ABX	Antibiotic treatment
ACTH	Adrenocorticotropic hormone
BBB	Blood–brain barrier
BDNF	Brain-derived neurotrophic factor
CFU	Colony-forming unit
CRP	C-reactive protein
DA	Dopamine
DNA	Deoxyribonucleic acid
DSM	Diagnostic and Statistical Manual of Mental Disorders
ERP	Exposure and response prevention
FDR	False discovery rate
FMT	Fecal microbiota transplantation
GABA	Gamma-aminobutyric acid
GWAS	Genome-wide association study
HIP	Hippurate
HPA axis	Hypothalamic–pituitary–adrenal axis
HTR2A	5-hydroxytryptamine receptor type 2A
IL	Interleukin
JBI	Joanna Briggs Institute
LPS	Lipopolysaccharide
MAOA	Monoamine oxidase A
mGluR5	Metabotropic glutamate receptor 5
mPFC	Medial prefrontal cortex
MR	Mendelian randomization
NGF	Nerve growth factor
OCD	Obsessive–compulsive disorder
OCI-R	Obsessive–Compulsive Inventory–Revised
OXTR	Oxytocin receptor
PERMANOVA	Permutational multivariate analysis of variance
pCS	p-Cresol sulfate
SERT	Serotonin transporter
SCFA	Short-chain fatty acid
SLC6A4	Solute carrier family 6 member 4 (serotonin transporter gene)
SNP	Single nucleotide polymorphism
SSRI	Selective serotonin reuptake inhibitor
SYRCLE	Systematic Review Centre for Laboratory Animal Experimentation
TNF-α	Tumor necrosis factor-alpha
Trp	Tryptophan
WGS	Whole-genome sequencing
Y-BOCS	Yale–Brown Obsessive Compulsive Scale