To observe the effect of bacteroides fragilis BF839 intervention on learning, memory and social novelty of fragile X-mental retardation gene 1 (Fmr1) knockout (KO) mice.

Thirty three-week-old Fmr1 KO mice were randomly divided into Fmr1 KO group (n=15) and Fmr1 KO+BF839 group (n=15). Mice in the Fmr1 KO group freely drank autoclaved tap water everyday; mice in the Fmr1 KO+BF839 group drank BF839 bacterial liquid (10 mL/d) everyday;11 wild-type mice freely drank autoclaved tap water everyday were set as controls (WT group). After 4 weeks of intervention, Morris water maze test was used to observe the differences in escape latency and frequencies of crossing the original platform among mice in each group; Three-chamber Social Interaction Test was used to observe the differences in contact frequencies and contact durations with unfamiliar mice among mice in each group.

On the 4^th d of experiment, the escape latency of mice in the Fmr1 KO group ([46.06±10.29] s) was significantly longer than that in the WT group ([33.39±12.02] s, P<0.05); the escape latency of mice in the Fmr1 KO+BF839 group ([28.39±9.07] s) was significantly shorter than that in the Fmr1 KO group (P<0.05); the escape latency of mice in the Fmr1 KO+BF839 group was slightly shorter than that in the WT group without significant difference (P>0.05). The frequencies of crossing through the original platform of mice in Fmr1 KO group (0.00[0.00, 1.00] time) was slightly less than that in WT group (1.00 [0.00, 1.00] time) without significant difference (P>0.05); that in the Fmr1 KO+BF839 group (1.50[1.00, 2.00] times) was significantly larger than that in the Fmr1 KO group and WT group (P<0.05). The contact frequencies of the mice in the Fmr1 KO group with unfamiliar mice (5.50[0.50, 12.75] times) was less than that in the WT group (7.00[4.00, 17.00] times) without significant difference (P>0.05); that in the Fmr1 KO+BF839 group (23.00[16.00, 36.00] times) was significantly increased as compared with that in the Fmr1 KO group and WT group (P<0.05). The contact duration of mice in the Fmr1 KO group with unfamiliar mice (9.50[0.50, 41.95] s) was significantly shorter than that in the WT group (142.00[65.00, 171.60] s, P<0.05); Fmr1 KO+BF839 group had significantly longer contact duration with unfamiliar mice (69.60 [50.40, 98.40] s) than Fmr1 KO group (P<0.05); the contact duration of mice in Fmr1 KO+BF839 group with unfamiliar mice was shorter than that in WT group without significant difference (P>0.05).

Early BF839 intervention can significantly improve the learning, memory abilities and social novelty of Fmr1 KO mice, and even restore the Fmr1 KO mice to normal levels, which suggests that BF839 may become a new tool for treatment of fragile X syndrome and autism.

Bacteroides fragilis 839; Fragile X syndrome; Autism; Learning and memory; Social novelty