Actinotignum-associated prosthetic aortic valve infective endocarditis complicated by postoperative mixed shock and dialysis-dependent acute kidney injury: a case report

Technical Summary: Actinotignum-associated Prosthetic Aortic Valve Infective Endocarditis Complicated by Postoperative Mixed Shock and Dialysis-Dependent Acute Kidney Injury

Problem Statement
Actinotignum spp. are facultatively anaerobic Gram-positive rods increasingly recognized as urogenital pathogens, particularly in elderly or urologically compromised patients. However, clinically significant infections are often underrecognized due to the organism's slow growth, fastidious culture requirements (needing anaerobic or CO₂-enriched conditions), and difficulty in identification by conventional phenotypic methods. While Actinotignum schaalii is the most frequently reported species, Actinotignum sanguinis and others are less characterized. Infective endocarditis (IE) caused by Actinotignum spp. is uncommon, and involvement of prosthetic valves is exceptionally rare. Furthermore, species-level identification within this genus often requires molecular confirmation, as Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) may yield ambiguous results between closely related species. The clinical challenge lies in the delayed recognition of the pathogen, the necessity for early surgical source control before microbiological confirmation in prosthetic valve endocarditis (PVE), and the management of complex postoperative complications such as mixed shock and acute kidney injury (AKI).

Methodology and Case Presentation
This case report details the clinical course of a 71-year-old male with a complex history including benign prostatic hyperplasia (BPH), recurrent urinary retention requiring clean intermittent self-catheterization, horseshoe kidney, chronic kidney disease (CKD), and a bioprosthetic aortic valve replacement (21-mm INSPIRIS) performed seven months prior. The patient presented with low back pain, oliguria, dark urine, severe AKI, hyperkalemia, and systemic inflammation.

• Diagnostic Approach: Initial workup suggested a urinary tract infection (UTI) complicated by AKI. Blood and urine cultures were obtained prior to antimicrobial initiation. Transthoracic echocardiography (TTE) revealed a large (20 × 9 mm) mobile vegetation on the bioprosthetic aortic valve.
• Microbiological Identification: Despite initial negative routine cultures, subsequent analysis utilized MALDI-TOF MS. Blood and urine isolates yielded Actinotignum sanguinis as the best match (score values 2.02–2.31). However, because 16S rRNA gene sequencing was not performed and lower-ranking matches included A. schaalii, the organism was conservatively identified as Actinotignum spp. Crucially, the organism was only detected after anaerobic culture on Brucella HK agar, as routine aerobic conditions failed to yield growth despite positive Gram staining.
• Surgical Intervention: Due to the large mobile vegetation and high embolic risk, emergency surgery was performed on Day 4, prior to microbiological confirmation. Intraoperative findings showed vegetations and granulation tissue but preserved valve structure and sewing ring stability. Consequently, a risk-adapted strategy of valve-preserving debridement was chosen over redo valve replacement to avoid prohibitive risks associated with aortic root reconstruction in friable tissue. An iatrogenic injury to the right coronary ostium necessitated a saphenous vein graft coronary artery bypass (SVG-RCA CABG).
• Postoperative Management: The patient developed a complex "mixed shock" phenotype characterized by low-output physiology, vasoplegia, anuria, severe hypoalbuminemia, and massive fluid overload. Management involved integrated antimicrobial therapy (transitioned from ceftriaxone/linezolid to ampicillin based on susceptibility), vasoactive support, albumin administration to restore effective intravascular volume, and renal replacement therapy (transitioning from continuous hemodiafiltration to intermittent hemodialysis).

Key Results

• Microbiology: Successful isolation of Actinotignum spp. from both blood and urine cultures required anaerobic incubation. The concordance of the pathogen in both sites supported a urogenital portal of entry.
• Surgical Outcome: Valve-preserving debridement successfully removed the vegetation without compromising the structural integrity of the bioprosthesis. Follow-up TTE on Day 27 confirmed the disappearance of the vegetation, preserved valve function, and improved transprosthetic gradients.
• Clinical Recovery: Despite severe postoperative complications, including a peak weight gain of over 14 kg due to fluid overload and a requirement for multiple vasoactive agents, the patient achieved hemodynamic stability. Renal function recovered sufficiently to discontinue hemodialysis by Day 54. The patient was discharged to a rehabilitation ward on Day 60, walking independently without neurological or physical sequelae.
• Complications: The patient experienced a secondary ventilator-associated pneumonia caused by Pseudomonas aeruginosa, which was managed with targeted antibiotics (meropenem).

Significance and Claims
The authors posit that this case offers several critical educational points regarding the management of Actinotignum-associated PVE:

1. Diagnostic Vigilance: In patients with urological abnormalities or instrumentation, Actinotignum spp. should be suspected in cases of bacteremia or IE, even if routine cultures are negative. Discordance between Gram staining (showing organisms) and routine culture results should prompt immediate anaerobic culture and extended incubation.
2. Identification Caution: While MALDI-TOF MS is valuable for genus-level recognition, species-level identification (e.g., distinguishing A. sanguinis from A. schaalii) requires caution and ideally molecular confirmation (16S rRNA sequencing), as commercial databases may not reliably discriminate between these taxa.
3. Surgical Strategy: In PVE, source control may need to proceed based on clinical and echocardiographic grounds before microbiological confirmation is available. Surgical strategies must be individualized; in this case, valve-preserving debridement was a viable, risk-adapted alternative to high-risk redo replacement, leading to infection control and functional recovery.
4. Shock Management: Postoperative deterioration in severe PVE may present as "mixed shock" (coexisting low-output physiology and vasoplegia) rather than isolated septic shock. Successful management requires recognizing the coexistence of fluid overload and effective intravascular hypovolemia (due to hypoalbuminemia), necessitating a multimodal approach involving volume resuscitation with albumin, vasoactive support, and careful renal replacement therapy.

The paper concludes that while this single case does not establish general efficacy for valve-preserving debridement or specific shock protocols, it highlights the potential severity of Actinotignum infections and the necessity for a multidisciplinary, integrated approach to diagnosis and management in complex prosthetic valve cases.