Gram-Negative Rods
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p480-482
2025-08-28
412
Bacteroides Fragilis Group
The anaerobic gram-negative rods typically are isolated from the mucosal surfaces of the human oral cavity and gastrointestinal tract (Figure 1).The Bacteroidaceae family consists of the saccharolytic, bile-resistant, nonpigmented Bacteroides fragilis group. B. fragilis is the most common organism isolated from clinical specimens, followed by B. thetaiotaomicron and B. ovatus. These organisms have been associated with a variety of infections.
Fig1. Gram stain of Bacteroides fragilis.
The gram-negative Bacteroides fragilis group grows in 20% bile, and the organisms are almost always resistant to all three special-potency antibiotic disks (Figures 2 and 3). Rare strains of B. fragilis are susceptible to colistin.
Fig2. Bacteroides fragilis on anaerobic blood agar.
Fig3. Bacteroides fragilis on a biplate containing anaerobic blood agar and Bacteroides bile esculin agar (BBE) (arrow). (Courtesy Anaerobe Systems, Morgan Hill, Calif.)
Nonpigmented Prevotella spp.
Prevotella spp. are ubiquitous in the oral cavity and are an important component of dental biofilms. Prevotella organ isms have also been identified in the esophagus and stomach. Most are bile-sensitive, kanamycin-resistant, gram-negative rods. Colistin susceptibility is variable, and almost all strains are catalase and indole negative.
Pigmented Porphyromonas and Prevotella spp.
The Porphyromonadaceae family comprises five genera, including the genera, Parabacteroides, Porphyromonas, Tannerella, Odoribacter, and Barnesiella. Porphyromonas generally is considered the pathogenic genus in the Porphyromonadaceae family. Most Porphyromonas spp. are asaccharolytic and pigmented. The Prevotellaceae includes saccharolytic organisms that have been isolated from a variety of body sites, including the oral cavity and feces. Colonies that fluoresce brick red or produce brown to black pigment are placed among the pigmented Prevotella (Figure 4) and Porphyromonas spp. (Figure 5). Some species appear coccobacillary on Gram staining.
Fig4. Prevotella disiens on laked kanamycin-vancomycin blood agar. Note black pigment (arrow).
Fig5. Porphyromonas spp. on anaerobic blood agar. Red fluorescence is seen under ultraviolet light (365 nm). (Courtesy Anaerobe Systems, Morgan Hill, Calif.)
Bacteroides ureolyticus
Bacteroides ureolyticus is asaccharolytic, reduces nitrate, and requires formate and fumarate for growth in broth culture. Its disk pattern is the same as for the fusobacteria; however, the colony morphology is different. B. ureolyticus forms small, translucent to transparent colonies that may corrode the agar (Figure 6), whereas the Fusobacterium colony generally is larger and more opaque (Figure 7). B. ureolyticus formerly was grouped with organisms that have been transferred to the genus Campylobacter (C. gracilis, C. concisus, C. recta, and C. curva).
Fig6. Bacteroides ureolyticus on anaerobic blood agar. Note pitting of agar (arrow). (Courtesy Anaerobe Systems, Morgan Hill, Calif.)
Fig7. Fusobacterium nucleatum subsp. nucleatum on anaerobic blood agar. Note bread crumb–like colonies and greening of agar.
The campylobacters are all microaerophiles, not anaerobes, and are discussed in Chapter 34. Curved or motile organisms that grow anaerobically but not in 5% CO2 should be retested in a microaerophilic atmosphere with approximately 6% oxygen.
Fusobacteriaceae
The Fusobacteriaceae family includes the genera Fusobacterium, Leptotrichia, and Sneathia. These organisms typically are isolated from the oral cavity as integral components of dental biofilms (Figure 8). The gram negative Fusobacterium spp. are sensitive to kanamycin, and most strains fluoresce chartreuse. Different species have characteristic cell and colony morphologies (Figure 9).
Fig8. Gram stain of Fusobacterium nucleatum subsp. nucleatum. Note pointed ends.
Fig9. Peptostreptococcus anaerobius on anaerobic blood agar.
Leptotrichia spp. are very large, fusiform rods with one pointed end and one blunt end. Colonies are large, gray, and convoluted. They are most often isolated from the oral cavity or urogenital tract.
Protobacteria
The phylum Protobacteria contains a variety of clinically significant organisms, including Bilophila wadsworthia and Sutterella wadsworthensis. B. wadsworthia is an anaerobic, asaccharolytic, bile-resistant, gram-negative rod. S. wadsworthensis is an asaccharolytic, bile-resistant, short, gram-negative rod. B. wadsworthia phenotypically resembles B. ureolyticus but is resistant to bile and is strongly catalase positive.
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