The Periodontal Pathogen Porphyromonas gingivalis Induces Expression of Transposases and Cell Death of Streptococcus mitis in a Biofilm Model

Abstract

Oral microbial communities are extremely complex biofilms with high numbers of bacterial species interacting with each other (and the host) to maintain homeostasis of the system. Disturbance in the oral microbiome homeostasis can lead to either caries or periodontitis, two of the most common human diseases. Periodontitis is a polymicrobial disease caused by the coordinated action of a complex microbial community, which results in inflammation of tissues that support the teeth. It is the most common cause of tooth loss among adults in the United States, and recent studies have suggested that it may increase the risk for systemic conditions such as cardiovascular diseases. In a recent series of papers, Hajishengallis and coworkers proposed the idea of the "keystone-pathogen" where low-abundance microbial pathogens (Porphyromonas gingivalis) can orchestrate inflammatory disease by turning a benign microbial community into a dysbiotic one. The exact mechanisms by which these pathogens reorganize the healthy oral microbiome are still unknown. In the present manuscript, we present results demonstrating that P. gingivalis induces S. mitis death and DNA fragmentation in an in vitro biofilm system. Moreover, we report here the induction of expression of multiple transposases in a Streptococcus mitis biofilm when the periodontopathogen P. gingivalis is present. Based on these results, we hypothesize that P. gingivalis induces S. mitis cell death by an unknown mechanism, shaping the oral microbiome to its advantage.T he human oral microbiome consists of over 600 individual taxa (1, 2). Its complexity and the fact that a large fraction of species is noncultivable have hampered our knowledge of the molecular and metabolic interactions that occur between the species of the bacterial biofilm and between bacteria and host.Periodontal disease is a polymicrobial inflammatory biofilm-mediated pathology that leads to a progressive loosening and eventual loss of teeth (3, 4), and it is responsible for half of all tooth loss in adults that occurs in moderate form in 39% of American adults and in severe form in 9% of adults. Polymicrobial diseases are increasingly being recognized as more frequent than previously thought (5). In these diseases there are complex interactions among the etiologic agents, which ultimately lead to disease. In the microbial etiology of periodontitis it is generally accepted that a consortium of bacteria, not a single microorganism, is involved in the disease. Nonetheless, it is well established that in destructive periodontitis, the "red complex" (Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia) are key players in the disease process (6).A more holistic treatment of periodontitis or any other polymicrobial disease requires a better understanding of the ecology of the microbial community and the mechanisms that cause the shift from a mature stable biofilm to a dysbiotic community responsible for disease. Maintaining a healthy biofilm would be a less invasive strategy than removing an ...

Polymicrobial synergy and dysbiosis in inflammatory disease

“…Filifactor alocis , Peptostreptococcus stomatis and species from the genera Prevotella , Megasphaera , Selenomonas , and Desulfobulbus ) that have now been shown by metagenomic analyses to exhibit as good (or better) a correlation with disease as traditional pathogenic species, such as P. gingivalis , Tannerella forsythia , and Treponema denticola [14–17]. Consistent with this notion, a recent metatranscriptomic study revealed that the majority of virulence factors upregulated in the microbiome of periodontitis patients is primarily derived from previously underappreciated species that were not traditionally implicated in periodontitis [18]. It is possible that pathobionts might outcompete keystone or keystone-like pathogens such as P. gingivalis and T. forsythia in late stages of periodontal disease pathogenesis.…”

“…Consequently, the distribution of P. gingivalis and S. cristatus in human subgingival plaque is negatively correlated, and S. cristatus attenuates P. gingivalis -induced bone loss in mice [35]. Such antagonistic interaction may lead to a ‘Red Queen’ effect, as P. gingivalis can induce cell death of another oral streptococcus, S. mitis [18]. Overall, however, the periodontal microbiota conforms more with the ‘Black Queen Hypothesis’ [36], whereby functions that are energetically costly and are discarded by ‘cheaters’, are nevertheless retained by a subset of community members as they benefit the entire community.…”

The Biofilm Community: Rebels with a Cause

“…During progression of periodontitis, there is a reduction in the fraction of streptococci present in the biofilm. Studies have shown that P. gingivalis induces S. mitis cell death (10-fold increase); and DNA fragmentation, and increases production of reactive oxygen species by up to 25-fold in an in vitro biofilm system by an unknown mechanism, thus shaping the oral microbiome to its advantage [28]. Therefore, it could be reasoned that successive biofilm formation may play a role in major periodontal species pathogenicity.…”

“…These are also clustered as the “red complex” species [28]. Studies investigating colonization of human gingival multi-layered epithelium by 10-species subgingival biofilm ( F. nucleatum, Campylobacter rectus, V. dispar, P. gingivalis, P. intermedia, T. forsythia, T. denticola, A. oris, S. anginosus and S. oralis ) or its 7-species variant lacking P. gingivalis, T. forsythia and T. denticola , evaluated the relative effects of the "red complex" species on the composition and cell-colonizing capacities of the remaining seven species [61].…”

Pathogenic Bacterial Species Associated with Endodontic Infection Evade Innate Immune Control by Disabling Neutrophils

“…Material adhering to the chamber material during S. mitis infection at 3 days, however, appears to be comprised primarily of neutrophils, and individual bacteria were present but not common. This strain has been shown to form biofilm under appropriate conditions in vitro (49), suggesting that biofilm forms in the chamber model as well. Biofilm formation may contribute to the persistence of the bacteria in the EP-infected chambers and may account to some extent for the difference in clearance between EP and S. mitis.…”