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Description : Human body is made up of over hundred trillion cells of which only around 10% are mammalian. The remainder are the microorganisms that comprise the resident microflora of the host. This resident microflora does not have merely a passive relationship with its host, but contributes directly and indirectly to the normal development of the physiology, nutrition and defence systems of the organism.

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Published on : May 22, 2021 | Views : 6000 | Downloads : 45

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ORAL MICROFLORA By :-Dr PREM SHANKAR CHAUHAN POST GRADUATE STUDENT
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CONTENTS INTRODUCTION THE MOUTH AS A MICROBIAL HABITAT FACTORS AFFECTING THE GROWTH OF MICROORGANISMS IN THE ORAL CAVITY HUMAN ORAL FLORA ACQUISITION OF THE RESIDENT ORAL MICROFLORA AGEING AND THE ORAL MICROFLORA DISTRIBUTION OF THE RESIDENT ORAL MICROFLORA METABOLISM OF ORAL BACTERIA BENEFITS OF THE ORAL MICROBIOTA
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INTRODUCTION Human body is made up of over 1014 cells of which only around 10% are mammalian. The remainder are the microorganisms that comprise the resident microflora of the host. This resident microflora does not have merely a passive relationship with its host, but contributes directly and indirectly to the normal development of the physiology, nutrition and defence systems of the organism. In general, these natural microfloras live in harmony with humans and animals and, indeed, all parties benefit from the association. Loss or perturbation of this resident microflora can lead to colonization by exogenous (and often pathogenic) microorganisms, thereby predisposing sites to disease.
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The oral cavity is a border area between the environment and the human organism, a gateway for food, an outpost of the immune system and above all a multilayered habitat inhabited by countless microorganisms. The mouth is continually exposed to organisms from the external environment ,beginning with the passage through the birth canal. In time a ecological balance is reached that serves to establish a resident microbial flora that remains fairly stable throughout life. Microorganisms in mouth were first described by Anton von Leeuwenhoek in 1683.
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THE MOUTH AS A MICROBIAL HABITAT Not all of the micro-organisms that enter the mouth are able to colonize. The properties of the mouth make it ecologically distinct from all other surfaces of the body, and dictate the types of microbe able to persist. Habitats that provide obviously different ecological conditions include mucosal surfaces (such as the lips, cheek, palate and tongue) and teeth. Ecological conditions within the mouth will also vary during the change from the primary to the permanent dentition and following the extraction of teeth, the insertion of prostheses such as dentures, and any dental treatment, including scaling, polishing and fillings. Transient fluctuations in the stability of the oral ecosystem may be induced by the frequency and type of food ingested, variations in saliva flow and periods of antibiotic therapy.
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Four features that help to make the oral cavity distinct from other areas of the body are: Teeth Specialized mucosal surfaces Saliva Gingival crevicular fluid
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TEETH The mouth is the only normally-accessible site in the body that has hard non-shedding surfaces for microbial colonization. Teeth (and dentures) allow the accumulation of large masses of microorganisms (predominantly bacteria) and their extracellular products, termed dental plaque. Teeth do not provide a uniform habitat but possess several distinct surfaces each of which is optimal for colonization and growth by different populations of microorganism.
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SPECIALIZED MUCOSAL SURFACES The oral cavity have specialized surfaces which contribute to the diversity of the microflora at certain sites. The papillary structure of the dorsum of the tongue provides refuge for many micro-organisms which would otherwise be removed by mastication and the flow of saliva The mouth also contains keratinized (e.g. the palate) as well as non-keratinized, stratified squamous epithelium which may affect the intra-oral distribution of some micro-organisms.
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SALIVA The mouth is kept moist and lubricated by saliva which flows over all the internal surfaces of the oral cavity. Saliva enters the oral cavity via ducts from the major paired parotid, submandibular and sublingual glands as well as from the minor glands of the oral mucosa (labial, lingual, buccal and palatal glands) where it is produced. Saliva contains several ions including sodium, potassium, calcium, chloride, bicarbonate and phosphate . Some of these ions contribute to the buffering property of saliva which can reduce the cariogenic effect of acids produced from the bacterial metabolism of dietary carbohydrates. Bicarbonate is the major buffering system in saliva but phosphates, peptides and proteins are also involved. The mean pH of saliva is between pH 6.75 and 7.25, although the pH and buffering capacity will vary with the flow rate.
