Oral microbiota versus oral mucositis during cancer treatment : a review

Introduction: Oral microbiota has been implicated on oral mucositis (OM) that occurs during cancer therapy, however without consensus. Objective: This study, aimed to establish, through a review, the association between oral microbiota and OM at head and neck cancer therapy (HNCT). Material and Methods: The search of PubMed was performed considering 2008-2018 period, and the descriptors “oral mucositis” and “oral microbiota” in subheadings etiology and microbiology into the Medical Subject Heading (MeSH) “Head and Neck Neoplasms”. The conducting question was “Is there an oral dysbiosis during HNCT associated with OM?” Results: 22 articles were selected under two steps of data extraction: articles that evaluated de oral microbiota during HNCT (n=13), and articles that also focused in OM (n=9). Conclusion: The evidence presented in the literature suggests associations of oral microbiota dysbiosis with the progression and worsening of radiation-induced OM. However, to define a microbial core for the disease, future standardized studies are required.


INTRODUCTION
Chemotherapy (CT) and radiotherapy (RT) treatments are commonly used in cancer therapy.
However, the agents used in these antitumor treatments do not discern healthy cell from disorderly grown cell. Therefore, the side effects to these therapies are recurrent and worrying to the patient health, being oral mucositis (OM) the most common side effect to the antitumor treatment of the head and neck region. 1,2 The establishment of OM is related to a cascade of impactful events for the patient and the community, raising the costs of cancer treatment and the mortality. 1,3 Its development ranges from increasing number of proinflammatory cytokine receptors to damage of affected tissues by cell apoptosis and necrosis. 4 Its symptoms of pain, difficulty in swallowing, trismus and decreased taste, detract from the nutrition of the patient, therefore, the predisposition to microbial virulence factors is greater, compromising the general state of health. 1,5 In fact, the oral microbiota has been implicated as an agent of progression and/or aggravation of the OM for years. 3,[6][7] Despite there are no effective interventions for OM therapy yet, 8 the knowledge of the oral microbiota of individuals at head and neck cancer therapy could be the first step to comprehend its role in the development and progression of the OM disease. Therefore, the aim of this study was to determine, through a review, the association between oral microbiota and OM at head and neck cancer therapy (HNC).

MATERIAL AND METHODS
The key question to conduct this review was "Is there an oral dysbiosis during HNC therapy associated with OM?". To answer the key question, a literature search of PubMed was carried out. The Medical Subject Heading (MeSH) "Head and Neck Neoplasms" with the subheadings etiology and microbiology, in combinations with the descriptors "oral mucositis" and "oral microbiota" were used. To restrict the results, the search was limited to studies published from 2008 up to 2018, with full text available and with analysis in humans. Case reports, experimental studies, review articles and letters to the editor were excluded.

RESULTS
The PubMed database is a reference database for the scientific literature including the health field. It contains more than 29 million references from MEDLINE, life science journals, and online books. 9 Considering only MEDLINE, it comprises scientific journal articles from over 70 countries around the world being every day updated. 10 Regarding the search terms used and their associations, several articles were retrieved, which were selected considering the flow process presented in figure 1. At the end of the process, 22 articles were selected, nine of them specifically worked with oral microbiota analysis during head and neck cancer treatment focusing in OM.
The 22 articles included in this review met the eligibility criteria, were identified to respond to the objective of the study and presented adequate  3 methodological relevance and quality. However, the process of data extraction were divided on two steps: first were analyzed the articles that evaluated oral microbiota during the head and neck cancer treatment (n=13), and then the articles that also focused in OM (n=9). These nine articles, named as "OM articles", were studied and a synthesis of the literature was performed with the data extracted summarized in table 1.
The sample size of the OM articles was quite similar, having the sample ranging from 19 to 49 patients. However, the age of the study group was very different, with some of them working with age up to 18 years, 11 and other from 18 years. 3 Furthermore, five studies are from countries of the European continent, among them Sweden, 11,12 Croatia, 13 Spain and Italy. 3,14 Three studies are from Asian countries, among them India, 15 China and Japan. 1,2,16 In addition, one study is from the American continent, more specifically Brazil. 5

