An isolate from a wound culture is a Gram-negative rod identified as Bacteroides fragilis. Anaerobic infection with B. fragilis is characterized by
Bacteroides fragilis is a constituent of normal intestinal flora and readily causes wound infections often mixed with aerobic isolates. These anaerobic, Gram-negative rods are uniformly resistant to aminoglycosides and usually to penicillin as well.Reliable laboratory identification may require multiple analytical techniques. Generally, wound exudates smell bad owing to production of organic acids by such anaerobes as B.fragilis. Black exudates or a black pigment (heme) in the isolated colony is usually a characteristic of Bacteroides (Porphyromonas) melaninogenicus, not B.fragilis. Potent neurotoxins are synthesized by the Gram-positive anaerobes such as Clostridium tetani and C. botulinum.
A foul-smelling discharge
A black exudate in the wound
An exquisite susceptibility to penicillin
A heme-pigmented colony formation
Bacteria