Unseen but unforgettable - refractive bacilli in the gram stain of a muscular abscess.

The epidemiology of tuberculosis has been altered due to the increased use of immunosuppressive therapies (chemotherapy and biologic agents), the human immunodeficiency virus (HIV) epidemic, and the changing migration patterns. Extrapulmonary tuberculosis occurs most often in individuals with HIV infection. Osteoarticular tuberculosis is rare in Western countries, with its diagnosis being performed through culture and/or molecular detection, however muscular tuberculosis without coexistent skeletal involvement is rare, with only 30 cases reported in the literature. The authors present the case of a 65-year-old man with a history of acute myeloid leukaemia, undergoing first-line treatment with venetoclax/azacitidine and awaiting an allogeneic hematopoietic stem cell transplant. He was admitted with a suspected left thigh skin infection, presenting with elevated C-reactive protein (19 mg/dL) and normal creatine kinase levels (48 U/L). Imaging evidence (CT scan) indicated myofasciitis with fluid layers between the muscle planes and ruling out compartment syndrome. Empiric antimicrobial therapy with piperacillin-tazobactam and amikacin was initiated following microbiological testing. During the hospitalisation, probable streptococcal aetiology was admitted due to increased antistreptolysin O titer (1212 UI/mL), prompting a switch to penicillin and clindamycin. Due to the worsening of the patient's clinical condition despite empiric antimicrobial therapy, as well as imaging evidence of a new intramuscular abscess, an ultrasound-guided aspirate was performed. The collected sample was evaluated by Gram staining, revealing numerous neutrophils along with several refractive (ghost-gram) bacillary elements, although bacteriological cultures were negative in both aerobiosis and anaerobiosis. These findings prompted further investigation, including a new staining with Auramine and Kinyoun methodologies, which confirmed the presence of acid-fast bacilli. The identification of Mycobacterium tuberculosis was confirmed via molecular biology methods, specifically RT-PCR. Early recognition of tuberculosis in immunosuppressive patients, regardless of clinical presentation, is crucial to a timely treatment. The lack of growth on routine culture media, coupled with its mycolic acid-rich cellular wall, which does not stain well with Gram methods, makes the diagnosis challenging. Despite the absence of clinical suspicion of osteoarticular tuberculosis, the identification of abnormal morphological findings at Gram led to a laboratory diagnosis and targeted therapeutic adjustment.