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Within a mouth, the flow rate and the concentration of components such as proteins and calcium and phosphate ions have circadian rhythms, with the slowest flow of saliva occurring during sleep. Thus, it is important to avoid consuming sugary foods or drinks before sleeping because the protective functions of saliva are reduced. The major organic constituents of saliva are proteins and glycoproteins, such as mucin, and they influence the oral microflora by: Adsorbing to the tooth surface to form a conditioning film (the acquired pellicle), which determines which microorganisms are able to attach. Acting as primary sources of nutrients (carbohydrates and proteins) for the resident microflora. Aggregating exogenous microorganisms, thereby facilitating their clearance from the mouth by swallowing. Inhibiting the growth of some exogenous microorganisms.
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GINGIVAL CREVICULAR FLUID (GCF) Serum components can reach the mouth by the flow of a serum-like fluid through the junctional epithelium of the gingivae. The flow of GCF is relatively slow at healthy sites, but increases during inflammation. GCF can influence the site by acting as a novel source of nutrients, while its flow will remove nonadherent microbial cells. GCF also contains components of the host defenses which play an important role in regulating the microflora of the gingival crevice in health and disease. The neutrophils in GCF are viable and can phagocytose bacteria within the crevice.
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FACTORS AFFECTING THE GROWTH OF MICROORGANISMS IN THE ORAL CAVITY Temperature Redox potential pH Nutrients Adherence and agglutination Anti-microbial agents. Host defence Host genetics
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TEMPERATURE The human mouth is kept at a relatively constant temperature (35–36°C) which provides stable conditions suitable for the growth of a wide range of microorganisms. Periodontal pockets with active disease (inflammation) have a higher temperature (up to 39°C) compared with healthy sites. Even such relatively small rises in temperature can significantly alter bacterial gene expression, and possibly the competitiveness of individual species. A rise in temperature down-regulated expression of some of the major proteases as well as the gene coding for the major subunit protein of fimbriae in the periodontal pathogen, Porphyromonas gingivalis Up-regulated synthesis of superoxide dismutase, which is involved in the neutralization of toxic oxygen metabolites.
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REDOX POTENTIAL/ANAEROBIOSIS Tissues or microbes that need a positive Eh for viability are termed "aerobes," and those that need a negative Eh are "anaerobes”. Anaerobic species require reduced conditions for their normal metabolism; therefore, it is the degree of oxidation-reduction (redox potential, Eh) at a site that governs their survival. Some anaerobes can survive at aerobic habitats by existing in close partnership with oxygen consuming species. Obligate anaerobes also possess specific molecular defence mechanisms that suitable for the survival and growth of a changing pattern of organisms, and partenable them to cope with low redox potential (highly reduced).
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The development of plaque in this way is associated with a specific succession of microorganisms . Early colonizers will utilize O2 and produce CO2; later colonizers may produce H2 and other reducing agents such as sulphur containing compounds and volatile fermentation products. Thus, as the redox potential is gradually lowered, sites become icularly anaerobes. Periodontal pockets are more reduced ( - 48 m V) than healthy gingival crevices in the same individuals (+ 73 m V). Gradients of O2 concentration and Eh will exist in the oral cavity, particularly in a thick biofilm such as plaque. The redox potential will also affect bacterial metabolism. e.g. the activity intracellular glycolytic enzymes and the pattern of fermentation products of Streptococcus mutants varies under strictly anaerobic conditions. Thus, modifications to the habitat that disturb such gradients may influence the composition and metabolism of the microbial community.
