Abscesses are usually caused by mixed anaerobic and facultative (able to grow with or without oxygen) bacteria. On average, abscesses have 2.5 species of bacteria, 1.6 of which are anaerobes and 0.9 of which are aerobic or facultative bacteria.116 Commensal bacteria from adjacent sites (oropharynx, intestine, and female genital tract) are the usual cause of abscesses, so the species found in the abscess reflect the species found in the normal flora. Since most anaerobes that cause abscesses are part of the normal flora, it is not surprising that these organisms do not produce significant toxins.
The bacteria found in head and neck abscesses reflect the oral and pharyngeal flora. Common anaerobes at this site include the Gram-negative bacilli Prevotella and Porphyromonas species, often mixed with the facultative S. aureus and S. pyogenes. Fusobacterium necrophorum, an oral commensal, causes Lemierre syndrome, characterized by infection of the lateral pharyngeal space and septic jugular vein thrombosis.
Abdominal abscesses are caused by the anaerobes of the gastrointestinal tract, including Gram-positive Peptostreptococcus and Clostridium species, as well as the Gram-negative Bacteriodes fragilis and Escherichia coli. Genital tract infections in women are caused by anaerobic Gram-negative bacilli, including Prevotella species that are found in Bartholin cyst abscesses and tubo-ovarian abscesses, often mixed with E. coli or Streptococcus agalactiae.
Morphology
The pus of abscesses is discolored and foul smelling owing to the presence of anaerobes, especially in lung abscesses, and the suppuration is often poorly walled off. Otherwise, these lesions pathologically resemble those of the common pyogenic infections. Gram stain will reflect the mixed infection including Gram-positive and Gram-negative rods and Gram-positive cocci mixed with neutrophils.