The impact of faecal microbiome transfer on gut health in autistic adolescents and young adults

All participants will undergo bowel cleansing on the day before treatment using an oral solution containing 70 g of Glycoprep-O (active ingredient macrogol 3350) (Fresenius Kabi Australia, Mount Kuring-gai, Australia). After bowel cleansing, participants will fast overnight for at least 8 hours; in the morning at clinic, participants will take the first dose of either encapsulated faecal microbiome transfer (FMT) or placebo (saline), 10 capsules each swallowed with water or diluted juice. Following another 8-hour overnight fast, in the next morning, participants will receive a second dose of 10 capsules of the same treatment. After each dose, participants will need to remain fasted for another 2 hours. The FMT treatment will be extracted from donor stools and be doubly encapsulated using delayed release hydroxypropyl methylcellulose capsules (DRCaps, Capsugel Inc, Sydney, Australia). The DRCaps mask taste, odour, and visual appearance; importantly, they are designed to remain intact during passage from the stomach to the intestine, ensuring FMT delivery to the proximal bowel. Thus, the use of invasive techniques (i.e. endoscopy) for FMT will not be required. Methods for faecal microbiome isolation, preparation, and double encapsulation will be carried out as detailed in the Gut Bugs Trial study protocol (Leong et al. BMJ Open 2019). Briefly, immediately after donation, stools are placed in normal saline, blended, and sieved to remove particulate matter. Samples are then differentially centrifuged to isolate the microbiota pellet. The use of low-speed centrifugation to pellet the microbiota cells is a feature of this methodology that reduces the risk of having free viruses included into the treatment capsules. The pellet is suspended in normal saline (containing 15% glycerol – a cryoprotectant) at 1 g wet weight/ml before being dispensed into size 0 DRcaps capsules. These capsules are closed and secondarily sealed within size 00 DRcaps capsules. Each capsule will contain 0.5 ml of faecal suspension corresponding to approximately 0.5 g of microbiota, so that our treatment dose of 20 capsules will administer approximately 10 g of microbiota. Capsules will be stored frozen at -80°C and will remain viable for at least 6 months.