Oral microbiota transplant: a potential new therapy for oral diseases

Abstract

Dental caries and periodontitis are amongst the most common diseases affecting humans worldwide. There is an evolving trend for dental and medical research to share knowledge on the etiology and promising therapies for human diseases. Inspired by the success of fecal microbiota transplant to manage gastro-intestinal disordes, oral microbiome transplant has been proposed but not yet tested in humans. This article critically reviews the potential of oral microbiome transplant for managing oral diseases.

Introduction

A recent systematic review has called attention to the fact that untreated dental caries is the most common disease, and severe periodontitis is the sixth most common disease affecting humans globally¹. Of further concern are the serious implications that these oral diseases can have on general health². The oral microbiome is comprised by hundreds of microbial species that co-inhabit and functionally interact in oral biofilms to cause disease or to maintain homeostasis^{3, 4}. Both caries and periodontitis are closely related to a dysbiosis of the microbial consortia driven by environmental changes, such as a sugar-frequent/acidic-pH environment in caries and a protein-rich/neutral-to-weakly alkaline-pH environment in periodontal disease^5–7. In caries, continuous acid production from the metabolism of dietary carbohydrates results in the emergence of acid-producing and acid-tolerant organisms in supragingival biofilms, a selective process that alters the pH homeostasis of biofilms and shifts the demineralization-remineralization equilibrium toward loss of tooth minerals. Accumulation of subgingival plaque leads to inflammation of the gum tissues, or gingivitis, which may progress to periodontitis. In periodontitis, certain members of the microbial community can destabilize the host immune response, which may result in destruction of periodontal tissues in susceptible individuals. Conventional therapies for caries and periodontitis aim at controlling the formation and metabolic activities of supra- and subgingival biofilms. Still, caries and periodontitis remain as major public health problems worldwide. Clearly, there is an urgent need to identify novel and more efficient strategies for intervention of these oral diseases that can be widely and safely utilized in a cost-effective manner.

There has been an increasing interest on therapeutic interventions that modulate microbial ecology to restore homeostasis of human biofilms, and thus health^{8, 9}. Such interest follows insights provided from the Human Microbiome Project revealing that ecological balance in biofilms play a significant role in health⁹. Fecal microbiota transplant (FMT) is an example of therapy based on altering the dysbiotic microbiota to restore microbial ecological balance. The remarkable success of FMT to treat persistent Clostridium difficile infections suggests that the gut microbiota has sufficient plasticity to undergo ecological interventions that improve health¹⁰. Specifically, C. difficile can be replaced by commensal and beneficial gut bacteria that has been killed or suppressed, usually by the continuing use of antibiotics. Inspired by the use fecal transplantation in medicine, oral microbiota transplant (OMT) has been hipothetically proposed by few dental researchers. This article critically reviews the potential of OMT as a new therapy for managing oral diseases such as caries and periodontitis.

Fecal Microbiota Transplant

The human gut microbiota is highly complex, and functions to support health in a similar way as the microbiota of other organ sytems¹¹. Treatment options for gastro-intestinal disorders include antibiotics, dietary changes, probiotics, prebiotics, and FMT¹². In particular, the FMT procedure involves administration of fecal material (stool) from a healthy donor to a patient with a disease or condition related to dysbiosis, or alteration of their normal gut microbiota. The donor may be an intimate, long-time partner, friend, or an unrelated volunteer. The stool suspension taken from the donor is mixed with saline or other solution, strained, and introduced into the gastrointestinal tract of the recipient via colonoscopy, enema or a nasogastric tube¹¹. FMT usually involves a single administration dose but the use of several doses has been proposed¹³. Differently than probiotic therapies in which few bacterial species are dispensed, fecal transplant introduces thousands of naturally occurring gut microorganisms in the colon. Theoretically, the native microbiota used in FMT is more likely to thrive in the acidic environment and during intestinal transit, to adhere to the intestinal mucosa, and to produce antimicrobial substances that contribute to their beneficial health effects.

FMT has been used to manage chronic inflammatory bowel diseases¹⁴, insulin sensitivity¹⁵, ulcerative colitis¹⁶, autism spectrum disorders (ASD)¹², however, better outcomes were shown when FMT was used to treat persistent C. difficile infection¹⁷. Figure 1 illustrates the use of FMT to increase the diversity of the gut microbiota and eradicate bacteria containing antibiotic resistant genes¹⁸. Current clinical and best practice guidelines with indications for fecal transplants, and protocols for donor selection and screening, stool preparation and methods of administration were reviewed elsewhere^{11, 19}. Although FMT is a promising approach to alter the gut ecosystem and improve gastrointestinal health, evidence of its true effectiveness remains questionable and concerns have been raised regarding short- and long-term safety and tolerability¹⁰. FMT remains classified as an experimental treatment and complications with regulatory agencies have limited the general use of this therapy²⁰.

Fig. 1.

The human gut microbiome naturally contains some bacteria that carry antibiotic resistance (ABR) genes. Following repeated courses of antibiotics, the diversity of the gut microbiota is reduced, allowing the bacteria containing ABR genes to flourish. This allows for opportunistic pathogens such as C. difficile to colonize and dominate the gut. Following fecal microbial transplantation, the diversity of the gut (figure from Millan and Madsen, 2016)¹⁸. Permission to reprint requested.