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pH Many micro-organisms require a pH around neutrality for growth, and are sensitive to extremes of acid or alkali. The pH of most surfaces of the mouth is regulated by saliva so that, in general, optimum pH values for microbial growth are provided at sites bathed by this fluid. Bacterial population shifts within the plaque microflora can occur following fluctuations in environmental pH after sugar consumption, the pH in plaque can fall rapidly to below pH 5.0 by the production of acids (predominantly lactic acid) by bacterial metabolism slowly to base-line values then pH recovers. Depending on the frequency of sugar intake, the bacteria in plaque will be exposed to varying challenges of low pH. Many of the predominant plaque bacteria from healthy sites can tolerate only brief conditions of low pH, and are inhibited or killed by more frequent or prolonged exposures to acidic conditions.
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This can result in the enhanced growth of, or colonization by, acid-tolerant species, especially mutans streptococci and Lactobacillus species, which are normally absent or only minor components in dental plaque at healthy sites. Such a change in the bacterial composition of plaque predisposes a surface to dental caries. In contrast, the pH of the gingival crevice becomes alkaline during the host inflammatory response in periodontal disease, e.g. following deamination of amino acids and ammonia production. The mean pH may rise to between pH 7.2 and 7.4 during disease, with a few patients having pockets with a mean pH of around 7.8. This degree of change may perturb the balance of the resident microflora of gingival crevice by favouring the growth and metabolism of periodontal pathogens, such as Porphyromonas gingivalis, that have pH optima for growth above pH 7.5.
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NUTRIENTS The association of an organism with a particular habitat is direct evidence that all of the necessary growth-requiring nutrients are present. The mouth can support a microbial community of great diversity and satisfy the requirements of many nutritionally demanding bacterial population.
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ENDOGENOUS NUTRIENTS The persistence and diversity of the resident oral microflora is due primarily to the metabolism of the endogenous nutrients provided by the host, rather than by exogenous factors in the diet. The main source of endogenous nutrients saliva, which contains amino acids, peptides, proteins and glycoproteins, vitamins and gases. In addition, the gingival crevice is supplied with GCF which, in addition to delivering components of the host defences, contains potential sources of novel nutrients, such as albumin and other host proteins and glycoproteins, including haeme containing molecules. Plaque bacteria proteases, and produce glycosidase interact synergistically and to breakdown these endogenous nutrients as no single species has the full enzyme complement to totally metabolize these nutrients.
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E X O G E N O U S N U T R I E N T S Superimposed upon these endogenous nutrients is the complex array of food stuffs ingested periodically in the diet. Fermentable carbohydrates are the main class of compounds that influence markedly the ecology of the mouth. Such carbohydrates can be broken down to acids. Sucrose can be converted by bacterial enzymes into two classes of polymer (glucans and fructans) which can be used to consolidate attachment or act as extra cellular nutrient storage compounds, respectively. Dairy products (milk, cheese) have some influence on the ecology of the mouth. The ingestion of milk or milk products can protect the teeth of animals against caries. This may be due to the buffering capacity of milk proteins or due to decarboxylation of amino acids after proteolysis since several bacterial species can metabolize casein. Xylitol is a sugar substitute that has been added to some confectionery; it cannot be metabolized by oral bacteria and, in addition, xylitol can be inhibitory to the growth of Streptococcus mutans.
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ADHERENCE AND AGGLUTINATION Chewing and the natural flow of saliva (mean rate = 19 ml/h) will detach microorganisms not firmly attached to an oral surface. Salivary components can aggregate certain bacteria which facilitates their removal from the mouth by swallowing. Bacteria are unable to maintain themselves in saliva by cell division because they are lost at an even faster rate by swallowing. The molecules responsible for agglutination are mucins. Mucins are high molecular weight glycoprotein's. These Mucins not only agglutinate oral bacteria, but can also interact with exogenous pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa, as well as viruses (e.g. aeruginosa influenza virus) Dental plaque formation involves an ordered colonization by a range of bacteria. The early colonizers interact with, and adhere to, saliva coated enamel, while later colonizers bind to already attached species (co-aggregation).
