This is a single centre, randomised, double-blind, parallel group study (3-Arm) of healthy subjects aged 18-30 years designed to demonstrate whether two active lactobacilli in the Experimental Gingivitis Model modify plaque accumulation, gingival status, immunological and microbial markers compared to placebo. Participants will be screened at a pre-baseline visit by a Study Dentist to determine their suitability for the study which will include healthy volunteers and exclude subjects with initial signs of gingival inflammation. Those volunteers considered to be suitable for the study will receive a pre-baseline dental prophylaxis. Participants will be required to return to the Study Centre 14 days following the screening visit and volunteers who comply with all the inclusion and exclusion criteria will be randomly allocated to one of three treatment groups at the baseline visit. Two of the groups will receive one of two probiotics containing either Lactobacillus (A) or Lactobacillus (B). The third group will receive placebo (P) which does not contain any active ingredient. Each subject will receive a fluoride toothpaste (Colgate Regular) and toothbrush (Oral-B) at the pre-baseline visit to reduce any carry-over (anti-microbial) effects from their normal toothpaste usage.

The initial brushing with the gum shield in place will be conducted under supervision by the study staff at the study centre to enable participants to be familiar with using the gum shield during their unsupervised brushing at home. Each participant will be given both verbal and written instructions, to enable them to correctly insert the gum shield under supervision; the placement of the gum shield will be checked by a study staff member, followed by supervised brushing of the upper teeth.

At the baseline visit those participants who fulfil the inclusion/exclusion criteria will receive a fluoride toothpaste (Colgate Regular) and toothbrush (Oral-B) for use throughout the duration of the study. All treatments will be unsupervised at the subject's home. Participants will be provided with a participant diary at the screening visit to check for compliance with tooth brushing throughout the study period. At the baseline visit those volunteers who successfully completed the inclusion/exclusion criteria will continue to complete the participant diary for compliance with twice daily tooth brushing, the test and control products and the placement of the gum shield over the lower teeth when brushing the upper teeth during the study. Those participants who fail to satisfy the inclusion/exclusion criteria will return the diary together with the fluoride toothpaste and toothbrush at the baseline visit.

Participants will be instructed to abstain from all methods of tooth cleaning in the lower (mandible) jaw for two weeks apart from the fluoride toothpaste provided by the study centre. A soft acrylic stent (gum shield) will be fabricated to cover the teeth in the lower jaw (mandible) whilst brushing the teeth in the upper (maxilla) jaw and will be removed 10 minutes after brushing. The soft stent can be easily inserted and removed without disturbance of the developing plaque whilst ensuring participants do not brush their lower teeth and thus disrupt plaque formation. Modified Gingival Index (MGI) (Lobene et al. 1986) assessments will be undertaken prior to the other assessments as this is non-invasive assessment. At baseline and after two weeks (+ 14 days) four sites/tooth (mesial, mid-buccal, distal and lingual) will be probed using the Gingival Bleeding Index \[GBI\]) as defined by Saxton \& van der Ouderaa (1989) to assess the health status of the gingiva. Probing following one week of plaque accumulation will be avoided as bleeding in the gingival crevice may influence the developing biofilm. Non-invasive samples of gingival crevicular fluid (GCF) will however, be collected at all-time intervals before the BOP measurements or plaque sampling as the volume of GCF (Kistler et al. 2013) has been shown to increase before the development of clinically evident inflammation e.g. GBI. The GCF samples will be collected from the mesio-buccal sites of 4 selected sites (two teeth in each quadrant of the lower \[mandible\] jaw) on perio strips (ProFlow Inc. Amityville, NY) (Thunell et al. 2010). The strip will be applied in the gingival sulcus for 30 seconds to collect a standardised GCF fluid sample. The GCF samples will be pooled from 4 sites of each participant as indicated above at each time point and analysed on an individual basis. The number of sites with clearly visible plaque following the disclosing of the teeth in the lower (mandible) jaw (modified Quigley-Hein) (Turesky et al. 1970) will be recorded. Plaque will be collected from the buccal and lingual surfaces of teeth in the lower (mandible) jaw excluding 3rd molars from each participant using a sterile curette and placed in tubes with saturated saline-DMSO buffer. The plaque samples, and GCF samples in tubes with Phosphate Buffered Saline (PBS), will be transported on dry ice and stored at -80°C. All assessments and sample collection will be undertaken by trained and calibrated personnel. After two weeks, all participants who complete the study will receive a dental prophylaxis following the collection of samples and will be asked to resume their normal oral hygiene regime.