## 1 Introduction  
Life expectancy significantly increased in industrialized countries in the last century, while natality progressively reduced [1]; consequently, the proportion of elderly subjects significantly increased, with evident healthcare issues in all fields of medicine, including dentistry [2, 3]. Edentulism (both partial and total) is among the most frequent dental problems of an aged population, and in many cases it is treated by removable dentures [4]. An inflammatory status of the oral mucosal areas covered by the denture is frequently observed in denture wearers. This inflammatory condition is commonly known as denture stomatitis (DS). The prevalence of DS was reported to range from 15% to over 70%, depending on variables including: continuous use of the denture, quality of oral and denture hygiene (frequently limited by dexterity of the subject), adequateness of the denture, and co-morbidities such as diabetes [5].  
The etiology of DS is multifactorial, but _Candida albicans_ and other fungi of the genus _Candida_ are believed to be key etiologic factors. Several members of the genus _Candida_ are common members of oral biofilms and are able to overgrow in the denture microenvironment, inducing inflammation and acting as promoters of opportunistic polymicrobial infections, frequently observed in DS [5; 6; 7; 8; 9]. Clinically DS is characterized by soreness, pain, discomfort, denture mobilization and is consequently the cause of multiple visits to the dental office. DS is, moreover, believed to be a condition potentially enhancing the risk of pulmonary and other systemic infections, particularly in institutionalized elderly subjects [10; 11; 12].  
The prevention and treatment of DS in many cases relies only upon an examination of the adequacy of the denture, improvement of oral hygiene measures and a discontinuation of nocturnal wearing of the denture [13]. Therapeutic strategies based upon the administration of topical or systemic antifungal agents and disinfectants [14], or upon incorporation of slow release antifungal/antimicrobial agents into denture base materials [15] were also proposed. Although antifungal drugs proved effective in relieving clinical signs and symptoms of DS, they are not without drawbacks. In fact, high rates of early post-treatment relapse and recurrence were reported [16], while prolonged therapeutic regimens or multiple cycles of treatment greatly enhance the risk of potential systemic adverse effects [17] and of induction of antifungal drug resistance in resident fungal strains [18], making the treatment of DS by their means challenging. In recent years the administration of specific probiotic preparations was investigated as a means to contrast oral colonization by _Candida_ spp. in denture wearers both in the presence and in the absence of DS, although with contrasting results [19; 20; 21; 22].  
The present work was consequently designed to preliminarily evaluate if a probiotic preparation, containing _Streptococcus salivarius_ strain K12 could prove effective in reducing clinical signs and symptoms of DS and in reducing oral colonization by _C. albicans_, and if the duration of clinical effects continued after the end of treatment.