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ANTIMICROBIAL AGENTS AND INHIBITORS Anti-plaque agents are distinguished from antimicrobials on the basis of their mode of action. Anti-plaque agents remove already attached cells, or prevent adhesion of new cells to the tooth surface. Unlike antimicrobials which are designed to kill (bactericidal) or inhibit the growth (bacteriostatic) of the bacteria. Both types of agent can be delivered from toothpastes (dentifrices) and mouthwashes. Antibiotics given systemically or orally for problems at other sites in the body will enter the mouth via saliva or GCF and affect the stability of the oral microflora Within a few hours of taking prophylactic high doses of penicillin's, the salivary microflora can be suppressed permitting the emergence of antibiotic resistant bacteria.
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HOST DEFENCES The health of the mouth is dependent on the integrity of the mucosa (and enamel) which acts as a physical barrier to prevent penetration by micro-organisms or antigens . The host has a number of additional defence mechanisms which play an important role in maintaining the integrity of these oral surfaces.
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HOST GENETICS Gender and race can influence disease susceptibility, and possibly also affect the microflora. In an adult periodontitis group, P. gingivalis and Peptostreptococcus anaerobius were associated more with black subjects whereas, Fusobacterium nucleatum was found more commonly in white individuals. The reasons for this are unknown, but may reflect some variation in the local immune response. The microflora of twin children living together was more similar than that of unrelated children of the same age. Further analysis showed that the micro flora of identical twins was more similar than that of fraternal twins, suggesting some genetic control.
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HUMAN ORAL FLORA Gram-positive facultative cocci Staphylococcus epidermidis Staph. aureus Streptococcus mutans Strep. sanguis Strep. Mitis Strep. Salivarius Strep. Faecalis Beta-hemolytic streptococci Gram-negative facultative rods Enterobacteriaceae Hemophilus influenzae Eikenella corrodens Actinobacillus Actinomycetemcomitans
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Gram-positive anaerobic cocci Peptostreptococcus sp Gram-negative anaerobic cocci Diphtheroids Corynebacterium Gram-positive anaerobic Rods Actinomyces israelii A. odonotolyticus A. Viscosus Lactobacillus Gram-negative aerobic or facultative cocci Eubacterium Neisseria sicca N. Flavescens
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Spirochetes Treponema denticola T. Microdentium Protozoa Entamoeba gingivalis Tirchomonas tenax Yeasts Candida albicans Geotrichum sp. Mycoplasma Mycoplasma orale M. pneumoniae
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ACQUISITION OF THE RESIDENT ORAL MICROFLORA The foetus in the womb is normally sterile. Despite the widespread possibility of contamination, the mouth of the newborn baby is usually sterile. However, from the first feeding onwards, the mouth is regularly inoculated with microorganisms and the process of acquisition of the resident oral microflora begins. Acquisition depends on the successive transmission of micro-organisms to the site of potential colonization. Initially, in the mouth, this is by passive contamination from the mother, from food, milk and water, and from the saliva of individuals in close proximity to the baby. S. salivarius, mutans streptococci and some other species transmitted from mother to child via saliva.
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Mutans streptococci found in children appeared identical to those of their mothers in 71 % of 34 infant-mother pairs examined. No evidence of father to infant transmission of mutans streptococci was observed, although transmission between spouses may occur with some periodontal pathogens, such as P. gingivalis. The first micro-organisms to colonize are termed pioneer species, and collectively they make up the pioneer microbial community. These pioneer species continue to grow and colonize until environmental resistance is encountered. This can be due to several limiting forces (including physical and chemical factors) which act as barriers to further development.
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In the oral cavity, physical factors include the shedding of epithelial cells (desquamation), and the shear forces from chewing and saliva flow. Nutrient requirements, redox potential, pH, and the antibacterial properties of saliva can act as chemical barriers limiting growth. One genus or species is usually predominant during the development of the pioneer community. The pioneer micro-organisms are S. salivarius, S. mitis and S. oralis. With time, the metabolic activity of the pioneer community modifies the environment providing conditions suitable for colonization by a succession of other populations, by: Changing the local Eh or pH. Modifying or exposing new receptors for attachment. Generating novel nutrients.
