HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Figures Only Full Text (PDF) Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ruby, J. Articles by Goldner, M. Search for Related Content PubMed PubMed Citation Articles by Ruby, J. Articles by Goldner, M.

Nature of Symbiosis in Oral Disease

¹ Department of Pediatric Dentistry, School of Dentistry, The University of Alabama at Birmingham, 1919 7th Ave. South, Birmingham, AL 35294, USA; and
² Département de biologie médicale, Faculté de Médecine, Université Laval, Québec, QC G1K 7P4, Canada

^* corresponding author, john_ruby{at}cs1.dental.uab.edu

Martin A. Taubman, Editor

KEY WORDS: symbiosis • amphibiosis • enrichment culture • indigenous oral bacteria • dental caries • periodontal disease

"Everything is everywhere; but the milieu selects...in nature and in the laboratory."

— L.G.M. Baas Becking, 1934 (Leadbetter, 1997)

INTRODUCTION

The mouth is inhabited by an indigenous "normal" microflora that is composed of over 500 species—the majority still uncultivable. Soon after birth, we begin to acquire a complex consortium of micro-organisms by salivary transmission associated with intimate human contact (Carlsson et al., 1975). Our oral flora is symbiotic with us, for we have a "life together". The microflora is usually benign and certainly is essential for our well-being. However, this protean entity may become parasitic, leading to the pathogenesis of infectious oral diseases, e.g., dental caries and periodontal disease.

How can the same microflora that engenders oral health also cause endogenous disease, i.e., a pathogenic infection caused by the indigenous microflora? The paradoxical relationship between the human host and the indigenous oral microflora can be understood as the synthesis of three processes: symbiosis, amphibiosis, and microbial enrichment culture.

SYMBIOSIS

The etymology of symbiosis is Greek (µß), meaning "life together". In 1879, the publication of Die Erscheinung der Symbiose, by Anton de Bary (de Bary, 1879), first introduced the term as "the living together of unlike named organisms". By the turn of the century, symbiosis was recognized as a universal biological phenomenon, culminating in Paul Portier’s treatise Les Symbiotes (Portier, 1918). The American Society of Parasitologists’ Committee on Terminology recommended that symbiosis should be defined within the context of microbiology, as de Bary intended—dissimilar organisms living together (Hertig et al., 1937). Current definitions of symbiosis support the ideas of these early biologists. Dorland’s Illustrated Medical Dictionary, 30th edition, defines symbiosis as: "...the living together or close association of two dissimilar organisms, each of the organisms being known as a symbiont. The association may be beneficial to both (mutualism), beneficial to one without effect on the other (commensalism), beneficial to one and detrimental to the other (parasitism)...". It is crucial that the original concept of symbiosis (living together) be maintained. Symbiosis as mutualism is restrictive and limits our use of language. Clear knowledge of this term is an essential first step if the infinite possibilities that characterize the dynamic nature of host-symbiont interactions are to be understood. Moreover, commensalism should be excluded as a characteristic of symbiosis, as stated, respectively, by Theodor Rosebury and Ron Gibbons: "From our present standpoint the word commensal is objectionable because of its imprecise overtones, especially when it implies a mutually neutral or passive relationship between host and microbe which we assume does not exist" (Rosebury, 1962), and "While commensalism may be theoretically possible, practically it rarely if ever exists, for it is hard to conceive of two organisms, be they micro- or macroorganisms, which live in close association without interaction and affecting each other" (Gibbons, 1969).

Roger Stanier suggested that the character of symbiosis was subject to change and adaptation, depending on environmental conditions, so as to explain the behavior of organisms in their natural environment (Stanier, 1953). For Stanier, symbiosis could be analyzed according to intimacy, advantage, and dependence. The nature of a particular symbiosis may shift under changing conditions in a reciprocal manner, with mutualism becoming parasitism and vice versa (Stanier et al., 1970).

