Microbes and Infection 17 (2015) 471e472 www.elsevier.com/locate/micinf
Editorial
Guest editorial for special oral microbes edition Technological advances, specifically improvements in sequencing and computational tools, combined with new findings about polymicrobial infections and uncultivable organisms, have revolutionized our understanding of the oral microbiota and its role in oral-dental health and disease. In 2007, the US National Institutes of Health launched the Human Microbiome Project (HMP) followed closely in Europe and China by the Human Intestinal Tract Project (MetaHIT). Both projects demonstrated the feasibility of using massively parallel short-read sequencing technologies for analysis of complex microbial ecosystems and have transformed our thinking on habitat diversity (who's there?) and metabolic function (what are they doing?). Following many decades of culture-dependent studies involving the oral microbiota, metagenomic projects like the HMP have confirmed that the oral cavity is one of the most taxonomically diverse body sites second only to the gut in overall microbial complexity. Building on a rich legacy of culture-dependent studies defining the physiology and genetics of oral bacteria in monoculture, basic immunology of oral infection and inflammation, and rudimentary animal models of virulence, investigators now are turning their attention from pure culture studies to the collective microbiota and those factors that favor either homeostasis or that drive dysbiosis (i.e. microbial imbalance). In the case of periodontitis, central to this dogma is the polymicrobial synergy and dysbiosis (PSD) model of oral microbial disease. Within the last decade evidence has mounted suggesting that Porphyromonas gingivalis may act as a classic keystone species assisting in the transformation of the oral microbiota from an otherwise benign state to a pathogenic state via dysbiosis, elevated community virulence, and inflammation. In a mouse model of periodontitis, P. gingivalis exploited components of the immune system such as complement and Toll-like receptor-2 (TLR2) mediated pathways to inhibit neutrophil killing but not neutrophil-mediated inflammation. P. gingivalis-induced activation of these pathways leads to downstream events preventing phagocytosis while stimulating an inflammatory response and ultimately limiting the clearance of the infection. This alternate mechanism provided “bystander” protection to otherwise susceptible bacterial species and promoted polymicrobial dysbiotic inflammation in this particular mouse model. Understanding how P. gingivalis
http://dx.doi.org/10.1016/j.micinf.2015.03.005 1286-4579/Published by Elsevier Masson SAS on behalf of Institut Pasteur.
subverts key immunological pathways in the host may allow for the identification of new targets and therapies to block unwarranted immune cell recruitment, and subsequent destruction of hard and soft tissues seen in periodontitis. Key to investigations studying polymicrobial populations relevant to the human oral cavity is the availability of robust biofilm and animal model systems that can recapitulate, in the laboratory to various degrees, those conditions representing states of health and disease. Advances in microfluidic culture technologies coupled with quantitative real-time imaging systems are facilitating in vitro studies with minimal requirements for saliva-based culture media. Likewise, exciting new animal models are under development for in vivo investigations that more closely mimic human biology. A remaining microbiological phenomenon relevant to the oral cavity is the Great Plate Count Anomaly. This fascinating anomaly became apparent at the dawn of modern microbiology having been observed in all polymicrobial systems examined to date. The anomaly represents the vast discrepancy seen between direct microbial cell counts and recoverable colony forming units on laboratory media. In the early days cell counts were performed by direct microscopic observation whereas today missing cultural diversity is detected via sequencing methods. In the oral cavity these elusive phylotypes are often referred to as “yet-to-be cultured”, “uncultivable” species, or microbial “dark matter”. In terms of the human oral cavity, these missing organisms represent the next grand frontier of oral microbiology, particularly if one considers their potential roles in homeostasis or dysbiosis. Several exciting projects are underway to recover these phylotypes in culture through various domestication and co-cultivation strategies and to understand the role of the uncultivables in oral health and disease. One recent example involves the recovery of the first human TM7 phylotype associated with human inflammatory mucosal disease. Designated TM7x, this small coccus has a distinctive lifestyle existing in close association (as an obligate epibiont) with Actinomyces odontolyticus XH001. Its genome (705 kb) exhibits remarkable synteny with environmental TM7 isolates and surprisingly displays a complete lack of amino acid biosynthetic capacity. Furthermore, TM7x, when physically associated with XH001, greatly reduces induction of tumor necrosis factor (TNF-a) gene expression in macrophages providing the first evidence
472
Editorial / Microbes and Infection 17 (2015) 471e472
that TM7x may modulate the host immune response, perhaps having a direct role in oral-dental associated diseases. We truly are at a novel point in the history of modern oral microbiology; one that affords a robust tool set to allow us to ask the really hard questions surrounding oral infection, host defense and oral-dental linked diseases, and the acceptance and need to begin addressing the vast uncultivable diversity of the oral microbiota. Conflict of interest The authors have no conflict of interest concerning the contents of this editorial.