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Eventually a stable situation is reached with a high species diversity; this is termed the climax community. A climax community reflects a highly dynamic situation and must not be regarded as a static state. The diversity of the pioneer oral community increases during the first few months of life, and several Gram-negative anaerobic species appear . When the infants were followed longitudinally during the eruption of the primary dentition, gram-negative anaerobic bacteria were isolated more commonly, and a greater diversity of species were recovered from around the gingival margin of the newly erupted teeth (infant mean age = 32 months). These findings confirmed that the eruption of teeth has a significant ecological impact on the oral environment, and its resident microflora. The acquisition of some bacteria may occur optimally only at certain ages. Studies of the transmission of mutans streptococci to children have identified a specific 'window of infectivity' between 19 and 31 months (median age = 26 months). This opens up the possibility of targeting preventive strategies over this critical period to reduce the likelihood of subsequent colonization in the infant.
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ALLOGENIC AND AUTOGENIC SUCCESSION Microbial succession -The establishment of a climax community at any oral site involves a series of phases of development during which the complexity of the microflora increases Two distinct types of succession have been identified In Allogenic succession, factors of non-microbial origin are responsible for an altered pattern of community development. For example, the frequency of detection of mutans streptococci and S. sanguinis in the mouth increases markedly once hard, non-shedding surfaces are present. This is usually following tooth eruption, but it can also occur after the insertion of dentures or removable orthodontic appliances, and acrylic obturators in children with cleft palate.
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The increase in number and diversity of obligate anaerobes once teeth are present is an example of autogenic succession in which community development is influenced by microbial factors. The metabolism of the aerobic and facultatively anaerobic pioneer species lowers the redox potential in plaque and creates conditions suitable for colonization by strict anaerobes. Other examples of autogenic succession are the development of food chains and food webs, whereby the metabolic end product of one organism becomes a primary nutrient source for another:
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AGEING AND THE ORAL MICROFLORA At Birth The mouth of full term foetus is usually sterile, transient flora from the birth canal may be acquired. Mouth then rapidly acquires organisms from mother and from the environment. It consists of several streptococcal and staphylococcal species with Lactobacilli, Bacillus, Neisseria and Yeasts. Streptococcus salivarius is the most common and forms the pioneer community with Staphylococcus albus.
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Infancy & Early Childhood The infant comes into contact with an ever-increasing range of microorganisms and some become established as part of commensal flora. The eruption of deciduous teeth provides a new attachment surface and turns Streptococcus sanguis and mutans as regular inhabitants of oral cavity. Anaerobes are few in number due to absence of deep gingival crevice. Actinomyces , Lactobacilli are found regularly.
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Adolescence The greatest number of organisms in mouth occur when permanent teeth erupt. These teeth have deep fissures, larger inter proximal spaces and deeper gingival crevice, allowing a great increase in anaerobes.
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Adulthood Its chief characteristic is its complexity of oral flora. There is an increase in Bacteroides and Spirochetes with maturity of dental plaque. As the teeth are lost the available sites for microbial colonisation decreases and several species diminish disproportionately in numbers. Edentulous persons harbour few Spirochetes or Bacteroides but carriage of Yeast increases. S.sanguis & mutans disappear
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Some variations in the oral microflora have been discerned in later life and can be attributed to both direct and indirect effects of ageing.
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DISTRIBUTION OF THE RESIDENT ORAL MICROFLORA The populations making up the resident microbial community of the oral cavity are not found with equal frequency throughout the mouth. The composition of these communities varies on distinct surfaces due to differences in the biological and physical properties of each site.
