BLIS study: a feasibility randomised controlled trial assessing compliance, acceptability and colonisation with different dosing regimens of the probiotic supplement Streptococcus salivarius K12 (Bactoblis) in adults in England.
SsK12 colonisation was more prevalent with daily dosing than weekly dosing, but colonisation was not maintained when dosing stopped, and both regimens were acceptable to participants, supporting the use of daily dosing in a future efficacy trial.
Key Findings
Results
SsK12 colonisation prevalence on day 2 was equal between weekly and daily dosing groups at 59.1%.
13 of 22 participants in both groups tested PCR-positive on day 2 (95% CI 38.6% to 79.6%)
Baseline colonisation was detected in 2 participants in the weekly group and 4 in the daily group, who were excluded from microbiology analysis
23 participants in the weekly group and 22 in the daily group without baseline colonisation were included in microbiology analysis
Swabs were self-taken whole-mouth swabs analysed by real-time PCR
Results
By day 7, SsK12 colonisation was substantially higher in the daily dosing group (85.7%) compared to the weekly dosing group (13.6%).
Daily group: 18/21 participants colonised (95% CI 70.8% to 100.0%) on day 7
Weekly group: 3/22 participants colonised (95% CI 0.0% to 28.0%) on day 7
The weekly group received only one dose prior to day 7, whereas the daily group received daily doses
Results
On day 14, colonisation remained higher in the daily dosing group (59.1%) than in the weekly dosing group (19.1%).
Daily group: 13/22 participants colonised (95% CI 38.6% to 79.6%) on day 14
Weekly group: 4/21 participants colonised (95% CI 2.3% to 35.8%) on day 14
Day 14 was the final day of the 14-day dosing period for both groups
Results
SsK12 colonisation was not maintained after dosing stopped, with only one participant (in the weekly group) testing positive on days 21 and 35.
Both days 21 and 35 were post-dosing timepoints
Only a single participant from the weekly group showed colonisation at day 21 and day 35
No participants in the daily group showed colonisation at these post-dosing timepoints
This indicates SsK12 does not establish persistent colonisation following a 14-day dosing course
Results
Both dosing regimens were highly acceptable to participants, with the daily regimen rated as easy to follow by 96.2% and the weekly regimen by 85.0%.
Weekly group: 17/20 participants (95% CI 69.4% to 100.0%) reported the regimen was easy to follow
Daily group: 25/26 participants (95% CI 88.8% to 100.0%) reported the regimen was easy to follow
Acceptability and compliance were secondary outcomes assessed via questionnaire data
Results
Overall swab return rates across both groups were moderate, with 65.4% of the weekly group and 70.4% of the daily group returning all swabs.
Weekly group: 17/26 participants returned all swabs
Daily group: 19/27 participants returned all swabs
Swabs were collected at baseline and on days 2, 7, 14, 21, and 35
53 participants total were recruited between 26 April and 5 December 2023
Results
Some participants had pre-existing baseline colonisation with SsK12 prior to the intervention.
2 participants in the weekly group and 4 participants in the daily group tested PCR-positive at baseline
These participants' swabs at all timepoints were excluded from the microbiology analysis
This finding highlights the need to screen for baseline colonisation in future trials
Methods
The study was a randomised, non-blinded feasibility trial conducted in primary care in the south of England, enrolling adults with a history of recurrent sore throat.
Participants were required to have had ≥2 episodes of sore throat within the 3 years before recruitment
26 participants were randomised to weekly dosing and 27 to daily dosing
Registered under trial identifier NCT04297878
The primary outcome was prevalence and duration of SsK12 colonisation determined by real-time PCR
What This Means
This research suggests that taking a probiotic lozenge called Streptococcus salivarius K12 (SsK12) daily for two weeks leads to much higher rates of the probiotic 'taking hold' in the mouth compared to taking it just once a week. By day 7 of the study, over 85% of people in the daily dosing group had detectable levels of SsK12 in their mouths, compared to only about 14% of those in the weekly group. Both regimens were well-tolerated and easy to follow, with over 85% of participants in both groups reporting the schedule was easy to stick to.
However, an important finding was that once people stopped taking the lozenges, the probiotic largely disappeared from their mouths within weeks. By days 21 and 35 — one and three weeks after dosing ended — only a single participant still tested positive for SsK12. This suggests the probiotic does not permanently establish itself in the mouth and would likely need to be taken on an ongoing basis to maintain any potential protective effect against sore throats.
This research matters because recurrent sore throat is a common and burdensome problem, and there is interest in whether probiotics like SsK12 could help prevent infections rather than just treating them after they occur. This feasibility study was designed to work out the best way to take SsK12 before conducting a larger trial to test whether it actually reduces sore throats. The findings support moving forward with a daily dosing schedule in that future trial, as it achieves better colonisation of the mouth, which is considered a likely prerequisite for any protective effect.
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Wilcox C, Cleary D, Augustine M, Anderson R, Francis N, Lown M, et al.. (2026). BLIS study: a feasibility randomised controlled trial assessing compliance, acceptability and colonisation with different dosing regimens of the probiotic supplement Streptococcus salivarius K12 (Bactoblis) in adults in England.. BMJ open. https://doi.org/10.1136/bmjopen-2026-120406