Ketamine is one of several clinically important drugs whose therapeutic efficacy is due in part to their ability to act upon ion channels prevalent in nearly all biological systems. In studying eukaryotic and prokaryotic organisms in vitro, we show that ketamine short-circuits the growth and spatial expansion of three microorganisms, Stachybotrys chartarum, Staphylococcus epidermidis and Borrelia burgdorferi, at doses efficient at reducing depression-like behaviors in mouse models of clinical depression. Although our findings do not reveal the mechanism(s) by which ketamine mediates its antifungal and antibacterial effects, we hypothesize that a function of L-glutamate signal transduction is associated with the ability of ketamine to limit pathogen expansion. In general, our findings illustrate the functional similarities between fungal, bacterial and human ion channels, and suggest that ketamine or its metabolites not only act in neurons, as previously thought, but also in microbial communities colonizing human body surfaces.