First reported case of invasive Bacteroides zoogleoformans infection in Japan

Japanese clinicians have reported what they describe as the first documented case of invasive human infection caused by Bacteroides zoogleoformans, an anaerobic bacterium first described in 1937 that had previously been found only in periodontal infections and animal bite wounds. The case, published in New Microbes and New Infections, involved a liver abscess and bloodstream infection in a 78-year-old man, and the identification of the organism required genetic sequencing after routine laboratory methods failed.

The patient had chronic hepatitis C, hepatocellular carcinoma, and type 2 diabetes mellitus, and presented with right upper quadrant pain and fever. Contrast-enhanced computed tomography revealed an approximately 8-centimeter mass in segment 4 of the liver with arterial dilation, suggesting recurrent hepatocellular carcinoma with possible rupture. Repeat imaging showed liquefaction and intralesional air within the embolized tumor, consistent with abscess formation in necrotic tissue. Percutaneous drainage yielded purulent material, and the patient experienced transient delirium suggestive of sepsis-associated encephalopathy.

Blood cultures collected on hospital day 2 turned positive only after 119 hours of incubation, and only in the anaerobic bottle of one of two culture sets. Subculturing produced small, smooth, grayish-white colonies after roughly 48 hours, and Gram staining revealed Gram-negative rods. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry returned Bacteroides uniformis, Prevotella heparinolytica, and Bacteroides fragilis among its highest-ranked matches, but all identification scores fell below the threshold for reliable identification. The laboratory therefore turned to 16S rRNA gene sequencing, which showed 100% sequence identity with B. zoogleoformans in the GenBank database, confirming the species. The isolate tested negative for beta-lactamase production and was susceptible to meropenem and cefmetazole. Despite extended incubation, no microorganisms were recovered from the abscess fluid itself.

Treatment proved prolonged. The patient’s maximum body temperature fell below 38 degrees Celsius by hospital day 5, but intermittent fever above that threshold persisted until hospital day 26, with sustained defervescence thereafter. Because of the severity of his condition, intravenous meropenem was given for 38 days before de-escalation to intravenous cefmetazole, 1 gram every 12 hours for 5 days, based on the susceptibility results. His clinical condition improved, and he was transferred to a long-term care hospital on hospital day 48. The researchers also performed whole-genome analysis of the isolate, and the genome sequence has been deposited in GenBank under BioProject accession PRJDB37915.

The authors emphasize that the pathogenicity of B. zoogleoformans remains unclear and that no well-characterized virulence factors have been reported. In this case, they suggest that necrotic tumor tissue following embolization, together with diabetes mellitus and chronic liver disease, likely created a hypoxic and immunocompromised environment that facilitated anaerobic proliferation and bloodstream invasion. Because the organism is rare and underrepresented in reference databases, routine identification methods may fail, and the authors argue that molecular approaches such as 16S rRNA gene sequencing are essential for accurate identification. Detection in a sterile site expands the known clinical spectrum of the species and suggests it should be considered a potential opportunistic pathogen in intra-abdominal infections, although the authors caution that further case reports are needed to determine whether this case is an incidental finding or whether B. zoogleoformans is an emerging pathogen.

Subject of Research: First reported case of invasive Bacteroides zoogleoformans liver abscess and bloodstream infection in Japan

Article Title: Bacteroides zoogleoformans liver abscess and bloodstream infection in Japan: the first reported case of invasive infection

Article References: Kitaya, S., Horiba, K., Hashino, M., Takemura, S., Matano, S., & Kanamori, H. (2026). Bacteroides zoogleoformans liver abscess and bloodstream infection in Japan: the first reported case of invasive infection. New Microbes and New Infections, 74, Article 101871. https://doi.org/10.1016/j.nmni.2026.101871

Image Credits: AI Generated

DOI: 10.1016/j.nmni.2026.101871

Keywords: Bacteroides zoogleoformans, liver abscess, bloodstream infection, anaerobic bacteria, 16S rRNA sequencing, MALDI-TOF MS, hepatocellular carcinoma, opportunistic pathogen, Japan, case report, Bacteroides, zoogleoformans