Metalloids: Semi as Metals yet Full of Antimicrobial Potential

In the periodic table, the metalloids boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), and tellurium (Te) demarcate a borderline between the metals and non-metals. Also referred to as semi-metals, their behaviour is indeed characterized by inorganic ions, organometallic complexes, and covalent compounds. Yet despite this unique medley of inorganic and organic molecules, reactivities and activities, and century-old traditional uses, their pharmaceutical applications today are often underestimated or associated with outright toxicity. Still, recent research has taken a closer look at (some of) these elements and has encountered an amazing assortment of natural metalloid-containing products, many with distinct antimicrobial activities. At the same time, metalloid-containing complexes have been synthesized and tested successfully as ionophores, in ligand-exchange reactions with amino acids, such as cysteine, or as redox modulators triggering oxidative stress. This has been complemented by an extensive synthesis of organic metalloid-based compounds, even including bioisosteric replacements for carbon. Today, a few metalloid compounds are still or already used in therapy; others are in clinical trials, and many more are in the pipeline, clearly demanding full attention for the semi-metals and their many colourful compounds.