Actinomyces are anaerobic, gram-positive bacilli that have a branching, filamentous growth pattern similar to the mycelial form of fungi (hence their "fungus-like" name).  They exist as normal commensals of the human gastrointestinal tract and rarely cause illness.  However, introduction of the bacteria into the submucosa during a mechanical trauma (eg, tooth extraction) can sometimes lead to invasive disease, particularly when the wound is associated with low oxygen tension (ie, due to necrosis or infection).

Infections with Actinomyces most commonly involve the cervicofacial region.  The bacteria grow slowly and without regard to tissue planes, resulting in a chronic, nontender, indurated perimandibular mass that enlarges over time and evolves into multiple abscesses and draining sinus tracts.  A characteristic finding is the presence of sulfur granules, yellow-orange granules in pus that resemble (but do not contain) sulfur.  Sulfur granules are intertwined Actinomyces filaments mixed with necrotic tissue.

(Choice A)  Pasteurella multocida is a gram-negative rod that exhibits bipolar staining (ends of bacilli stain more than the middle).  This organism causes a rapidly progressive soft tissue infection following an animal (usually a cat) bite.  Although Pasteurella infection may cause draining cutaneous sinus tracts, infections are acute and not associated with tooth extraction or sulfur granules.

(Choice B)  Staphylococcus aureus is the leading cause of acute suppurative parotitis, which is associated with firm, erythematous swelling in the area of the mandibular angle.  However, S aureus soft tissue abscesses progress rapidly and are painful and tender.

(Choice C)  Sporothrix schenckii is a conidia-forming, dimorphic fungus that causes cutaneous sporotrichosis when contaminated soil is inoculated into skin or soft tissue.  Manifestations include an ulcerating papule at the site of inoculation with proximal spread along the lymphatics leading to additional lesions.

(Choice E)  Pseudomonas is an oxidase-positive, gram-negative bacillus that can produce a green pigment.  Cutaneous Pseudomonas infections are most common after hot tub use and are usually associated with acute folliculitis (tender, pruritic papules or nodules).

Educational objective:
Actinomyces species are gram-positive, branching, filamentous bacteria that may cause cervicofacial infections when introduced to the submucosa during mechanical trauma (eg, tooth extraction).  Manifestations include a slowly enlarging, nonpainful, chronic mandibular mass that evolves into multiple abscesses and sinus tracts draining yellow-orange sulfur granules.