AMPHIBIOSIS

Considering the breadth of symbiosis, how does its changing and adaptive nature, as noted by Stanier, relate to the endogenous character of oral disease? We should recognize that symbiosis is the archetype of all relationships between the host and microbial symbionts in Nature. In an evolutionary sense, the continuous adaptation, both temporally and spatially, of organisms within biofilms—a dense aggregate of microorganisms adhering to a surface in an organized fashion—would enhance the fitness of the microbial community to the ever-changing oral environment of the host. Microbial adaptation as a requirement for symbiotic existence was recognized by Rosebury in Microorganisms Indigenous to Man, wherein he referred to this phenomenon as ’amphibiosis’ (Rosebury, 1962). An amphibiont or a symbiont undergoing dynamic change holds a spectral position between probiosis (mutualistic symbiosis) and antibiosis or pathogenicity (parasitic symbiosis). Endogenous oral disease is caused by heterogenous microbial populations within biofilms; thus, amphibiosis can be viewed as a process of change within microbial consortia as a whole. Therefore, amphibiotic shifts within microbial populations in dental plaque biofilms are the essential determinants in the causation of endogenous infectious disease by a parasitic microflora, or the maintenance of oral health by a mutualistic microflora. The role of amphibiosis in endogenous oral disease is congruent with Philip Marsh’s "ecological plaque hypothesis", proposed in the early 1990s (Marsh, 1994), which states clearly that microbial populations associated with dental caries and periodontal inflammation are the result of fluxes in pH and Eh [oxidation-reduction potential, a measure in millivolts (mV) of the reducing-potential of an environment] (Bartlett and Gorbach, 1981).

ENRICHMENT CULTURE, A CRITICAL DETERMINANT OF ORAL PATHOGENESIS

Heterotrophic energy requirements provide strong selective pressures that determine the composition of oral biofilm communities. Martinus Beijerinck (1851–1931), of the Delft School of Microbiology, predicted that one could select for specific micro-organisms from natural environments through the use of specific culture media and incubation conditions that favored the growth of certain organisms while constraining the growth of others (Brock, 1961; Madigan et al., 2000). Sergei Winogradsky (1856–1953) isolated soil bacteria directly from their natural habitats, using glass columns that modeled natural anaerobic ecosystems and served to enrich for a microorganism’s specific metabolic needs (Brock, 1961; Madigan et al., 2000). Importantly, L.G.M. Baas Becking noted, in 1934, that enriched bacterial growth occurs not only in the laboratory, but also in Nature (Leadbetter, 1997).

The ability to acquire, transport, and catabolize organic compounds efficiently in the oral cavity is a necessary ecological determinant for microbial growth, and provides relevant selective advantages to organisms that are most adaptive to a particular microenvironment. Furthermore, the metabolic constraints imposed by the anaerobic milieu in an oral biofilm result in energy metabolism that yields minimal ATP via substrate-level phosphorylation. The generation of ATP in dense microbial biofilm populations such as dental plaque has to be immediate and rapid for growth and survival in these competitive environments. Availability of specific nutrients exerts strong selective pressures for microbial survival and determines species dominance and composition in the various oral ecosystems. The establishment of cariogenic-saccharolytic organisms—e.g., Streptococcus mutans and lactobacilli—in supragingival plaques, and periodontopathic-proteolytic organisms—e.g., Treponema denticola, Tannerella forsythensis and Porphyromonas gingivalis ("red complex")—in subgingival pockets is a functional equivalent of enrichment culture. Dietary sugars would enrich for lactic acid bacteria in supragingival dental plaques, while protein and heme in gingival crevicular fluid (GCF) would enrich for proteolytic organisms in the subgingival ecosystem.

DENTAL CARIES

The primary etiologic agents of supragingival dental caries are the mutans streptococci (S. mutans and Streptococcus sobrinus) and lactobacilli, while in root-surface caries, Actinomyces spp. are involved. Oral cavities with healthy dentitions usually have decreased numbers of mutans streptococci in plaque, low lactobacilli counts in saliva, and high plaque counts of Streptococcus sanguinis. This is the result of restricted sugar intake and reverses with the addition of sugar to the diet. Continuous-culture studies have substantiated these observations, and species fluctuations appear to be pH-driven, i.e., selected for by the environment. The intermittent consumption of dietary sugar establishes an enrichment culture within the oral cavity that is selective for acidogenic/aciduric chemo-organotrophic micro-organisms. These organisms have evolved metabolic pathways for efficient energy production and storage during cycles of feast and famine. Therefore, dental caries could be viewed as a microbial adaptation to high-energy fluxes occurring in the oral cavity. Micro-organisms that have evolved metabolically to compete efficiently at low pH are the lactic acid bacteria. Stanier et al.(1970) noted, in The Microbial World:

"The capacity of lactic acid bacteria to produce [acidogenic] and tolerate [aciduric] a relatively high concentration of lactic acid is of great selective value, since it enables them to eliminate competition from most other bacteria in environments that are rich in nutrients. This is shown by the fact that lactic acid bacteria can be readily enriched from natural sources through the use of complex media with a high sugar content. Such media, of course, support the growth for many other chemo-organotrophic bacteria, but the competing organisms are almost completely eliminated as growth proceeds by the accumulation of lactic acid, formed through the metabolic activity of the lactic acid bacteria."

