Oropharyngeal Candidiasis (OPC) is a common fungal infection affecting populations with immature or weakened immune systems including neonates, the elderly, AIDS patients, and those taking immunosuppressive drugs [1–4]. Like other diseases in which commensal microbes become pathogenic, OPC is associated with a dysbiotic state, an alteration in the composition or abundance of the oral health-associated host microbiota, contributing to a permissive environment for Candida spp. infection [5, 6]. An aging population, the rise in the use of immune-compromising treatments and the increasing prevalence of drug-resistant Candida strains make it necessary to find alternative treatments for OPC.

Probiotics are defined by the FAO/WHO as “live microorganisms which, when consumed in appropriate amounts in food, confer a health benefit on the host” [7]. Their use is believed to discourage pathogenic microbes and help the host microbiota to return to a health-associated, balanced state after disruptions [8]. Research has provided evidence supporting a positive role of some probiotic organisms in promoting gut, oral and vaginal health, but clinical studies showing no effect of probiotics also exist [9–11].

In this mini-review we explore the current evidence for the role of probiotics in the prevention and treatment of OPC. We will not summarize all work in this area, a task well-accomplished by recent systematic reviews [9, 12–14]. Rather, our intention is to critically appraise the state of the science on this topic by integrating the information gleaned from studies in vitro and in animal models with the results of human clinical trials, where possible. We discuss the pitfalls of each type of research and the difficulties encountered in translating results from laboratory research to benefits for patients, then suggest areas that need more research attention in the future.

Three recent systematic review and meta-analysis studies explored probiotics as treatment or prevention for OPC [15–17]. These studies restricted their analyses to randomized, controlled clinical trials (RCTs) which measured Candida burdens in colony-forming units (CFUs) from saliva samples or swabs of oral mucosal tissues. The meta-analyses included between 3 [18–20] and 8 clinical trials [21–25] that compared a probiotic to a control or placebo. All three meta-analyses found significant reduction in Candida counts in the oral environment as a result of probiotic treatment, although this did not always lead to significant clinical improvements. This suggests that the development of probiotic approaches holds promise in changing current clinical guidelines of OPC prevention or treatment, especially for immunocompromised patients and for patients with refractory or recurring infections.

Experimental work has provided evidence of several anti-Candida activities of Lactobacillus spp. that negatively affect C. albicans virulence characteristics including growth, yeast to hyphae transition, adhesion, and biofilm development [44, 45]. Antifungal mechanisms include production of acids, hydrogen peroxide, bacteriocins, and biosurfactants, as well as competition for adhesion sites. Probiotics can also activate protective functions of the immune system, affecting cytokine profiles, innate and adaptive immune responses, and Candida recognition by epithelial and immune cells [44, 45]. However, which of these protective mechanisms are active in the human oral environment is not fully understood. In the oral environment some of the probiotic protective functions may be compromised by dietary habits and nutrient availability, antagonistic interactions with a complex resident microbiota, and the salivary flow which may limit colonization and persistence.

With respect to persistence in the oral cavity, the impact of probiotic consumption on the oral microbiome is relatively unexplored. The question of whether probiotic organisms become stably incorporated into the human host oral microbiome is not often addressed in clinical trials. However, there is some evidence that while ingested organisms are often detected in the oral cavity of participants during probiotic treatment, they do not persist once treatment ceases [24, 46]. Nevertheless, because OPC is precipitated by an underlying dysbiosis in the oral microbiome [5, 6], it would be important to demonstrate that probiotics can encourage the recovery of a “healthy” microbiome in the oral environment after disruptions caused by antibiotics, cancer treatment or chronic disease, as they appear to do in the gut [8]. These important questions are beginning to be explored using next generation sequencing technology [47, 48] and require more investigation using preclinical models of infection.

It is clear that complex interactions between the fungus, the resident microbiota and the host immune system determine whether C. albicans remains commensal or becomes pathogenic in the oral environment [49, 50]. T helper-17 (Th17) cell-mediated immunity is crucial in maintaining commensal colonization of the oral mucosa by C. albicans [51, 52]. The host aryl hydrocarbon receptor (AhR) binds ligands that are products of tryptophan catabolism and activates IL-17/IL-22 production by Th17 cells. Lactobacilli metabolize tryptophan to produce an AhR ligand, indole-3-aldehyde, which triggers IL-22 production to protect from C. albicans infection and dampen inflammation [53]. Further exploration in this area is needed to reveal how probiotic organisms contribute to metabolic control of protective immune responses in the oral environment.

