Black pigmented bacteria

Bacteroides, Tannerella,Porphyromonas andPrevotella

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
Prevotella.

Bacteroides fragilis

Habitat and transmission
Bacteroides species are the most predominant flora in the intestine outnumbering Escherichia coli.
They cause serious anaerobic infections such as intra-abdominal sepsis, peritonitis, liver and brain abscesses, and wound infection.

Strictly anaerobic, Gram-negative, non-motile, non-sporing bacilli, but may appear pleomorphic.
The polysaccharide capsule is an important virulence factor.
Culture and identification
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
(1)biochemical tests,
(2)growth inhibition by bile salts,
(3) antibiotic resistance tests and
(4)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.

Tannerella forsythia (formerly Bacteroides forsythusand 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.

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 CO2 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.

Porphyromonas gingivalis

Habitat and transmission
Found almost solely at subgingival sites, particularly in advanced
periodontal disease: considered a consensus periodontal
pathogen. 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 ,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); 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.

PREVOTELLAE

Habitat and transmission
The predominant ecological niche of all Prevotella species appears to be the human oral cavity.
Strains of P. intermedia are associated more with periodontal disease,
While P. nigrescens is isolated more often from healthy gingival sites. Culture and identification
Non-motile, short, round-ended, Gram-negative rods; brownblack 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 P. gingivalis.
The pathogenicity of other subdivided species
awaits clarification. Oral non-pigmented species such as P. buccae, P. oralis and P. 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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