31: Normal oral flora, the oral ecosystem and plaque biofilms

Chapter 31 Normal oral flora, the oral ecosystem and plaque biofilms

Normal oral flora

Oral flora comprises a diverse array of organisms and includes eubacteria, archaea, fungi, mycoplasmas, protozoa and possibly a viral flora that may persist from time to time. These organisms usually live in harmony in a range of habitats including the teeth, gingival sulcus, tongue, cheek, hard and soft palate and tonsils. Collectively the oral flora have been termed oral microbiota, and more recently, the oral microbiome. Bacteria are by far the predominant group of organisms, and there are probably some 500 to 700 common oral species or phylotypes of which only 50 to 60% are cultivable. The remaining unculturable flora are currently being identified using molecular techniques. This, together with the fact that the oral cavity has a wide range of sites (habitats) with different environmental conditions, makes the study of oral microbiology complex and difficult. Interestingly, despite the enormous diversity and complexity of the oral flora, many organisms commonly isolated from neighbouring ecosystems such as the gut and skin are not found in the mouth, indicating the unique and selective ecology of the oral cavity with regard to microbial colonization.

The main bacterial genera found in the oral cavity are well characterized using mostly traditional culture-based techniques. Oral bacteria can be classified primarily as Gram-positive and Gram-negative organisms, and secondarily as either anaerobic or facultatively anaerobic according to their oxygen requirements. Some oral microbes are more closely associated with disease than others, and a proportion of these appear to be uncultivable. The following is a synopsis of the major bacterial genera isolated from the oral cavity. Students should refer to the appropriate chapters in Part 3 for detailed information on these organisms.

A note on the nomenclature of oral flora

Due to continuing advances in molecular technology, especially those based on 16S ribosomal RNA (rRNA) sequences, microbial taxonomy is always in a state of flux. This poses a challenge to both the student and the scientist alike. Despite these changes, some prefer using the traditional nomenclature, while others use the new terminology, leading to further confusion. Hence in the following text, both the old and the recent taxonomic changes of oral bacteria are highlighted.

Flora of the oral cavity

Gram-positive cocci

Genus Streptococcus

Gram-positive cocci in chains, non-motile, usually possessing surface fibrils, occasionally capsulate; facultative anaerobes; variable haemolysis but α-haemolysis most common; selective medium: mitis salivarius agar (MSA).

mutans group

• Main species: Streptococcus mutans serotypes c, e, f, k; Streptococcus sobrinus serotypes d, g; Streptococcus criceti (previous Streptococcus cricetus) serotype a; Streptococcus ratti (previous Streptococcus rattus) serotype b. Oral isolates from monkeys: Streptococcus ferus; Streptococcus macacae; Streptococcus downei serotype h.
• Cultural characteristics: high, convex, opaque colonies; produce profuse extracellular polysaccharide in sucrose-containing media (Fig. 11.3); selective medium: MSA + bacitracin agar.
• Main intraoral sites and infections: tooth surface, dental caries.

salivarius group

• Main species: Streptococcus salivarius; Streptococcus vestibularis.
• Cultural characteristics: large, mucoid colonies on MSA due to the production of extracellular fructans (polymer of fructose with a levan structure). Streptococcus vestibularis do not produce extracellular polysaccharide from sucrose; they produce urease and hydrogen peroxide, which lowers the pH and contributes to the salivary peroxidase system, respectively.
• Main intraoral sites and infections: dorsum of the tongue and saliva; Streptococcus vestibularis mainly reside in the vestibular mucosa (hence the name); not a major oral pathogen.

anginosus group

• Main species: Streptococcus constellatus; Streptococcus intermedius; Streptococcus anginosus.
• Cultural characteristics: carbon dioxide-dependent; form small, non-adherent colonies on MSA.
• Main intraoral sites and infections: gingival crevice; dentoalveolar and endodontic infections.

mitis group

• Main species: Streptococcus mitis, Streptococcus sanguinis (previously Streptococcus sanguinis); Streptococcus gordonii, Streptococcus oralis, Streptococcus cristatus (previously Streptococcus crista), Streptococcus parasanguinis, Streptococcus oligofermentans, Streptococcus sinensis, Streptococcus australis, Streptococcus peroris, Streptococcus infantis.
• Cultural characteristics: small, rubbery (Streptococcus sanguinis) or non-adherent (Streptococcus oralis and Streptococcus mitis) colonies on MSA.
• Main intraoral sites and infections: mainly dental plaque biofilms, tongue and cheek, dental caries (?), infective endocarditis (except Streptococcus mitis).

