Breakthrough nocardiosis emerged despite trimethoprim/sulfamethoxazole prophylaxis
A renal transplant recipient in India developed disseminated nocardiosis while receiving trimethoprim/sulfamethoxazole prophylaxis, adding another warning to the growing problem of resistant opportunistic infection in heavily immunosuppressed hosts. The case is clinically important because the infection broke through the very agent most often used to prevent it. In a transplant patient, that failure changes the threshold for diagnostic imaging, tissue sampling, and susceptibility testing as soon as pulmonary or neurologic symptoms appear.
The patient was a 39-year-old woman taking tacrolimus, mycophenolate sodium, prednisolone, and trimethoprim/sulfamethoxazole after renal transplant. She remained well for 14 months before fever, hemoptysis, anorexia, and weight loss developed over 45 days. Chest computed tomography showed right upper lobe consolidation with pleural effusion, a pattern that could fit fungal disease, nocardiosis, or other chronic infections in an immunocompromised lung. In this setting, broad differential diagnosis is not academic; delayed recognition can allow a focal pulmonary process to seed the brain.
Pulmonary aspergillosis did not explain the full syndrome
Lung biopsy culture grew Aspergillus fumigatus, and serum galactomannan was strongly positive at 8.16 optical density index, meeting 2020 European Organization for Research and Treatment of Cancer and Mycoses Study Group criteria for invasive pulmonary aspergillosis. She received voriconazole and liposomal amphotericin B, yet her condition worsened with altered sensorium and bilateral lower-limb weakness, prompting referral to a higher center. The clinical shift mattered: a fungal diagnosis did not close the case, because mixed opportunistic infection is common in transplant recipients and can mask a second pathogen with a different treatment requirement.
On examination, she had asymmetric upper motor neuron weakness in the lower limbs, neck stiffness, and coarse bilateral crackles. Brain imaging showed multiple ring-enhancing supratentorial lesions with central diffusion restriction and surrounding edema, findings that fit cerebral nocardiosis in an immunosuppressed patient. Neurologic involvement in this setting should be treated as an extension of pulmonary disease until proven otherwise, especially because hematogenous spread to the central nervous system is a recognized pattern for Nocardia.
Tissue staining and culture confirmed a second invasive pathogen
Repeat transbronchial lung biopsy showed suppurative inflammation with weakly acid-fast, branching, beaded filamentous bacteria on Fite stain and on hematoxylin and eosin sections. Culture on Sabouraud dextrose agar yielded dry, chalky colonies suggestive of Nocardia species. Species-level confirmation could not be completed because matrix-assisted laser desorption/ionization time-of-flight mass spectrometry failed twice and 16S rRNA sequencing was unavailable. In real-world microbiology, those failures matter because morphology can suggest the genus, but susceptibility patterns often determine whether the patient survives the first treatment plan.
Disc diffusion testing on cation-adjusted Mueller–Hinton agar, interpreted with Comité de l’Antibiogramme de la Société Française de Microbiologie thresholds, showed resistance to cotrimoxazole and ciprofloxacin, with susceptibility to linezolid, minocycline, imipenem, and amikacin. The isolate’s trimethoprim/sulfamethoxazole resistance is the pivotal microbiologic finding here. Prophylaxis had not just failed; it had failed against a resistant breakthrough organism, leaving the patient exposed while clinicians initially faced an apparently active but incomplete antimicrobial regimen.
Drug selection changed after cytopenias and seizure risk
The working diagnosis became disseminated trimethoprim/sulfamethoxazole-resistant nocardiosis with pulmonary and cerebral involvement. Tacrolimus and mycophenolate were withheld, low-dose prednisone was continued, and imipenem plus linezolid were started together with voriconazole. Clinical response appeared within 1 week, consistent with effective source control once the correct antibacterial coverage was in place. For a patient with brain lesions, fast improvement after targeted therapy is a bedside signal that the diagnosis and the drug selection are aligned.
Treatment then required further adjustment. After 1 month, cytopenias developed, and linezolid was switched to minocycline. Because of seizure risk, imipenem was changed to meropenem, and meropenem was continued for 3 months alongside minocycline. Voriconazole was stopped after 6 months. These modifications reflect a familiar problem in complex opportunistic infection: the first effective regimen may still be limited by toxicity, neurologic risk, and the need for prolonged therapy in disseminated disease.
Radiologic clearance took six months and graft function was preserved
At 6 months, imaging showed complete radiologic resolution of both intracranial and pulmonary lesions, with preserved graft function. The patient remained well at 1-year follow-up and continued on secondary prophylaxis with minocycline. In a transplant recipient, preserved graft function after severe disseminated infection is a meaningful outcome because it shows that antimicrobial recovery and organ stability can coexist when immunosuppression is carefully adjusted.
The case fits a wider pattern now seen across transplant programs. Nocardiosis affects 0.04%–3.5% of solid-organ transplant recipients, pulmonary disease disseminates in up to 44% of cases, and central nervous system involvement occurs in 20%–44%. Concurrent infection with Nocardia and Aspergillus accounts for only 5%–7% of posttransplant cases, yet that overlap is exactly what complicated this presentation. In tropical settings, where chronic pulmonary infections can mimic each other on imaging, fungal and mycobacterial coinfection can delay the moment when Nocardia enters the differential diagnosis.
India has reported notable resistance among transplant-associated nocardiosis isolates. One renal-transplant series found 9 of 16 patients receiving trimethoprim/sulfamethoxazole prophylaxis and 4 N. farcinica isolates resistant. A South India 10-year analysis reported 20.8% resistance among 48 cases, and a North India center reported 12 nocardiosis cases among renal transplant recipients, with 60% of isolates resistant and 58.3% breakthrough infection while patients were receiving prophylaxis. Those figures are not interchangeable across settings, but they all point in the same direction: prophylaxis does not guarantee coverage when the organism has already crossed the resistance threshold.
Brain imaging belongs in every case of pulmonary nocardiosis, even without neurologic symptoms, because occult central nervous system disease is common and often changes treatment duration and drug choice. Species-level identification also remains difficult, and susceptibility testing can be technically demanding when control strains and molecular tools are not available. This case leaves one concrete clinical lesson: in a transplant recipient with persistent pulmonary disease, neurologic deficits, and poor response to standard therapy, trimethoprim/sulfamethoxazole resistance must be considered early, and the laboratory result can be the difference between prolonged diagnostic drift and recovery.