## 4 Discussion  
The present work was designed to preliminarily evaluate the potential effectiveness of combining scrupulous oral and denture hygiene with the administration of a probiotic preparation, containing _Streptococcus salivarius_ K12 for the treatment of DS in patients wearing well-fitting full arch dentures. DS is a multifactorial inflammatory disease of the oral mucosal areas covered by the denture, in which an overgrowth of _C. albicans_ and other opportunistic fungal species normally found in the oral biofilm, is believed to play a key etiologic role [5; 6; 26]. DS is the cause of multiple visits to the dental office for pain, sensation of dryness of the mouth, discomfort and denture mobilization and is also believed to enhance the risk of pulmonary and other systemic infections [10; 11; 12]. _C. albicans_, as many other fungal species, is a normal colonizer of mucosal surfaces including the oral mucosa [8], although in healthy subjects its counts are low. Some clinical conditions, as for example the presence of acrylic bases of removable prostheses and a diminished hygienic skill favored by ageing, are known to promote a significant overgrowth of resident fungal species and of _C. albicans_ in particular, thus promoting the onset of DS [6; 7; 8; 16]. The availability of effective preventative and therapeutic approaches for DS is consequently important. As a direct consequence of the multifactorial nature of DS, its treatment may rely on different approaches, including examination and correction of the denture base, discontinuation of nocturnal wearing of the denture, improvement of oral and denture hygiene measures [13]. Other strategies could be the administration of topical or systemic antifungal agents and disinfectants [14], or the incorporation of slow release antifungal/anticribbial agents into denture base materials [15]. Despite its multifactorial etiology, the main etiological factor and the principal cause of extensive inflammation observed in DS is certainly the colonization of the denture base and of the underlying mucosa by _Candida_ spp. and other fungal species [26]. Consequently, the administration of antifungal drugs is highly effective in relieving clinical signs and symptoms of DS. Nevertheless, the use of such drugs is not without drawbacks, since short term treatments are affected by high rates of early post-treatment relapse and recurrence [16]. Furthermore, prolonged or repeated therapeutic regimens enhance the risk of systemic adverse effects [17] and are likely to induce antifungal drug resistance in resident fungal strains [18]. It is well known that the administration of probiotics can be beneficial for patients suffering for mucosal candidiasis [27]. Previous studies have already shown that the assumption of some, but not all [22], probiotic preparations can reduce symptoms of DS [19; 20; 21], and _Candida_ colonization levels in healthy denture wearers [19; 20] and in subjects affected by DS [21]. However, common probiotic strains of the genus _Lactobacillus_ have few chances to exert prolonged protective activity since they are not adapted to be oral colonizers. To overcome this limitation, studies were recently performed to isolate probiotic _Lactobacillus_ strains directly from the oral cavity [28]. As an alternative strategy, probiotic strains were searched among normal pioneer colonizers of the oral cavity as _S. salivarius_[29]. _S. salivarius_ strain K12 is considered as the prototype probiotic strain of this species. It was originally selected for its ability to inhibit _Streptococcus pyogenes_, but it was subsequently shown to possess several other interesting activities, including inhibitory activity on _Candida_ spp. [29; 30]. Although data on this matter are still incomplete, several probiotic strains were shown to be able to inhibit biofilm growth, and expression of virulence and resistance genes of _C. albicans_[20; 30; 31; 32; 33]. These considerations prompted us to preliminarily evaluate the effectiveness of a probiotic preparation containing _S.