Table 2.

Studies on the relationships between microbiota and obsessive-compulsive disorder (OCD).

Authors and Year	Type of Study	Findings
Kantak et al., 2014 [132]	Animal study (BALB/cJ house mice)	-Pretreatment with probiotics (L. rhamnosus GG) or with fluoxetine attenuated the OCD-like behaviours induced by RU 24969 in comparison with saline pretreatment; -The effects of L. rhamnosus pretreatment and fluoxetine pretreatment on OCD-like behaviours were comparable
Sanikhani et al., 2020 [135]	Animal study (rats)	-Treatment with L. casei Shirota, with fluoxetine and with the combination of both reduced OCD-like symptoms induced by quinpirole hydrochloride; -L. casei shirota might modulate gene expression (Increassed BDNF, decreassed 5-HT2A receptors) in the OFC
Kobliner et al., 2018 [136]	Case report	S. boulardii administration, aimed at reducing GI symptoms, resulted in an amelioration of OCD and SIB in a boy with ASD, OCD, tics, SIB, a history of GI disturbances and global immune dysregulation
Rees et al., 2014 [140]	Review	Antibiotics altering the composition of intestinal flora could be the causative factor of PANDAS rather than GABHS

Legend: BDNF—brain-derived neurotrophic factor; GABHS—group A beta-haemolytic streptococcus; GI—gastrointestinal; L. casei Shirota—Lactobacillus casei Shirota; L. rhamnosus GG—Lactobacillus rhamnosus GG; OFC—orbitofrontal cortex; PANDAS—paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections; S. boulardii—Saccharomyces boulardii; SIB—self-injurious behaviour.