Anaerobe
(Genus) | Type | Gram reaction | Commonly affected body site(s) and conditions | Colony growth | Comments |
| Actinomyces | Obligate (strict) | GPR, sometimes filamentous or branching; sulfur granules may be present | Abdominal
Thoracic
Cervicofacial | Small, smooth, flat, gray-white colonies. A. israelii and A. gerencseriae are white and may resemble a "molar tooth" | Cause of actinomycosis May also be responsible for chronic localized infections |
| Bacteroides | Obligate (strict) | GNR, sometimes pleomorphic | Abdominal abscesses
Brain and other deep abscesses
Aspiration pneumonia
Skin/soft tissue infections (perirectal abscesses, decubitus ulcers) | Circular, gray-white colonies; raised or convex B. thetaiotaomicron colonies are punctiform and black (due to esculin hydrolysis) on blood bile esculin agar | Most frequently encountered anaerobe |
| Bifidobacterium | Obligate (strict) | GPR, pleomorphic (sometimes short, irregular V- or Y-shapes with branching) | Pediatric:
Otitis media
Abdominal abscesses
Peritonitis | Small, white colonies; shiny, irregular edge; convex | No spores |
| Clostridium | Obligate (strict) | GPR C. perfringens may appear as large boxcar-shaped rods (No spores seen) Other Clostridium species may demonstrate spores, and may also appear gram-negative | Gas gangrene
Food poisoning
Tetanus
Skin/soft tissue infections (perirectal abscesses, decubitus ulcers) | Gray to grayish-yellow; some colonies appear large with irregular margins Many times, colonies on blood are nonhemolytic Exceptions:
C. botulinum has variable beta hemolysis with a circular to scalloped margin C. perfringens with double-zone of hemolysis C. tetani may present with a narrow zone of beta hemolysis C. septicum growth is known as "medusa-head" in appearance Growth of C. septicum, C. sordelli, and C. tetani may swarm on agar | C. tetani is a cause of tetanus C. perfringens is a cause of food poisoning C. botulinum is a cause of botulism |
Cutibacterium
(formerly Propionibacterium nomenclature for many—but not all—of the Propionibacterium spp. changed in 2016) | Facultative;
"aerotolerant anaerobe" because it possesses enzymes that help it to detoxify oxygen | GPR, pleomorphic Some appear as coccobacilliary forms | Acne
Endophthalmitis
Chronic blepharitis | Young colonies are gray-white and small; larger with age | Often found in mixed infections Normal inhabitant of skin |
| Fusobacterium | Obligate (strict) | GNR, fusiform/filamentous, long/thin with tapered ends Some appear coccoid, pleomorphic, and quite variable | Peritonsillar abscess
Liver abscess
Pulmonary (aspiration pneumonia) infections
May be found in deep animal bite wounds and tissue infections | Colonies of F. nucleatum may appear white (breadcrumb-like) F. varium and F. necrophorum are known for their "fried egg" appearance Fusobacterium species may cause a "greening" of the medium | No spores |
| Peptostreptococcus | Obligate (strict) | GPC, chains or pairs | Otitis media
Sinusitis
Aspiration pneumonia
Abdomen
Pelvic inflammatory disease
Bone, joint, soft tissue infections | Gray-white colonies; medium in size | May cause infection in any body site |
| Porphyromonas | Often obligate (strict), but sometimes grows in low concentrations of oxygen | GNR, pleomorphic, coccobacilli P. gingivalis was previously classified as Bacteroides—Gram stain is consistent with Bacteroides | Periodontal disease
Pelvic inflammatory disease | Dark brown to black on anaBAP; some are mucoid | Nonmotile |
| Prevotella | Obligate (strict) | GNR, sometimes pleomorphic, coccobacilli | Head and neck infections
Peritonsillar
Lung abscess
Perineal and perianal infections (e.g., pilonidal abscess) | White, circular, convex colonies on anaBAP Nonhemolytic May demonstrate black pigment on LKV | Many used to be classified as Bacteroides and colonies may appear similar |
| Veillonella | Obligate (strict) | GNC | Abdominal abscesses
Aspiration pneumonia
Vaginal tract infections
Meningitis
Dental caries (biofilms)
Endocarditis
Infected prosthetic devices (biofilms) | Slow-growing colonies Small, smooth, almost transparent, grayish-white colonies on anaBAP | No spores
Nonmotile
No capsules
Catalase variable
Oxidase negative
Indole negative
Nitrate reduction is positive Note: Vancomycin-resistant |
