Discussion:
Actinomycosis is an infrequent invasive bacterial disease that has been recognized for over a century. Actinomyces  spp. are filamentous Gram-positive bacilli, mainly belonging to the human commensal flora of the oropharynx, gastrointestinal tract, and urogenital tract. To date, multiple different clinical features of actinomycosis have been described, as various anatomical sites (such as the face, bone, joint and …) can be affected. (19,20,21)
Osseous actinomycosis usually results from a direct invasion of bone from adjacent soft-tissue infection. Involvement of the jaw and vertebral column has been frequently reported, but involvement of the bones of the wrist or hand is rare. (20)
The use of broad-spectrum antibiotics can indeed impact the microbiome, making it challenging to extract and identify Actinomyces. (13) To ensure efficient recovery, specific anaerobic transport methods and pre-reduced anaerobically sterilized (PRAS) media are necessary.
Your emphasis on strict anaerobic conditions during incubation is well-placed. Actinomyces species are more resistant to ambient oxygen compared to other anaerobic bacteria, making anaerobic incubation essential. (14) Gram staining can be more sensitive than culture, especially in patients who have received antibiotics. Actinomyces are Gram-positive rods that do not produce spores, except for A. meyeri, which is tiny and nonbranching. (10)
The slow growth of Actinomyces, requiring up to 15-20 days for full maturation, necessitates extended incubation periods. Negative cultures should be incubated for at least 10 days before concluding as negative. In bone infections, where bacterial growth may be limited, even longer incubation periods may be necessary. On chocolate blood agar media at 37°C, Actinomyces can be grown.
Histological examination plays a crucial role in identifying actinomycosis. (8) The formation of sulfur granules, characterized by the presence of small aggregates stabilized by a protein-polysaccharide complex, is a hallmark of Actinomyces invasion. (15) Histopathology reveals the presence of sulfur granules in approximately 75% of patients, which appear basophilic with eosinophilic distal branches when stained.
While the presence of sulfur granules is highly suggestive of actinomycosis, it is important to note that they can also occur in other infectious disorders. Gram staining, revealing Gram-positive filamentous branching bacteria near the periphery of the granules, provides further evidence of actinomycosis. (16)
When it comes to treatment, beta-lactams, particularly penicillin G or amoxicillin, are highly effective against Actinomyces species and are the preferred drugs of choice. Piperacillin-tazobactam, imipenem, and meropenem are also effective antibiotics, but their use should be reserved for serious and resistant cases to prevent the development of resistant microflora. (17)
Metronidazole and aminoglycosides have limited efficacy against Actinomyces, while fluoroquinolones like ciprofloxacin and moxifloxacin are generally considered ineffective. (18) However, doxycycline has shown clinical effectiveness, despite initial beliefs of low activity. Macrolides and clindamycin are also effective treatment options for actinomycosis.
The patient’s case presents a complex and challenging scenario. After the initial appearance of pustules on the dorsal side of the left hand, the lesion progressed and recurred despite previous treatment attempts. The subsequent histopathologic evaluation confirmed the diagnosis of actinomycosis, and further imaging studies revealed osteomyelitis in the carpal bones and proximal metacarpals, along with multiple collections and joint destruction. The patient reported experiencing numbness and paresthesia in the hand, which was likely due to compression of nerves and vessels in the carpal tunnel by the tumor-like abscesses and lesions.
Orthopedic consultation recommended extensive debridement and possible amputation due to the severity of the condition. However, the patient did not provide consent for surgery. In light of this, antibiotic treatment with amoxicillin and TMP-SMX was initiated. Fortunately, the patient has responded well to the treatment, as evidenced by the shrinking of the lesion, reduction in vascular engorgement, and resolution of paresthesia.
It is crucial to closely monitor the patient’s progress and continue the antibiotic therapy to ensure complete remission and prevent any further complications. Regular follow-up examinations and imaging studies will help assess the effectiveness of the treatment and guide further management decisions.