even outnumbered. Outnumbered by trillions of tiny individuals living, laughing and eating on and inside you. Actually, the numbers are a little scary: its 10 : 1 microorganisms vs. humans, and, in terms of genes, we are even talking about an incredible 100 : 1 ratio. So, dear advocates of individualism, we are literally more bug than human. And as our little cohabitants have followed us on our journey through evolution, we better continue to cooperate in order to stay healthy. As it is the case with any cooperation, a secret doubt arises: Are we friend or foe? The term microbiome was coined by Nobel Prize laureate Joshua Lederberg and defines the ecological community of commensal, symbiotic and pathogenic microorganisms that share our body space (Lederberg 2016). Usually, the microbiome wants to be our friend and therefore has many things to offer that you might not be aware of. In fact, only recently has the scientific community embraced the crucial part the microbiome plays for our health. It defines or is at least involved in diverse somatic functions: it is part of the barrier system of our skin, and it is involved in food digestion and the production of bioactive compounds. If nothing else, it boosts and supports our immune system from the moment of birth and even prenatally (Dreno et al. 2016, FernandezNavarro et al. 2016, Tamburini et al. 2016). Granted, this is what we expect this intrinsic armada to fight for, but this powerful system is also very susceptible to manipulations, changing former friends into enemies. It was reported that an infection with the protozoan Toxoplasma gondii might be implicated in brain disorders such as schizophrenia and was shown to influence human behaviour (Barragan et al. 2015, Xiao & Yolken 2015). Such an interference is as well conceivable for many different strains of our microbiome. Since the second brain, also referred to as the ‘brain– gut axis’, came into focus of research, more and more unconscious interactions have been decrypted. Depression, obesity, anxiety, emotional processing, cravings, digestion and stress coping, only to mention a few (Farzi et al. 2015, Khan et al. 2015), are influenced by interactions of the brain with the gut (Zakrisson 2015). Functions of the kidneys or the immune system might as well be greatly dependent on the microbiome. Most of our microbiota are found in our digestive system and here is where we find most of the microbiome-associated diseases. The oral application of strains of Lactobacillus reuteri was described to protect mice against colitis (Ahl et al. 2016, Petrella 2016). Moreover, the gut microbiome was proposed to be a pathogenic factor in inflammatory bowel disease (Hirata et al. 2016). Associations with asthma (Singanayagam et al. 2017), neurologic diseases and even HIV-1 pathogenesis (Dillon et al. 2016) were added to the list, which will very likely continue to grow as research continues (Umbrello & Esposito 2016). Once our physiological microbiotic barrier is harmed and out of balance, pathological organisms such as Helicobacter pylori (Hp) are gaining ground. Research results on the association of Hp and peptic ulcer disease were awarded the Nobel Prize in Physiology or Medicine in 2005. Besides, remarkable studies have shown that the interaction between Hp and gastrin may explain the pathogenesis of most diseases in the upper gastrointestinal tract (Waldum et al. 2016). Nevertheless, what influences the balance of our microbiome and what in turn makes us sick? Previous research often focused on the impact of factors such as weight (Brown et al. 2015), gender (de Alencar Franco Costa et al. 2015), age (Leung & Vanhoutte 2015), circadian rhythm and environmental changes (Dalgaard 2016) on disease state and outcome. By now, an internal imbalance due to our Western lifestyle, including nutrition and living conditions, may be involved (Annalisa et al. 2014, Amato et al. 2015). Western lifestyle does not only mean unhealthy food choices, stress and physical inactivity (Schulze 2015, Brook et al. 2016) but also describes the misusage of antibiotics, excessive sterile environments and even the dubious trend of barring us from the outside world (Gilbert 2016). Obviously, this comes to no good. So, how can we keep a proper relationship with our allies? First of all, we need to get to know each other: the information obtained from the human microbiome project, the follow-up of the human genome project, is undoubtedly more than interesting. However, it does not teach us how to treat or heal a diseased bacterial ecosystem. A sound understanding of the precise role of the microbiota in the pathogenesis of a specific disease is necessary prior to determining whether microbiota manipulation represents a novel treatment therapy (Young 2016). The trend of personalized medicine, raised in the field of genetics and cancer, might have reached a new era, and we