bacteriod

Dr. Fatima Malik

Bacteroides, Tannerella, Porphyromonas and Prevotella

The genera described in this chapter are obligatory 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.
• Bacteroides spp. is 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. is 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. is 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. includes asaccharolytic 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.

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 intra-abdominal 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 penicillin, first-generation cephalosporin and aminoglycosides. Penicillin resistance is due to ?-lactamase production. As Bacteroides spp. are normal gut commensals, infections are endogenous and diseases are virtually impossible to prevent.

Porphyromonas: Porphyromonas gingivalis

Habitat and transmission
Found almost solely at subgingival sites, particularly in advanced periodontal disease: considered a consensus periodontal pathogen. As mentioned above, P. gingivalis, T. forsythia and Treponema denticola are considered the three agents of ‘red complex’ bacteria almost always associated with periodontal disease. P. gingivalis is sometimes recovered from the tongue and tonsils.







Characteristics
Non-motile, asaccharolytic, short, pleomorphic, Gram negative coccobacilli.
Culture and identification
Grows anaerobically, with dark pigmentation, on media containing lysed blood, identified by biochemical
characteristics using commercially available kits. 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; 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 haemsequestering proteins and a haemolysin 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, T. forsythia, Treponema denticola and P. gingivalis are considered the three agents of ‘red complex’ bacteria almost always associated with periodontal disease


Prevotella
This genus includes a number of pigmented as well as nonpigmented species that are moderately saccharolytic; The pigment is thought to be related to breakdown products of the blood. 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. lnon-pigmented species such as Prevotella buccae, Prevotella oralis and Prevotella dentalis are isolated on occasion from
healthy subgingival plaque. Some of the latter are associated with disease, and increase in numbers and proportions during periodontal disease. It Produce acetic and succinic acid from glucose. Prevotella
melaninogenica is the type species



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, while 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
P. intermedia is closely associated with periodontal disease and shares a number of virulence properties exhibited by P. gingivalis. These organisms are classified as belonging to the ‘orange complex’ bacteria associated with the developmental stages of periodontal disease, and precedes the arrival of the ‘red complex’ group of bacteria. The pathogenicity of other subdivided species awaits clarification. Oral non-pigmented species such as Prevotella buccae, Prevotella oralis and Prevotella dentalis are isolated on occasion from healthy subgingival plaque. Some of the latter are associated with disease, and increase in numbers and proportions during periodontal disease.
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• Tannerella, Porphyromonas and Prevotella form of the microflora of the dental plaque, colon and the female genital tract.
• Bacteroides spp. is 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.
• P. gingivalis is found almost solely at subgingival sites and is a key periodontopathic organism (i.e. a periodontopathogen).
• The virulence of P. gingivalis is partly due to its many proteases (which destroy immunoglobulins, complement and haemsequestering proteins), a haemolysin and a collagenase.
• Strains of Prevotella intermedia are associated more with periodontal disease, while Prevotella nigrescens is isolated more often from healthy gingival sites.