Dental caries is the result of selection for a cariogenic microflora by frequent sugar ingestion by the host. Throughout human history, our "sweet tooth" has influenced our dietary preferences, and it still does today. Consider that 14 billion gallons (576 12-ounce servings per person) of soft drinks were consumed by Americans in 1997 (Jacobson, 2004). Each 12-ounce/355-mL can of Coca-Cola^® contains 39 grams of sucrose and/or high-fructose corn syrup at pH 2.47. High levels of mutans streptococci in supragingival dental plaque reflect microbial enrichment, with dietary sugar as the predominant heterotrophic energy source. Mutans streptococci and lactobacilli species may be considered quintessential "cariogens", since they are highly adapted to an anaerobic saccharose-containing milieu and are more able to out-compete other microbial inhabitants of the oral cavity. Increased dental plaque cariogenicity would be a manifestation of an amphibiotic shift, from a mutualistic symbiosis associated with dental health, to a parasitic symbiosis associated with dental caries (Fig. 1).

View larger version (35K): [in this window] [in a new window]   Figure 1. Dental caries as enrichment culture.

The microflora found in the healthy gingival sulcus are usually Gram-positive facultative anaerobes, and when dental plaque accumulates if oral hygiene is curtailed, Gram-negative species, along with motile forms, increase proportionally with the severity of gingival inflammation. The onset of gingivitis coincides with altered species selectivity and the emergence of obligate anaerobes. Since gingival inflammation intensifies with an increase in GCF, there is a reduction in Eh that is indicative of a transition to an anaerobic environment. The mean Eh for a healthy gingival sulcus is +73 mV, whereas the mean Eh for a periodontal pocket is –48 mV (Kenney and Ash, 1969); however, localized infected areas with characteristic low Eh may exist adjacent to healthy areas, indicating topographic variability of the subgingival flora. As compared with the saccharolytic nature of supragingival plaque exposed to dietary carbohydrates, the subgingival microflora is primarily a proteolytic microflora. Bleeding associated with gingival inflammation releases red blood cells into the sulcular environment, where they undergo lysis by bacterial hemolysins. The availability of iron in the form of heme, heme proteins, and GCF, a rich source of serum protein, is highly selective for proteolytic micro-organisms. Laboratory studies have focused on the changes in species of subgingival bacteria when enriched on human serum; the prolonged growth of subgingival consortia in a chemostat led to the enrichment of proteolytic micro-organisms, characterized by their progressive protein degradation (ter Steeg et al., 1987). Successions of microbes would be linked to their collective metabolic reactions affecting Eh. For example, facultative anaerobes that are aerotolerant would perform the initial reduction of available oxygen species—highly reactive electron acceptors. As the syntrophic metabolic activity of these anaerobic consortia continues, heterotrophic organic compounds would be fermented to acetate, butyrate, ethanol, formate, lactate, propionate, and succinate, each creating a niche for continued energy extraction. Further reduction of these fermentative end-products would ultimately lead to methane production by methanogenic bacteria at the bottom of the energy sink. There would be a drop in Eh as the ratio of reduced to oxidized compounds increases, indicating that Eh is the primary selective pressure for anaerobes in the subgingival micro-environment (Fig. 2). In addition, enhanced expression of virulence could occur in the subgingival microflora as Eh levels fall. Porphyromonas gingivalis demonstrated increased hemagglutination and Arg-gingipain activity at a low redox potential. Thus, modulation of virulence could be the direct result of low Eh.

View larger version (35K): [in this window] [in a new window]   Figure 2. Periodontal disease as enrichment culture.

CONCLUSION

The established microbial consortia of the oral cavity colonize or infect a variety of microhabitats; collectively, they contain a composite of all the micro-organisms necessary for either localized health or endogenous disease, as determined by the microbial populations selected for and enriched by the oral milieu. Oral health or disease is an adventitious event resulting from microbial communities adapting to prevailing conditions at a given moment. These microbes should not be considered as opportunistic; this anthropomorphic adjective implies human intention-microbes simply adapt. Understanding the processes of adaptation that result in dental caries and periodontal disease will facilitate the development of holistic strategies for their prevention. We need to know "how" to cause changes that select for oral microbes that result in health instead of disease. This will occur when we look at oral health and disease from an ecological perspective, with amphibiosis as the central determining feature. Theodor Rosebury recognized the need for a general theory of the indigenous micro-biota over 40 years ago, in Microorganisms Indigenous to Man:

