# Polymethylmethacrylate (PMMA) Hosts Microbiome Associated with Oral Malodor
## 4 Discussion  
Recent advancements in molecular microbiology including the \"Human Oral Microbiome Database (HOMD)\" project [23] and the application of deep sequencing technology has opened up new ways for examining the vast microbial population of the oral cavity (i.e., oral microbiome) in health and disease. In the current study, we implemented these new tools in order to investigate the microbial population of temporary dental crowns, specifically regarding oral malodor production. Results of the current study demonstrated several characteristic differences between the microbiome associated with malodorous temporary dental crowns as compared with non-malodorous ones both regarding composition as well as diversity. At phyla level, the phylum _Firmicutes_ demonstrated higher abundance in the malodor negative group. This observation could be partially explained by the increased abundance of the genera  
Figure 5: Heat map clustered according to genera found within the samples and ordered according to their relative abundance. Subjects (malodor—red; non-malodor—blue) are clustered according to genera found in each sample. Veillonella and _Streptococcus_ within this group. Although some studies reported _Veillonella_ spp. as an important hydrogen sulfide producer [24], it is not traditionally considered a malador producing genus since it does not normally produce any malodors when cultivated in growth media. Genus level analysis on the other hand provided a clearer explanation for the differences between the groups. Some genera that are well recognized for their malador producing properties such as _Prevotella_, _Tannerella_, _Treponema_ and _Porlynomonas_[19] showed higher abundance in the malador-positive samples. Despite the fact that these bacteria are associated with periodontal disease, they can often be found in patients without (or at risk of developing) periodontal disease since their initial colonization is on the tongue [25]. These Gram-negative, anaerobic, proteolytic bacteria are known to putrefy proteins and produce malodorous metabolites such as volatile sulfides, diamines (e.g., cadaverine and putrecine), volatile fatty acids and indoles. The oral cavity hosts hundreds of different bacterial species and shows high variability in microbial composition between different individuals. This, in addition to the relatively high prevalence of halitosis, suggests that oral malador is not produced by a specific bacterium but is most likely the result of a biochemical \"joint effort\" of several bacterial species. The wide view given by the 16S deep sequencing microbiome database analysis used in the current study provided some support for this assumption by demonstrating the differences between malador positive and negative samples with an emphasis on two key features: diversity and clustering. The observation that a more richly diverse bacterial population shows higher levels of malador production has been previously reported over 75 years ago [12]. Furthermore, there is ample evidence for the co-dependence of various oral bacterial species that rely on one another for metabolism and growth as well as malador production [26]. In the current study, results of the microbial population analysis demonstrated a higher diversity in samples extracted from malodorous temporary dental crowns. Furthermore, clustering analysis using PCoA and heat map centroid clustering demonstrated two definite clusters: one dominantly representing the non-malador group and another more scattered, comprised mainly by malador samples. Results of the present study showed that whereas no significant differences were noted in clinical parameters, medication or patient habits between the malador positive and negative groups, only in the malador positive group there was a significant correlation between malador scores and service time. This observation might suggest that there is a pre-inclination for this condition and once it exists it may worsen over time.