Photo Quiz: Gram-Variable Staining in Anaerobic Bacteremia

A 62-year-old male patient was admitted to the surgical intensive care unit (ICU) for secondary peritonitis and septic shock. His medical history included laryngeal cancer, adrenal insufficiency, and a recent total colectomy for acute mesenteric ischemia. An urgent exploratory laparotomy revealed an anastomotic leak secondary to the colectomy. Anastomotic takedown and peritoneal lavage were performed, with a Gram stain of the peritoneal fluid showing a polymicrobial flora. Antimicrobial therapy with piperacillin-tazobactam, amikacin, and caspofungin was initiated.

On day 3 of the ICU stay, one blood culture set was sampled for hypothermia (34.7°C). Peritoneal fluid samples collected during surgery yielded Escherichia coli and Candida albicans after 48 h of aerobic incubation at 35°C; anaerobic cultures were not routinely performed. Antimicrobial therapy was switched to cefotaxime, metronidazole, and fluconazole, taking into account the antimicrobial susceptibility results and keeping broad antianaerobic coverage.

On day 8 of the ICU stay, one anaerobic blood culture bottle was positive after 4 days 13 h of incubation in the BacT/Alert Virtuo automated system (bioMérieux, Marcy l’Etoile, France). Gram staining revealed Gram-variable bacilli (Fig. 1). Given the long time to positivity of the blood culture, the morphology with Gram staining, and the clinical history, an obligate anaerobic bacterium was suspected.

FIG 1.

Gram stain of an anaerobic blood culture, revealing Gram-variable bacilli. The image was obtained at a magnification of ×1,000.