The genera described in this chapter are obligately anaerobic, short Gram-negative rods or coccobacilli. Historically, only the Bacteroides genus was known, but the application of new taxonomic techniques has resulted in the definition of three additional genera: Tannerella, Porphyromonas and Prevotella. Together, they comprise a substantial proportion of the microflora of the dental plaque, intestine and the female genital tract (Table 17.1):
■ Bacteroides spp. are mainly restricted to species found predominantly in the gut and are the most common agents of serious anaerobic infections; Bacteroides fragilis is the main pathogen.
■ Tannerella spp. are black-pigmented, anaerobic rods, strongly implicated as a major pathogen of periodontal disease. Tannerella forsythia is frequently isolated with Porphyromonas gingivalis, indicating an ecological relationship between them.
■ Porphyromonas spp. are asaccharolytic pigmented species and form part of the normal oral flora. They are agents of periodontal disease and hence considered as periodontopathic organisms.
■ Prevotella spp. include saccharolytic oral and genitourinary species; some species are periodontopathic.
Collectively, Tannerella, Porphyromonas and Prevotella species are referred to as black-pigmented anaerobes, as some organisms from these genera form a characteristic brown or black pigment on blood agar (Fig. 17.1).
Bacteroides
Bacteroides fragilis
Habitat and transmission
Bacteroides species are the most predominant flora in the intestine (1011 cells per gram of faeces), far outnumbering Escherichia coli. They cause serious anaerobic infections such as intraabdominal sepsis, peritonitis, liver and brain abscesses, and wound infection.
Characteristics
Strictly anaerobic, Gram-negative, non-motile, non-sporing bacilli, but may appear pleomorphic. The polysaccharide capsule is an important virulence factor.
Culture and identification
These organisms have stringent growth requirements; they demonstrate slow growth on blood agar and appear as grey to opaque, translucent colonies. They grow well in Robertson's cooked meat medium supplemented with yeast extract.
Identified by biochemical tests, growth inhibition by bile salts, antibiotic resistance tests and gas-liquid chromatographic analysis of fatty acid end products of glucose metabolism.
Pathogenicity
Mainly the result of its endotoxin and proteases. No exotoxin has been reported. Other organisms, such as coliforms, are commonly associated with sepsis. The latter facultative anaerobes utilize oxygen in the infective focus and facilitate the growth of the anaerobic Bacteroides strains. Consequently, many Bacteroides infections are polymicrobial in nature.
Treatment and prevention
Sensitive to metronidazole and clindamycin. Resistant to penicillins, first-generation cephalosporins and aminoglycosides. Penicillin resistance is due to β-lactamase production. As Bacteroides spp. are normal gut commensals, infections are endogenous and diseases are virtually impossible to prevent.
Table 17.1 Anaerobic Gram-negative bacilli of clinical interest
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Organism |
Main colonization sites |
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Bacteroides |
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B. fragilis group |
Colon |
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B. fragilis |
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B. ovatus |
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B. vulgatus |
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B. distasonis |
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B. capillosus |
Colon, oropharynx |
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B. ureolyticus |
Oropharynx, intestine, genitourinary tract |
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Tannerella |
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T forsythia |
Oropharynx |
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Porphyromonas |
|
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P. gingivalis |
Oropharynx |
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P endodontalis |
Oropharynx |
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Prevotella |
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P. intermedia |
Oropharynx |
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P. nigrescens |
Oropharynx |
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P. melaninogenica |
Oropharynx |
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P. loescheii |
Oropharynx |
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P. pallens |
Vagina, oropharynx |
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P. corporis |
Vagina, oropharynx |
Fig. 17.1 Black-pigmented colonies of periodontopathogen Porphyromonas gingivalis on blood agar. The pigment is thought to be related to breakdown products of the blood.
Tannerella
Tannerella forsythia (formerly Bacteroides forsythus and Tannerella forsythensis)
Habitat and transmission
Both supragingival and subgingival sites but more common in the latter; the degree of isolation strongly related to increasing pocket depth and, increasingly, recovered from sites that converted from periodontal health to disease and sites with periodontal breakdown, hence considered a consensus periodontal pathogen. Indeed, Tannerella forsythia, Treponema denticola and Porphyromonas gingivalis are considered the three agents of red complex bacteria almost always associated with periodontal disease (see Chapter 33). A number of other epithets have been given to this group of pathogens such as pathobionts, keystone pathogens and inflammophilic bacteria. As these bacteria are always present in inflamed regions of the periodontium, they are thought to have co-evolved not only to endure inflammation but also to take advantage of it. Inflammatory by-products drive the selection and enrichment of these pathogenic communities by providing a source of nutrients in the form of tissue breakdown products (e.g., degraded collagen peptides and haeme-containing compounds)
Characteristics
Non-motile, pleomorphic, spindle-shaped Gram-negative rods.
Culture and identification
Grows anaerobically, but sometimes requires up to 14 days for visible growth. Growth enhanced by co-cultivation with Fusobacterium nucleatum. Media supplemented with N-acetylmuramic acid enhances growth.
Pathogenicity
Periodontal pathogen in both human and animals; induces apoptotic cell death; invades epithelial cells in vitro and in vivo. Its endotoxin, fatty acid and methylglyoxal production are considered virulence factors; increased levels found in ligature-induced periodontitis and peri-implantitis in dogs.
