Bacteroides thetaiotaomicron can convert some lithocholic acid to its ethyl ester. ![]() A few strains of Gram-negative anaerobic bacilli can convert primary bile acids to secondary bile acids. Fusobacterium necrophorum also is active in deconjugating bile acids but is active primarily on taurine conjugates. Rarely, P melaninogenica may deconjugate bile acids, but in general this species, P oralis, and F nucleatum are inhibited by bile acids and do not deconjugate them. Most strains of the B fragilis group can deconjugate bile acids and are equally active whether the bile acid is conjugated with glycine or with taurine. Such resistance also has been transferred from B distasonis to B fragilis. Plasmid-mediated antibiotic resistance has been transferred from strains of B fragilis to other strains of this species, to B thetaiotaomicron, and to Escherichia coli. For the most part, the biologic and clinical significance of these plasmids is not known however, some code for resistance to such antimicrobial agents as clindamycin, erythromycin, tetracycline, chloramphenicol, ampicillin, and cephalothin. Plasmids have been found in about half the Bacteroides strains studied. Bacteriocins also are produced by strains of B fragilis and B thetaiotaomicron. They are species specific and active against most strains. The endotoxin of F nucleatum also is variable in its biologic activity, but often exhibits strong activity, comparable to that of S enteritidis.īacteriophages active against B fragilis are not uncommon. Although biologic activity varies also, many strains do show strong biologic activity, comparable to that of Salmonella enteritidis. The endotoxin varies from strain to strain in its content of 2-ketodeoxyoctanate and sugars. The lipopolysaccharide of F necrophorum is located in a multilayered external coat. Note the filaments with swollen central portions, large round bodies, and irregular staining. Microscopic morphology of F mortiferum from broth culture. The cells of this species are usually spindle shaped, are 5 to 10 μm long, and are often seen in pairs, end to end. Fusobacterium nucleatum, although not producing infections as serious as those caused by F necrophorum, is a virulent organism and is much more common clinically. Cells of F necrophorum often are elongated or filamentous, are curved, and possess spherical enlargements and large, free, round bodies. The guanine-plus-cytosine content ranges from 26 to 34 percent. Members of the genus Fusobacterium ( Figs.20-6 and 20-7) may be spindle shaped or may have parallel sides and rounded ends. Serologic methods have not been reliable for characterizing Gram-negative anaerobic rods. Prevotella melaninogenica endotoxin contains no heptose or 2-ketodeoxyoctanate, and it and the endotoxin of P oralis both show weak biologic activity. ![]() Poor biologic activity of endotoxin also has been demonstrated for the closely related species B thetaiotaomicron, B ovatus, B vulgatus, and B distasonis. This endotoxin exhibits little biologic activity in various test systems and little chemotactic activity what activity there is is complement-mediated by the alternative pathway. Numerous studies of the endotoxin of Gram-negative anaerobic bacilli have determined that the B fragilis endotoxin contains little or no lipid A, 2-ketodeoxyoctanate, or heptose. These genera contain numerous other species that rarely or never infect humans.Ĭolony morphology of P melaninogenica. Fusobacterium nucleatum is the Fusobacterium species most often found as a pathogen, but F necrophorum occasionally produces serious disease. Porphyromonas species seem to be much less pathogenic except in dental infections. Among the bile-sensitive Prevotella species, the ones most commonly encountered clinically are P melaninogenica, P oris, and P buccae. Within the Bacteroides group, B fragilis is the most common pathogen, followed by B thetaiotaomicron and other members of the B fragilis group. These genera are prevalent in the body as members of the normal flora ( Fig.20-1), constituting one-third of the total anaerobic isolates from clinical specimens, and may become involved in infections throughout the body ( Fig.20-2). ![]() In most clinical infections, only the genera Bacteroides, Prevotella, and Fusobacterium need be considered. At present there are over two dozen genera of Gram-negative anaerobic bacilli.
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