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LIPS AND PALATE The lips form the border between the skin microflora (which consists predominantly of staphylococci, micrococci and Gram positive rods such as Corynebacterium and Propionibacterium spp.), and that of the mouth (which contains many Gram negative species and few of the organisms commonly found on the skin surface). Facultatively anaerobic streptococci comprise a large part of the microflora on the lips. Candida albicans can colonize damaged lip mucosal surfaces in the corners of the mouth (‘angular cheilitis’). The microflora of the normal palate can show large variations between subjects. The majority of the bacteria are streptococci and actinomyces; veillonellae, haemophili and Gram negative anaerobes are also regularly recovered but at lower levels. Candida are not regularly isolated from the normal palate except when dentures are worn; in this situation, the mucosa of the palate can become infected with C. albicans (denture stomatitis)
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CHEEK Streptococci are the predominant bacteria from the cheek (buccal mucosa), especially members of the mitis-group, and H. parainfluenzae is also commonly isolated . Simonsiella spp. are isolated primarily from the cheek cells of humans and animals. Molecular techniques have confirmed the presence of a range of streptococcal species on buccal cells, as well as Granulicatella and Gemella spp. Fluorescent in situ hybridization (FISH) was combined with confocal microscopy, have shown that some of the species implicated in periodontal disease (A. actinomycetemcomitans, P. gingivalis, F. nucleatum, P. intermedia, T. forsythia) can gain refuge inside buccal epithelial cells in healthy people, where they persist as intracellular polymicrobial communities
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TONGUE The dorsum of the tongue with its highly papillated surface has a large surface area and supports a higher bacterial density and a more diverse microflora than other oral mucosal surfaces Streptococci are the most numerous group of bacteria (approximately 40% of the total cultivable microflora) with salivarius- and mitis-group organisms predominating. Anaerobic streptococci have also been isolated while Rothia mucilagenosa is found almost exclusively on the tongue. Other major groups of bacteria (and their proportions) include Veillonella spp. (16%), Gram positive rods (16%) of which Actinomyces naeslundii and A. odontolyticus are common, and haemophili (15%). Both pigmenting (Prevotella intermedia, P. melaninogenica) and non-pigmenting anaerobes can be recovered from the tongue and this site is regarded as a potential reservoir (along with the tonsils) for some of the organisms implicated in periodontal diseases.
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SALIVA Although saliva contains up to 108 microorganisms ml−1 it is not considered to have its own resident microflora. The normal rate of swallowing ensures that bacteria cannot be maintained in the mouth by multiplication in saliva. The organisms found are derived from other surfaces, especially the tongue as a result of oral removal forces (saliva and GCF flow, chewing, oral hygiene). The microbial profile of saliva (in particular, the level of mutans streptococci and/or lactobacilli) has been used as an indicator of the caries susceptibility of an individual, and commercially-available kits for their culture are available. People with high counts of these potentially cariogenic bacteria are considered to be ‘at-risk’, and can be targeted for intense oral hygiene, antimicrobial therapy and dietary counselling.
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TEETH The microbial community associated with teeth is referred to as dental plaque. Its composition varies at each tooth surface due to the local environmental conditions . Plaque is described on the basis of the sampling site by terms such as smooth surface, approximal, fissure, or gingival crevice plaque, while the terms supragingival and subgingival plaque are used to describe samples taken above or below the gum margin, respectively. Gram positive rods and filaments (mainly Actinomyces species) are among the major groups of bacteria cultured from plaque. Mutans streptococci and members of the mitis- and anginosus- groups of streptococci are found in highest numbers on teeth.
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METABOLISM OF ORAL BACTERIA The persistence of the resident oral microbiota is dependent on the ability of microbes to attach to oral surfaces, obtain nutrients and multiply. The pattern of metabolism is closely related to whether the resident microbiota displays a pathogenic or commensal relationship with the host.
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CARBOHYDRATE METABOLISM Most attention has been paid to the metabolism of carbohydrates because of the relationship between dietary sugars, acid production and dental caries. Sucrose (a disaccharide of glucose and fructose) is the most widely used dietary sweetening agent, and can be: Broken down by extracellular bacterial invertases (α-glucosidases) and the resultant glucose and fructose molecules taken up directly by bacteria. Transported intact as the disaccharide or disaccharide phosphate, and cleaved inside the cell by an intracellular invertase or a sucrose phosphate hydrolase. Used extracellularly by glycosyltransferases. Glucosyltransferases (GTF) produce both soluble and insoluble glucans (with a release of fructose), which are important in biofilm maturation and in the consolidation of bacterial attachment to teeth. Fructosyltransferases (FTF) produce fructans (and liberate glucose), which can be metabolised by other microorganisms
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SUGAR TRANSPORT AND ACID PRODUCTION All substrates have to be transported across the cytoplasmic membrane and into the bacterial cell if they are to be used for biomass production or as an energy source. Oral bacteria can transport carbohydrates by: The phosphoenolpyruvate phosphotransferase (PEP-PTS) transport system The multiple sugar metabolism (Msm) system A glucose permease
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Once sugars have been transported into the bacterial cell, they can be used either in anabolic pathways to generate biomass, or they can be broken down to organic acids to generate energy. Bacteria catabolise sugars by glycolysis to pyruvate; the fate of pyruvate will depend on the particular organism and the availability of oxygen. Most oral bacteria metabolise pyruvate anaerobically to organic acids. Oral streptococci convert pyruvate to lactate by lactate dehydrogenase when sugars are in excess, whereas formate, acetate and ethanol are the products of metabolism by mutans streptococci and S. sanguinis (but not S. salivarius) under carbohydrate limitation. Other bacterial genera produce acetate, butyrate, propionate, and formate as primary products of metabolism.
