Probiotics for Depressive Symptomatology and Neural Activity.

A standardised measure of cognitive reactivity. This questionnaire consists of 34 items, in which participants rate on a scale from 0 to 4, how strongly they feel each statement relates to them when they feel somewhat sad.

A standardised measure of cognitive reactivity. This questionnaire consists of 34 items, in which participants rate on a scale from 0 to 4, how strongly they feel each statement relates to them when they feel somewhat sad.

A robust tool consisting of 21 items used to measure the severity of depression. It focuses on symptoms experienced in the last 2 weeks, such as cognitive and affective symptoms.

A short diagnostic interview designed to screen for 17 psychiatric disorders and the risk of suicide.

A self-report questionnaire measuring the tendency to focus on negative thinking, distressing symptoms, and their causes when people feel down, sad, or depressed.

P300 event-related potential (ERP) amplitude and latency during the n-back task. Resting-state and active-state alpha and theta power oscillations.

An exploratory measure of the N2 ERP may be analysed alongside the P300.

This is a measure of executive function and sustained attention. Participants will be shown a circle with 8 equally spaced radii, 2 of which form a bold bisecting line. Numbers are chosen randomly from a set of 1-4 & 6-9 and displayed sequentially in a clockwise direction. A response of higher or lower than 5 is made for trials below the line, and a response of even or odd for numbers above the line. Accuracy and reaction time (RT) on correct trials, as well as accuracy and RT on trials where there is a 'switch cost' from stimuli moving from below to above the bold line and vice versa, are the main outcome measures.

This is an executive function and working memory task. There are varying difficulties of this task, but this trial will use the 0-back and 2-back versions. The 0-back task involves participants being shown a sequence of letters and having to respond when the current letter matches that of the previous letter. In the 2-back task, participants will have to respond when the current letter matches the letter from two trials ago.

This is a condensed self-report questionnaire used to measure changes in mood, assessed across 6 sub-scales (tension-anxiety, depression-dejection, anger-hostility, vigour-activity, fatigue-inertia, and confusion-bewilderment) with 40 total items. A total mood disturbance score is provided.

Visual Analogue Scale of positive/negative feelings via a daily assessment. This includes two questions asking participants to rate on a scale from 0 to 100 the extent to which they felt positive or negative feelings.

The Quality of Life (SF12) is a questionnaire which asks for participants' views about their health. This information keeps track of how they feel and how well they can do their usual activities.

This is a 15-item self-reported questionnaire assessing the severity of gastrointestinal symptoms. It uses a 7-point Likert scale, covering five symptom domains: reflux, abdominal pain, indigestion, diarrhoea, and constipation symptoms.

This is a self-report questionnaire used to measure the severity of generalised anxiety disorder. It has seven items and asks individuals to rate the frequency of anxiety symptoms over the past two weeks, with scores ranging from 0 to 21 to determine anxiety levels. Baseline scores may be used as a covariate.

Information regarding the first day of the most recent menstruation, regularity and contraception will be recorded. It is not a direct outcome measure, but data may be used as a covariate.

This is a validated tool for analysing the average habitual dietary intake of micro and macronutrients of an individual in the UK, which is particularly important to explore in order to understand how the probiotic supplement may interact with diet and gut microbiome diversity. Data will be processed using the FETA software.