Anaerobic streptococci

• Main species: Peptostreptococcus anaerobius, Micromonas micros (previously Peptostreptococcus micros), Finegoldia magnus (previously Peptostreptococcus magnus) and Peptoniphilus asaccharolyticus (previously Peptostreptococcus asaccharolyticus); group acronym GPAC – Gram-positive anaerobic cocci.
• Cultural characteristics: strict anaerobes, slow-growing, usually non-haemolytic.
• Main intraoral sites and infections: teeth, especially carious dentine, periodontal and dentoalveolar abscesses in mixed culture.

Genus Stomatococcus

• Main species: Stomatococcus (formerly Micrococcus) mucilagenosus.
• Cultural characteristics: coagulase-negative; forms large colonies adherent to blood agar surface, facultative anaerobes.
• Main intraoral sites and infections: tongue mainly, gingival crevice; not a major opportunistic pathogen.

Genera Staphylococcus and Micrococcus

See Chapter 11.

Gram-positive rods and filaments

These organisms are common isolates from dental plaque biofilms and include actinomycetes, lactobacilli, eubacteria and propionibacteria.

Genus Actinomyces

Short, Gram-positive pleomorphic rods:

• Main species: Actinomyces israelii, Actinomyces gerencseriae, Actinomyces odontolyticus, Actinomyces naeslundii (genospecies 1 and 2), Actinomyces myeri, Actinomyces georgiae. The most important human pathogen is Actinomyces israelii.
• Cultural characteristics: ferments glucose to give characteristic patterns of short-chain carboxylic acids useful for speciating; strict or facultative anaerobes.
• Main intraoral sites and infections: Actinomyces odontolyticus, earliest stages of enamel demineralization, and the progression of small caries lesions appear related; Actinomyces naeslundii implicated in root surface caries and gingivitis; Actinomyces israelii is an opportunistic pathogen causing cervicofacial and ileocaecal actinomycosis (Chapter 13). Actinomyces gerencseriae and Actinomyces georgiae are minor components of healthy gingival flora.

Genus Lactobacillus

Gram-positive bacilli:

• Main species: Lactobacillus casei, Lactobacillus fermentum, Lactobacillus acidophilus (others include Lactobacillus salivarius, Lactobacillus rhamnosus).
• Cultural characteristics: catalase-negative, microaerophilic; complex nutritional requirements; aciduric, optimal pH 5.5–5.8. Selective medium: Rogosa agar.
• Main intraoral sites and infections: common oral inhabitants, but comprise less than 1% of the oral flora. Dental plaque biofilms, usually in small numbers; advancing front of dental caries. As levels of salivary lactobacilli correlate well with intake of dietary carbohydrates, they are used to detect the cariogenic potential of the diet.

Genus Eubacterium

Pleomorphic, Gram-variable rods or filaments:

• Main species: Eubacterium brachy, Eubacterium nodatum, Eubacterium saphenum. (Note: Eubacterium timidum and Eubacterium lenta, previously in this group, have now been reclassified as Mogibacterium timidum and Eggerthella lenta, respectively).
• Cultural characteristics: obligatory anaerobes, characterization ill-defined.
• Main intraoral sites and infections: plaque biofilms and calculus; implicated in caries and periodontal disease but role unclear; comprise over 50% of the anaerobes of periodontal pockets; Eubacterium yurii is involved in ‘corn-cob’ formation in dental plaque (see Fig. 31.1).
image

Fig. 31.1 Scanning electron micrograph of supragingival plaque showing corn-cob formation: cocci aggregated around an axial filamentous organism (×5000).

