Answer to January 2022 Photo Quiz

The answer is anaerobic bacteremia due to Enterocloster clostridioformis. E. clostridioformis was recently reclassified from Clostridium clostridioforme based on 16S rRNA phylogeny (1).

Direct microscopic examination revealed cigar-shaped Gram-variable bacilli. Clostridium strains belonging to the so-called RIC group (Clostridium ramosum, Clostridium innocuum, and E. clostridioformis) usually appear Gram negative or Gram variable (2). E. clostridioformis organisms are cigar shaped, whereas C. innocuum and C. ramosum form slender rods.

Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) identification was performed directly from the blood culture using the MBT Sepsityper (Bruker Daltonics, Wissembourg, France). E. clostridioformis was identified with a score of 2.01. Results of the nitrocefin test (Cefinase disks; Thermo Fisher Scientific, Waltham, MA, USA) were positive for subcultures, revealing β-lactamase production. Antimicrobial susceptibility testing using gradient diffusion showed resistance to benzylpenicillin and moxifloxacin but susceptibility to amoxicillin-clavulanate, piperacillin-tazobactam, meropenem, vancomycin, and metronidazole, according to the EUCAST guidelines for Gram-positive anaerobes (3). The patient recovered after 2 weeks of treatment with cefotaxime, metronidazole, and fluconazole.

The clinical relevance of Clostridium bacteremia has been challenged in the literature. Proposed criteria for clinical relevance include the presence of at least one of the following characteristics: (i) more than one positive blood culture, (ii) appropriate focus or source of infection (e.g., intraabdominal), or (iii) sepsis without another evident source of infection (4). Using these criteria, the clinical relevance of Clostridium bacteremia ranged from 68% to 83% (4, 5). For clinically relevant bacteremia, mortality rates at 30 days were higher in cases of hypothermia or inadequate empirical antimicrobial therapy (5).

Accurate identification of Clostridium strains from the RIC group is important because these Clostridium species are usually more resistant to antimicrobial agents. Indeed, E. clostridioformis strains are often resistant to benzylpenicillin (2). Moreover, resistance to β-lactam-β-lactamase inhibitor combinations (amoxicillin-clavulanate and piperacillin-tazobactam) has also been described (6). E. clostridioformis might act as a reservoir of antibiotic resistance genes in the gut microbiota (7). Despite its Gram-negative appearance, E. clostridioformis is usually susceptible to vancomycin, whereas C. ramosum and C. innocuum present low-level intrinsic resistance (2, 8). MALDI-TOF MS identification directly from positive blood cultures seems particularly interesting for anaerobic bacteria because of their slow growth.