Molecular imaging of microbiota-gut-brain axis: searching for the right targeted probe for the right target and disease

The highly bidirectional dialogue between the gut and the brain is markedly stimulated and influenced by the microbiome through integrated neuroendocrine, neurological and immunological processes. Gut microbiota itself communicate with the host producing hormonal intermediates, metabolites, proteins, and toxins responsible for a variety of biochemical and functional inputs, thereby shaping host homeostasis. Indeed, a dysregulated microbiota-gut-brain axis might be the origin of many neuroimmune-mediated disorders, e.g. autism, multiple sclerosis, depression, Alzheimer's and Parkinson's disease, which appear months or even years prior to a diagnosis, corroborating the theory that the pathological process is spread from the gut to the brain. A much deeper comprehension of how commensal microbe can be manipulated to interfere with disease progression is crucial for developing new strategies to diagnose and treat diseases. In recent years, the potential of positron-emission-tomography (PET) in the field of bacteria detection has gained attention. The uptake of several PET tracers has been evaluated to investigate infection pathophysiology, e.g. sterile or pathogen-mediated infection, monitoring of progression, or as a surrogate endpoint in clinical trials. In this minireview, we briefly describe the role of microbiome-gut-brain axis in health and disease and we discuss the imaging modalities and agents that could be applied to study the dynamic interactions between microbiome, gut and brain. These are key aspects in understanding the biochemical lexicon underpinning the microbiome-host crosstalk that would enable the development of diagnostics and therapeutics by targeting the human microbiota.