‘Psychobiotics’ is your new word this month: live microorganisms with mental health benefits. We've previously noted the links between the gut microbiome and changes in cognition and psychopathology. Now, Allen and colleagues test an intervention – using the bacterium Bifidobacterium longum 1714 – in 22 healthy volunteers who undertook cognitive assessments, resting electro-encephalography (EEG), and a social stress test at baseline, post-placebo, and post-psychobiotic. The active intervention was associated with reduced cortisol levels and subjective anxiety in response to the stress test, as well as lower levels of self-reported stress. It also resulted in enhanced frontal midline EEG mobility, which is representative of prefrontal cortical activity, and modest improvements in visuospatial memory performance. The authors propose a precision strategy, to rationally test particular candidate bacteria strains – rather than the ‘probiotic cocktail’ often administered – against specific psychopathology.