Porphyromonas
Porphyromonas gingivalis
Habitat and transmission
Found almost solely at subgingival sites, particularly in advanced periodontal disease: considered a consensus periodontal pathogen. As mentioned earlier, Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola are considered the three agents of 'red complex', inflammophilic (i.e., inflammation-loving) bacteria almost always associated with periodontal disease (see Chapter 33). Porphyromonas gingivalis is occasionally recovered from the tongue and tonsils. Porphyromonas endodontalis is mainly isolated from endodontic infections, whereas Porphyromonas catoniae is found in healthy gingivae or shallow pockets.
Characteristics
Non-motile, asaccharolytic, short, pleomorphic, capsulate, Gram-negative coccobacilli. Six serotypes are recognized.
Culture and identification
Grows anaerobically, with dark pigmentation, on media containing lysed blood (Fig. 17.1); identified by biochemical characteristics using commercially available kits (e.g., AnIdent); DNA and molecular probes are now used to identify these organisms directly from plaque samples.
Pathogenicity
An aggressive periodontal pathogen in both humans and animals (e.g., guinea pig, monkey, beagle dogs); its fimbriae mediate adhesion and the capsule defends against phagocytosis. Produces a range of virulence factors including collagenase, endotoxin, fibrinolysin, phospholipase A, many proteases that destroy immunoglobulins, gingipain, a fibroblast-inhibitory factor, complement and haem-sequestering proteins, and a haemolysin.
Prevotella
belonging to the orange complex bacteria associated with the developmental stages of periodontal disease, and precede the arrival of the red complex group of bacteria (see Chapter 33). The pathogenicity of other subdivided species awaits clarification. Oral non-pigmented species such as Prevotella buccae, Prevotella oralis and Prevotella dentalis are isolated occasionally from healthy subgingival plaque. Some of the latter are associated with disease, and increase in numbers and proportions during periodontal disease.
Key facts
This genus includes a number of pigmented as well as nonpigmented species that are moderately saccharolytic; all produce acetic and succinic acid from glucose. Prevotella melaninogenica is the type species (Table 17.1).
Prevotella spp.
Habitat and transmission
The predominant ecological niche of all Prevotella species appears to be the human oral cavity. Strains of Prevotella intermedia are associated more with periodontal disease, whereas Prevotella nigrescens is isolated more often from healthy gingival sites.
Culture and identification
Non-motile, short, round-ended, Gram-negative rods; brown- black colonies on blood agar (when pigmented). Molecular techniques are required to differentiate some species.
Pathogenicity
Prevotella intermedia is closely associated with periodontal disease and shares a number of virulence properties exhibited by Porphyromonas gingivalis. These organisms are classified as
• Tannerella, Porphyromonas and Prevotella form a substantial proportion of the microflora of the dental plaque, colon and the female genital tract.
• Bacteroides spp. are the predominant flora in the intestine.
• Collectively, Tannerella, Porphyromonas and Prevotella species are referred to as black-pigmented anaerobes.
• Tannerella forsythia is a key periodontopathogen and induces apoptotic cell death.
• Tannerella forsythia, Treponema denticola and Porphyromonas gingivalis are considered the three agents of ‘red complex’ bacteria almost always associated with periodontal disease.
• Porphyromonas gingivalis is found almost solely at subgingival sites and is a key periodontopathic organism (i.e., a periodontopathogen).
• The virulence of Porphyromonas gingivalis is partly due to its many proteases (which destroy immunoglobulins, complement and haem-sequestering proteins), a haemolysin and a collagenase.
• Strains of Prevotella intermedia are associated more with periodontal disease, whereas Prevotella nigrescens is isolated more often from healthy gingival sites.
Review questions (answers on p. 365)
Please indicate which answers are true, and which are false.
17.1 Bacteroides spp. are:
A. facultative anaerobes
B. outnumbered by Escherichia spp. in the intestine
C. spore formers
D. capsulated
E. capable of growing in a media rich in bile salts
17.2 Porphyromonas gingivalis:
A. are Gram-negative pleomorphic rods
B. are non-capsulated
C. form dark colonies on lysed blood
D. are aggressive periodontal pathogens
E. are isolated from many extraoral sites
17.3 Which of the following organisms is/are likely to be isolated from a subgingival plaque sample cultured anaerobically?
A. Fusobacterium nucleatum
B. Escherichia coli
C. Pseudomonas aeruginosa
D. Aggregatibacter actinomycetemcomitans
E. Haemophilus influenzae
Further reading
Holt, S. C., & Ebersole, J. L. (2005). Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia: The 'red complex', a prototype polybacterial pathogenic consortium in periodontitis. Periodontology 2000, 38, 72-122.
Marsh, P., Lewis, M., Williams, D., et al. (2009). The resident oral microflora. In Oral microbiology (5th ed., pp. 24-44). Edinburgh: Churchill Livingstone. Chapter 3.
Shah, H. N., Mayrand, D., & Genco, R. J. (Eds.), (1993). Biology of the species Porphyromonas gingivalis. Boca Raton: CRC Press.
Tanner, A. C. R., & Izard, J. (2006). Tannerella forsythia, a periodontal pathogen entering the genomic era. Periodontology 2000, 42, 88-103.