Genus Propionibacterium

Gram-positive bacilli:

• Main species: Propionibacterium acnes (includes Propionibacterium propionicus, formerly Arachnia propionica).
• Cultural characteristics: strict anaerobe; morphologically indistinguishable from Actinomyces israelii but produces propionic acid from glucose, unlike Actinomyces israelii.
• Main intraoral sites and infections: root surface caries, plaque biofilms. Possible involvement in dentoalveolar infections.

Other notable Gram-positive organisms

Rothia dentocariosa, a Gram-positive branching filament, is a strict aerobe, found in plaque and occasionally isolated from infective endocarditis.

Bifidobacterium dentium is a Gram-positive strict anaerobe regularly isolated from plaque biofilms; its role in disease is unclear.

Gram-negative cocci

Genus Neisseria

Gram-negative diplococci:

• Main species: Neisseria subflava, Neisseria mucosa, Neisseria sicca.
• Cultural characteristics: asaccharolytic and non-polysaccharide-producing, facultative anaerobes.
• Main intraoral sites and infections: isolated in low numbers from tongue, saliva, oral mucosa and early plaque; may consume oxygen in early stages of plaque formation and provide conditions conducive for the growth of anaerobes; rarely associated with disease.

Genus Veillonella

Small, Gram-negative cocci:

• Main species: Veillonella parvula, Veillonella dispar, Veillonella atypica.
• Cultural characteristics: strict anaerobes; selective medium: Rogosa vancomycin agar. Lack glucokinase and fructokinase and hence unable to metabolize carbohydrates; they therefore use lactate produced by other bacteria and raise the pH of plaque, and are thus considered to be beneficial in relation to dental caries.
• Main intraoral sites and infections: isolated from most surfaces, including the tongue, saliva and plaque biofilms. No association with disease.

Gram-negative rods – facultative anaerobic and capnophilic genera

Genus Haemophilus

Gram-negative coccobacilli:

• Main species: Haemophilus parainfluenzae, Haemophilus segnis, Haemophilus aphrophilus, Haemophilus haemolyticus, Haemophilus parahaemolyticus.
• Cultural characteristics: all isolates are facultative anaerobes; growth is enhanced on heated blood agar (chocolate), requires haemin (X factor) and nicotinamide adenine dinucleotide (V factor) for growth.
• Main intraoral sites and infections: plaque biofilms, saliva and mucosae; dentoalveolar infections, acute sialadenitis, infective endocarditis.

Genus Aggregatibacter

Gram-negative coccobacilli, microaerophilic or capnophilic (carbon dioxide-dependent).

• Main species: Aggregatibacter actinomycetemcomitans (serotypes a–e).
• Cultural characteristics: freshly isolated strains contain fimbriae that are lost on subculture. Produces many virulence factors: leukotoxin, epitheliotoxin, cdt, collagenase, protease that cleaves immunoglobulin G (IgG).
• Main intraoral sites and infections: periodontal pockets; implicated in aggressive forms of periodontal disease (e.g. localized and generalized aggressive periodontitis). Often isolated from cervicofacial Actinomyces infections as co-pathogens.

Genus Eikenella

Gram-negative coccobacilli:

• Main species: Eikenella corrodens.
• Cultural characteristics: factor X-dependent and microaerophilic; produces corroding colonies on blood agar.
• Main intraoral sites and infections: plaque biofilms; dentoalveolar abscesses, infective endocarditis; possibly implicated in some forms of chronic periodontitis.

Genus Capnocytophaga

Carbon dioxide-dependent, Gram-negative fusiform rods with ‘gliding’ motility:

• Main species: Capnocytophaga gingivalis, Capnocytophaga sputigena, Capnocytophaga ochracea, Capnocytophaga granulose, Capnocytophaga haemolytica.
• Cultural characteristics: capnophilic, medium-sized colonies with an irregular spreading edge.
• Main intraoral sites and infections: plaque, mucosal surfaces, saliva; infections in immunocompromised, destructive periodontal disease (?). Some strains produce IgA1 protease.