The microbial analysis of OM articles
The studies selected had used different methodologies, both those of conventional culture with the microorganisms being analyzed by cultures in selective media, 3,5,12-15 as microbial analysis by 16S sequencing. 1,2,11 Six studies have analyzed bacteria and two studies analyzed mainly fungi. [1][2][3][11][12][13][14][15] One study analyzed both microbiological groups. 5 The kind of sample of OM articles From the nine manuscript selected, three studies collected the sample with oropharyngeal swab before and after RT. 2,13,15 One study collected the supragingival plaque before and after CT. 14 One study collected the supra and subgingival plaque before and after RT. 5 One study performed the injury swab before and during RT. 1 One study performed mucosal smear before, during and after RT. 3 One study used strips of paper placed on the mucosa or OM if occurred. 11 And one study used the mucosal swab and scraped the dorsum tongue during the treatment. 12 Considering the above, some distinct shifts were expected when comparing the manuscripts studied, because the sample region was distinct. However, the divergence of the manuscript goes beyond of the local of sample. It includes the RT and/or CT protocol, age of the cases studied, sampling time and microbial analysis type, which varied very much as described above in the results section and summarized in table 1.

DISCUSSION
In the mouth region the representativeness of the different microorganisms varies according to their different sites. This depends on specific characteristics such as nutrient availability, oxidation reduction potential, pH, contact with saliva, access to host defense molecules, and others. 16 In the oral microbiota of healthy individuals the predominant groups in the mucosa are Streptococcus and Haemophilus, in supra and subgingival biofilms are Actinomyces and Prevotella, respectively. 3,18 In the gingival sulcus, there is predominant colonization of obligate anaerobes, besides Prevotella, also stand out Veillonella, Corynebacterium, Fusobacteriumand Rothia. 4 In the lips we can find facultative anaerobes like Streptococcus and in smaller quantity Veillonella, Neisseria and Candida. In the cheeks and tongue, we find Streptococcus, Actinomyces and Haemophillus. 4 The microorganisms found in saliva come from other sites of the mouth region, not being considered as having a resident microbiota. 16 Other microorganisms besides bacteria can also be found frequently in the oral cavity, such as yeasts of the genus Candida. 19 The microbiota has important functions for the host. Among these functions can be mentioned: auxiliary of human being to synthesis compounds that degrade toxic products, immune system modulating and serving as a barrier against pathogens colonization by inhibitory substancesproduction. 5 However, when in disequilibrium with the host, this same microbiota, due to its great diversity and virulence potential, can become pathogenic by its amphibiotic character. 2,5 Therefore, resident microorganisms have the ability to live in harmony with the host, but eventually, due to some imbalance factor, are able to change the virulence factors expression presenting the aggression capacity.
Furthermore, this change in the microbiota, called dysbiosis, is correlated with a higher risk of developing several diseases, including OM, since it may deregulate the immune system responses. 5,20 Neoplasm, as well as their cytotoxic therapies, leads to a myelosuppression in which bone marrow activity is decreased. Thus, the defense system cells production of the individual being treated for cancer is reduced. 4 The myelosuppression state may influence the diversity of the microbiome, [21][22][23] and distinct antitumor treatments result in different oral microbiome changes but in a specific manner in each host. [24][25] In the surgical treatment, the prevalence of T. forsythia tended to decrease. 24 The RT induces changes in the oral microbiota and this is significantly different when associated with the mutational changes of the cancer, 1,5,22,23,26 changing progressively with tumor progression. 23,27 In the treatments of RT with or without adjuvant CT, there is an increase of bacteria from some groups  • OM developed in 66.6% patients in CT group and 0% in the control group.
• No significant differences were found in bacterial alterations between tree times of sampling in CT group.

•
In the control group, the bacterial count remained unchanged during the observation period.
• There are no microbial changes in dental plaque in patients within 7 days of the first CT cycle.

•
The correlation be- The most severe cases of OM have a tendency of a greater growth of Neisseria in the tongue and a decrease of growth of Prevotella, S. Aureus, Candida and Gramnegative. 12 Furthermore OM severity is related to xerostomia, candidiasis, poor hygiene, absence of previous dental treatment, 5 sub-and supragingival colonization of Enterobacteriaceae and Candida. 5,12 It is known that RT and/or CT treatment leads to alteration in the oral microbiota, favoring the