The effects of probiotic organisms on epithelial barrier function and host immunity have been extensively studied in the context of gastro-intestinal diseases [11, 54]. Probiotics affect host immunity at both the local and systemic levels. Systemic immune effects of probiotic consumption could influence the host's ability to control Candida in the oral environment without the requirement for probiotic organisms to take up residence there. For example, consumption of Lactobacillus strains in mice was tied to improvement in the function of both peritoneal and alveolar macrophages and protection against both lung and peritoneal infection by C. albicans [55]. These distant protective effects may extend to the oral environment.

Experimental evidence suggests that probiotic organisms prime epithelial and immune cells to respond differently to Candida challenge. Pretreatment with Lactobacillus strains before exposure to C. albicans causes changes in cytokine production and pattern recognition receptor expression in macrophages [56]. In epithelial (HeLa) cell lines, pretreatment with L. crispatus altered β-defensin and cytokine production and toll-like receptor expression in response to C. albicans [57]. To shed light on the role of probiotics in the oral environment, more work must be done using oral epithelial cell lines, primary epithelial cells and mucosal organotypic cultures [58]. It will also be important to differentiate between direct effects of the probiotic organisms on epithelial tissues vs. effects secondary to Candida growth and morphology, which can also alter immune responses [59].

Evidence from RCT meta-analyses suggests that probiotic treatments may work better in preventing OPC, compared to treating established infection in susceptible patient categories [15–17]. This may indicate that short-term administration of probiotics cannot lead to changes in the oral microbiota that are sufficient in magnitude or duration to treat this infection. Interestingly, in these meta-analyses when studies that involved elderly patients or denture wearers were considered separately [18–20, 23], a stronger beneficial effect was found compared to including all studies and all susceptible patient groups together [15, 16]. A recent RCT, not included in the meta-analyses, confirmed this trend [60]. In other patient populations (children and adults), oral Candida numbers showed low or no responsiveness to probiotic treatments [21, 24, 25]. This evidence suggests that probiotics must be better targeted to specific vulnerable populations such as patients who are HIV positive, undergoing cancer treatment, or taking immunosuppressive drugs. In vivo animal models mimicking these conditions may be useful in developing a better-targeted, host immune susceptibility type-combined with a specific probiotic strain approach in treatment or prevention.

In the 8 RCTs mentioned above, the dose, timing, duration and delivery methods for probiotics were varied. Dosages in adult populations ranged from 7.2 x 10⁷ to 2 x 10¹⁰ CFUs per day. To put these numbers in perspective, a dosage of 10¹⁰ CFU per day was required to colonize the digestive tract and to treat acute gastroenteritis in children [61, 62], suggesting that better outcomes might have been achieved with higher doses in OPC studies with adults. Delivery methods included lozenges, cheese, yogurt, milk and the application of probiotic organisms directly onto denture surfaces. The timing of delivery ranged from once per day to 3 x per day and the duration of treatment ranged from 4 weeks to 3 months. No pilot studies were conducted to compare the efficacy of particular dosages, timing, delivery or duration and there is no indication of whether any of these variables are essential for success. Again, preclinical animal models could provide useful insights on these variables.

Based on recent studies, it is clear that relatively few clinical trials of probiotics for OPC were based on sound in vitro experimental data and/or appropriate preclinical animal models of infection. Because of the beneficial immunomodulatory effects of several probiotic strains, we suggest that further research is needed in animal models that recapitulate the specific host immune deficiency, to better target the probiotics to the specific underlying immune-compromising condition of susceptible patients.

It will also be important to further study the mechanisms of inhibition of Candida spp. by probiotic organisms and particularly to determine whether these mechanisms are active in the oral environment. Animal models should be used to determine the best methods for delivery of probiotics to the oral cavity before being applied in humans. Molecules with anti-Candida activity that are produced by probiotic species must be identified and their activity tested in appropriate animal models. Further, it will be essential to explore immune effects of probiotics both at the local level of the oral mucosa and at the systemic level and to determine how these changes in immune responses impact the susceptibility or severity of OPC.

LA researched the literature and wrote and edited the article. AD-B provided guidance and focus and edited the article. Both authors contributed to the article and approved the submitted version.

This study was funded by NIH/NIDCR Grant RO1 DE013986 and NIH/NIGMS Grant RO1 GM127909.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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