Approach to a Theory of the Indigenous Biota

"Evidence of neglect in the development of our knowledge of the indigenous microorganisms is pointed out....It will be apparent that the need is not merely or necessarily more research. What seems to be lacking is coordination. Individual aspects of the larger subject are explored, often enthusiastically and effectively, but too often as well in a kind of isolation, a lack of awareness of the kinship of the findings to other closely or more distantly related observations. What is needed is a general theory of the indigenous microbiota. The data now at hand and the level of their organization...are insufficient to formulate such a theory but may serve as a step in that direction." (Rosebury, 1962)

There is a great deal of information on oral microbiota that has been accumulated, but the challenge of a "general theory" has still to be met.

ACKNOWLEDGMENTS

We thank Mr. David Fisher, Medical Education and Design Services, The University of Alabama at Birmingham, Birmingham, AL for helping design and produce Figures 1 and 2; and we hold Dr. Charles Cox, Aphelion Futures, Fenton, MI in high regard for his generous funding support so that Figures 1 and 2 may be reproduced in color. We also appreciate the valuable editing and critical suggestions of Dr. Robert Calmes, Professor Emeritus, University of Kentucky, Lexington, KY.

Received October 1, 2005; Last revision July 1, 2006; Accepted October 31, 2006

REFERENCES

Bartlett J, Gorbach S (1981). Anaerobic infections of the head and neck. In: Oral biology. Roth G, Calmes R, editors. St. Louis: Mosby, pp. 401–428.

Brock T (1961). Milestones in microbiology. Englewood Cliffs, NJ: Prentice-Hall.

Carlsson J, Grahnén H, Jonsson G (1975). Lactobacilli and streptococci in the mouth of children. Caries Res 9:333–339.[ISI][Medline]

de Bary A (1879). Die erscheinung der symbiose. Strasbourg: Karl J. Trubner.

Gibbons R (1969). Significance of the bacterial flora indigenous to man. Annu Meet Am Inst Oral Biol 27–36.

Hertig M, Taliaferro W, Schwartz B (1937). The terms symbiosis, symbiont and symbiote. J Parasitol 23:326–329.

Jacobson M (2004). Liquid candy. Washington: Center for Science in the Public Interest. http://www.cspinet.org/sodapop/liquid_candy.htm

Kenney E, Ash M (1969). Oxidation reduction potential of developing plaque, periodontal pockets and gingival sulci. J Periodontol 40:630–633.[ISI][Medline]

Leadbetter E. (1997). Prokaryotic diversity: form, ecophysiology, and habitat. In: Manual of environmental microbiology. Hurst C, Knudsen G, McInerney M, Stetzenbach L, Walter M, editors. Washington: ASM, pp. 14–24.

Madigan M, Martinko J, Parker J (2000). Biology of microorganisms. 9th ed. Upper Saddle River, NJ: Prentice-Hall.

Marsh P (1994). Microbial ecology of dental plaque and its significance in health and disease. Adv Dent Res 8:263–271.[Abstract]

Newman H (1990). Plaque and chronic inflammatory periodontal disease. J Clin Periodontol 17:533–541.[ISI][Medline]

Portier P (1918). Les symbiotes. Paris: Masson.

Rosebury T (1962). Microorganisms indigenous to man. New York: McGraw-Hill.

Stanier R (1953). Adaptation, evolutionary and physiological: or Darwinism among micro-organisms. In: Adaptation in micro-organisms. Third Symp. Soc. Gen. Microbiol., London: Cambridge.

Stanier R, Doudoroff M, Adelberg E (1970). The microbial world. 3rd ed. Englewood Cliffs, NJ: Prentice-Hall.

ter Steeg P, van der Hoeven J, DeJong M, van Munster P, Jansen J (1987). Enrichment of subgingival microflora on human serum leading to accumulation of Bacteroides spp., Peptostreptococci and Fusobacteria. Antonie van Leeuwenhoek 53:261–271.[ISI][Medline]

Wilson M, Gibson M, Strahan D, Harvey W (1992). A preliminary evaluation of the use of a redox agent in the treatment of chronic periodontitis. J Periodontal Res 27:522–527.[ISI][Medline]

This Article Figures Only Full Text (PDF) Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ruby, J. Articles by Goldner, M. Search for Related Content PubMed PubMed Citation Articles by Ruby, J. Articles by Goldner, M.