Gram-negative rods – obligate anaerobic genera

These comprise a large proportion of the plaque biofilms. The classification of this group of organisms is fraught with difficulties, but the advent of new tests such as lipid analysis and molecular approaches have eased the problem to some extent. Most of the oral anaerobes were previously classified under the genus Bacteroides. However, advances in taxonomic methods have shown that they belong to two major genera, now termed Porphyromonas and Prevotella, which differ in their ability to metabolize sugar. Some of these organisms produce characteristic brown-black pigments on blood agar and are referred to collectively as ‘black-pigmented anaerobes’ (see Fig. 17.1).

Genus Porphyromonas

Gram-negative pleomorphic rods, non-motile; six serotypes based on capsular polysaccharides (K antigen); asaccharolytic:

• Main species: Porphyromonas gingivalis, Porphyromonas endodontalis, Porphyromonas catoniae.
• Cultural characteristics: strict anaerobes, require vitamin K and haemin for growth.
• Main intraoral sites and infections: gingival crevice and subgingival plaque in small numbers. Associated with chronic periodontitis and dentoalveolar abscess; Porphyromonas gingivalis is highly virulent in experimental infections, producing proteases, a haemolysin, collagen-degrading enzymes and cytotoxic metabolites; its capsule is an important virulent attribute; fimbriae helps adhesion. Porphyromonas endodontalis is mainly recovered from infected root canals.

Genus Prevotella

Gram-negative pleomorphic rods, non-motile; moderately asaccharolytic, producing acetic, succinic and other acids from glucose:

• Main species: pigmented species include Prevotella intermedia, Prevotella nigrescens, Prevotella loescheii, Prevotella corporis, Prevotella melaninogenica, non-pigmented species include Prevotella buccae, Prevotella oralis, Prevotella oris, Prevotella oulora, Prevotella veroralis, Prevotella dentalis (Bacteroides forsythus, another non-pigmented species considered an important periodontal pathogen, has now been reclassified as Tannerella forsythensis).
• Cultural characteristics: strict anaerobes, usually require vitamin K and haemin for growth.
• Main intraoral sites and infections: periodontal pockets, dental plaque; chronic periodontitis and dentoalveolar abscess.

Genus Fusobacterium

Slender, cigar-shaped Gram-negative rods with rounded ends (see Fig. 18.1):

• Main species: Fusobacterium nucleatum, Fusobacterium alocis, Fusobacterium sulci, Fusobacterium periodonticum.
• Cultural characteristics: require rich media for growth and are often asaccharolytic, strict anaerobes, usually nonhaemolytic; F. nucleatum can produce ammonia and hydrogen sulphide from cysteine and methionine and is implicated as an odorigenic organism in halitosis.
• Main intraoral sites and infections: most common isolate is F. nucleatum; normal gingival crevice, tonsils (F. alocis and F. sulci) or periodontal infections (F. periodonticum); acute ulcerative gingivitis, dentoalveolar abscess.

Genus Leptotrichia

Gram-negative filaments with at least one pointed end:

• Main species: Leptotrichia buccalis.
• Cultural characteristics: strict anaerobes, with colonies resembling fusobacteria.
• Main intraoral sites and infections: dental plaque. No known disease association.

Genus Wolinella

Gram-negative curved bacilli, motile by polar flagella:

• Main species: Wolinella succinogenes (Wolinella recta and Wolinella curva are now assigned to the Campylobacter genus).
• Cultural characteristics: strict anaerobe.
• Main intraoral sites and infections: gingival crevice. Possible involvement in destructive periodontal disease.

Genus Selenomonas

Gram-negative curved cells with tufts of flagella:

• Main species: Selenomonas sputigena, Selenomonas noxia, Selenomonas flueggei, Selenomonas inflexi, Selenomonas diane.
• Cultural characteristics: strict anaerobe.
• Main intraoral sites and infections: gingival crevice. No known disease association.

Genus Treponema

Motile Gram-negative helical cells, in three main sizes (large, medium and small):

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Related posts:

  1. 16: Vibrios, campylobacters and Wolinella
  2. 18: Fusobacteria, Leptotrichia and spirochaetes
  3. 17: Bacteroides, Tannerella, Porphyromonas and Prevotella
  4. 9: The immune response
  5. 21: Viruses of relevance to dentistry
  6. 4: Viruses and prions

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Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 31: Normal oral flora, the oral ecosystem and plaque biofilms

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