Robert D. Lunsford* Amanda A. Melillo Martha J. Somerman National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD, USA *Corresponding author. E-mail address: [email protected] (R.D. Lunsford) 7 March 2015 Available online 20 March 2015
Editorial
Guest editorial for special oral microbes edition Technological advances, specifically improvements in sequencing and computational tools, combined with new findings about polymicrobial infections and uncultivable organisms, have revolutionized our understanding of the oral microbiota and its role in oral-dental health and disease. In 2007, the US National Institutes of Health launched the Human Microbiome Project (HMP) followed closely in Europe and China by the Human Intestinal Tract Project (MetaHIT). Both projects demonstrated the feasibility of using massively parallel short-read sequencing technologies for analysis of complex microbial ecosystems and have transformed our thinking on habitat diversity (who's there?) and metabolic function (what are they doing?). Following many decades of culture-dependent studies involving the oral microbiota, metagenomic projects like the HMP have confirmed that the oral cavity is one of the most taxonomically diverse body sites second only to the gut in overall microbial complexity. Building on a rich legacy of culture-dependent studies defining the physiology and genetics of oral bacteria in monoculture, basic immunology of oral infection and inflammation, and rudimentary animal models of virulence, investigators now are turning their attention from pure culture studies to the collective microbiota and those factors that favor either homeostasis or that drive dysbiosis (i.e. microbial imbalance). In the case of periodontitis, central to this dogma is the polymicrobial synergy and dysbiosis (PSD) model of oral microbial disease. Within the last decade evidence has mounted suggesting that Porphyromonas gingivalis may act as a classic keystone species assisting in the transformation of the oral microbiota from an otherwise benign state to a pathogenic state via dysbiosis, elevated community virulence, and inflammation. In a mouse model of periodontitis, P. gingivalis exploited components of the immune system such as complement and Toll-like receptor-2 (TLR2) mediated pathways to inhibit neutrophil killing but not neutrophil-mediated inflammation. P. gingivalis-induced activation of these pathways leads to downstream events preventing phagocytosis while stimulating an inflammatory response and ultimately limiting the clearance of the infection. This alternate mechanism provided “bystander” protection to otherwise susceptible bacterial species and promoted polymicrobial dysbiotic inflammation in this particular mouse model. Understanding how P. gingivalis
http://dx.doi.org/10.1016/j.micinf.2015.03.005 1286-4579/Published by Elsevier Masson SAS on behalf of Institut Pasteur.
subverts key immunological pathways in the host may allow for the identification of new targets and therapies to block unwarranted immune cell recruitment, and subsequent destruction of hard and soft tissues seen in periodontitis. Key to investigations studying polymicrobial populations relevant to the human oral cavity is the availability of robust biofilm and animal model systems that can recapitulate, in the laboratory to various degrees, those conditions representing states of health and disease. Advances in microfluidic culture technologies coupled with quantitative real-time imaging systems are facilitating in vitro studies with minimal requirements for saliva-based culture media. Likewise, exciting new animal models are under development for in vivo investigations that more closely mimic human biology. A remaining microbiological phenomenon relevant to the oral cavity is the Great Plate Count Anomaly. This fascinating anomaly became apparent at the dawn of modern microbiology having been observed in all polymicrobial systems examined to date. The anomaly represents the vast discrepancy seen between direct microbial cell counts and recoverable colony forming units on laboratory media. In the early days cell counts were performed by direct microscopic observation whereas today missing cultural diversity is detected via sequencing methods. In the oral cavity these elusive phylotypes are often referred to as “yet-to-be cultured”, “uncultivable” species, or microbial “dark matter”. In terms of the human oral cavity, these missing organisms represent the next grand frontier of oral microbiology, particularly if one considers their potential roles in homeostasis or dysbiosis. Several exciting projects are underway to recover these phylotypes in culture through various domestication and co-cultivation strategies and to understand the role of the uncultivables in oral health and disease. One recent example involves the recovery of the first human TM7 phylotype associated with human inflammatory mucosal disease. Designated TM7x, this small coccus has a distinctive lifestyle existing in close association (as an obligate epibiont) with Actinomyces odontolyticus XH001. Its genome (705 kb) exhibits remarkable synteny with environmental TM7 isolates and surprisingly displays a complete lack of amino acid biosynthetic capacity. Furthermore, TM7x, when physically associated with XH001, greatly reduces induction of tumor necrosis factor (TNF-a) gene expression in macrophages providing the first evidence
472
Editorial / Microbes and Infection 17 (2015) 471e472
that TM7x may modulate the host immune response, perhaps having a direct role in oral-dental associated diseases. We truly are at a novel point in the history of modern oral microbiology; one that affords a robust tool set to allow us to ask the really hard questions surrounding oral infection, host defense and oral-dental linked diseases, and the acceptance and need to begin addressing the vast uncultivable diversity of the oral microbiota. Conflict of interest The authors have no conflict of interest concerning the contents of this editorial.
Robert D. Lunsford* Amanda A. Melillo Martha J. Somerman National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD, USA *Corresponding author. E-mail address: [email protected] (R.D. Lunsford) 7 March 2015 Available online 20 March 2015