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ACID TOLERANCE AND ADAPTATION TO LOW PH Microbial survival in acidic environments depends on the ability of a cell to maintain intracellular pH homeostasis. The mechanisms by which S. mutans achieves this include: • proton extrusion via membrane associated, proton translocating ATP synthase • acid end-product efflux. These mechanisms ensure that the intracellular pH remains higher (i.e., more alkaline) than that of the external environment during acid production. Acid tolerant organisms such as mutans streptococci and lactobacilli have higher levels of ATP synthase activity, and the pH optimum for their activity is lower than for less tolerant species such as S. sanguinis or A. naeslundii.
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BENEFITS OF THE ORAL MICROBIOTA Humans have coevolved with these resident microbes, and it is clear that this is a genuinely mutualistic relationship. The host provides a warm and nutritious environment for microbial growth, whereas evidence has emerged to show that the natural microbial residents are essential for the normal development of the physiology, nutrition and defences of the host.
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COLONISATION RESISTANCE One of the main beneficial functions of the oral microbiota is its ability to prevent colonisation by exogenous (and often pathogenic) microorganisms. This property, termed ‘colonisation resistance', is because of resident microorganisms being more effective at: Attachment to host receptors; Competition for endogenous nutrients; Creating unfavorable conditions to discourage attachment and retard multiplication of invading organisms; and Producing antagonistic substances (hydrogen peroxide, bacteriocins, etc.), which are inhibitory to exogenous microbes.
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WAYS TO BOOST COLONISATION RESISTANCE Prebiotics are molecules that stimulate the growth and/or activity of those members of the resident microbiota that contribute to the well-being of their host. Prebiotics such as non-digestible fibre compounds (e.g., inulin, oligofructose) have been delivered to the gastrointestinal tract to stabilise the composition and boost the metabolism of the gut microbiota, especially that of beneficial bifidobacteria and lactobacilli. Probiotics are live microorganisms, typically bifidobacteria and lactobacilli, that are claimed to provide health benefits when consumed. They can be consumed in dairy products such as yoghurts, and most applications are again targeted at the gut microbiota. Commonly claimed benefits of probiotics include the decrease of potentially pathogenic gastrointestinal microorganisms, the reduction of gastrointestinal discomfort, the strengthening of the immune system and the recovery of the intestinal microbiota in subjects who have received antibiotic treatment or who have suffered a gastric illness.
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HOST-MICROBE CROSS-TALK The host is able to detect microorganisms, and has evolved systems to tolerate beneficial resident microorganisms without initiating a damaging inflammatory response, while also being able to mount an efficient defence against pathogens. The attachment of bacteria to specific receptors on host cells can trigger substantial changes in gene expression in both the prokaryotic and eukaryotic cells. The host has developed strategies to distinguish between commensal and pathogenic bacteria. Certain oral streptococci have been shown to suppress epithelial cell inflammatory responses by functional modulation of immunity via Toll-like receptor expression and signalling, and also by suppressing inflammatory responses by inhibiting activation of NFĸB or by increasing the secretion of antiinflammatory cytokines, such as interleukin-10 (IL-10). Beneficial oral streptococci may also actively stimulate beneficial pathways in epithelial cells, including type I and II interferon responses, promote wound healing, enhance mucin production, and induce positive effects on then cytoskeleton.
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