Oral Microbiota Transplant

Involuntary transmission of oral microrganisms from one individual to another via saliva is a common life occurrence^{21, 22}. Whereas OMT is not part of this natural event, this therapy aims at transfering oral biofilms from a healthy donor to a patient with either caries or periodontitis. Human OMT has been hypothetically suggested by Floyd Dewhirst and Diane Hoffmann (unpublished data, https://www.law.umaryland.edu/programs/health/events/microbiota/documents/Dewhirst.pdf) and others^{23, 24}, but thus far no actual oral transplantation has been reported. The procedure that was hypothetically proposed by Dewhirst and Hoffmann involve: 1) collection of supragingival plaque from a caries-free donor (potentially a relative of the recipient patient), 2) storage of plaque into saline, and 3) the use a nylon swab to transfer the collected plaque to the teeth of a caries-active patient. According to their proposed protocol, the donor should have a healthy oral microbiome that excludes cariogenic bacteria such as Streptococcus mutans and presents minimal pH drop in response to sugar challenge.

Pozhitkov and collaborators proposed to introduce health-associated oral microbiota into the oral cavity of periodontitis patients²⁴. First, they confirmed that the microbiome of subjects with periodontitis were distinct from those of healthy or edentulous patients. Next, they tested an in vitro antimicrobial protocol to be used on the oral cavity of the recipient patient prior to OMT. It was shown that application of sodium hypochlorite (NaOCl) followed by its neutralization with sodium ascorbate buffer may be a valid option for suppressing the disease-associate microbiota to allow for a more pronounced microbial shift to a healthier microbiota. In that same study, the authors suggested an OMT procedure consisting of: (1) collecting sub- and supra-gingival plaque from a healthy donor (spouse or a partner), (2) performing deep cleaning, root planning and applying a broad-spectrum antimicrobial agent to the periodontitis patient, and finally (3) neutralizing the antimicrobial agent immediately following by a rinsing with a microbial suspension harvested from the healthy donor in the periodontitis patient²⁴.

What needs to be considered

The oral cavity is a complex ecosystem in which a rich and diverse microbiota has evolved since birth. The most abundant taxa in oral biofilms display remarkable phenotypic plasticity, e.g. health-associated and disease-associated bacteria can morph rapidly in response to oral environmental changes²⁵. In another words, the composition and metabolic activities of microbial communities fluctuate according to the constant environmental changes in pH, nutrient availability, oxygen tension and redox environment, shedding effects of oral surfaces, and composition of salivary and crevicular fluids. These changes in the environment, whether imposed by diet, behavior, systemic conditions or medications may disturb the homeostasis and lead to endogenous infections or susceptibility to exogenous infections. Evidently, inter-microbial species interactions and immuno-stimulatory effects are expected to play a key role in OMT therapy. Transplanted oral biofilms must exhibit the capacity to effectively: a) endure the selective pressure of the environment, b) colonize the oral sites, c) compete with the disease-microbiota for adhesion sites and nutrient sources, d) produce substances like bacteriocin and hydrogen peroxide to inhibit the growth of pathogens, and e) modulate local and systemic immune functions.

Safety concerns related to the potential application of OMT are similar to those for oral probiotics²⁶. As with probiotics, transplanted biofilms must not cause disease and should possess a high degree of genetic stability. At this point, the mechanisms of action and ideal vehicles for OMT have not been extensively discussed. For example, it is critical to determine whether oral biofilms should be transplanted directly from a healthy donor to a diseased patient, or pre-treated with methods that eliminate (or attempt to reduce the proportions of) pathogenic organisms prior to transplantations, or even if biofilms created in vitro but composed by naturally occurring commensals organisms would be the best option for OMT. Biofilms composed by clinical strains with beneficial and health-associated properties may be proven effective at interacting and replacing disease-associated biofilms^{27, 28}. Other topics for discussion include the need for disinfecting the oral cavity of the receipt patient prior to OMT²⁴, and whether one dose or multiple doses of oral transplants would be necessary for an effective and permanent colonization of the oral cavity in order to restore and maintain health.

Evidence from oral microbiome studies point out to a progressive increase in complexity and diversity from birth to adulthood²³. In health, the oral microbiome appears to be more stable than those of other body niches like the gut, but still with a substantial degree of within-individual variability^{29, 30}. In disease, microbial diversity appears to be lower in caries than health, which may reflect the ecological pressure of low environmental pH. Contrasting with caries, periodontal diseases are associated with an increase in microbial diversity, which could be the result of impaired local immune function, increased availability of nutrients or a reflection of the diverse environmental niches at the periodontal pocket^{23, 31}. Hence, it is important to keep in mind that while the goal of OMT for caries therapy may be to increase bacterial diversity, the goal of OMT for periodontitis may not be the same since the bacterial diversity is already high²³.

Ongoing and future metagenomics and metabolomics studies ought to increase our understanding of the oral microbiota dynamics and provide new insights on how a dysbiotic microbiota can be successfully replaced by a health-beneficial flora⁶. Moreover, studies involving other kingdoms, such as viruses, fungi, Archaea and protozoa should provide a more realistic picture of the complex interactions contributing to the compositional and functional stability of the oral ecosystem. Undoubtedly, well-conducted in vitro and animal studies as well as clinical trials with a proper study design are much needed to clarify the questions raised by this review. Future clinical trials must be conducted using clinical (carious lesions and loss of periodontal attachments) outcomes as endpoints measurements rather than microbial measurements alone, and extensive follow-up times should be included. Of great importance, if OMT is to be implemented in the future, the success will also be dependent on the association of this therapy with other conventional therapies aimed at reducing the risk of caries and periodontitis.

Conclusion

Despite limited scientific and clinical evidence, oral microbiota transplant holds promise as a new therapy for managing caries and periodontitis. OMT may represent a cost-effective approach and have the ability to better reach difficult to access high-risk populations. However, clinical recommendations for the use of OMT cannot be provided at this point based on the current state of knowledge. It is crucial to have a better understanding of the retentiveness of transplanted oral biofilms while maintaining the natural balance of the resident oral microbiota with the host immune responses. Understudied issues include best practices for optimal donor selection, sample preparation, vehicles, follow-up timing, and number of administrations.