Weissella viridescens Bacteremia Associated with Acute Cholangitis: A Case Report

Dear Editor,

Weissella spp. are gram-positive, non-spore-forming, catalase-negative coccobacilli, formerly categorized as Lactobacillus spp., Leuconostoc spp., and Pediococcus spp. [1]. While a few cases of human infection caused by Weissella confusa have been reported [2], only one case involving Weissella viridescens with limited microbiological characteristics has been described to date [3]. Recent reports have illustrated the utility of whole-genome sequencing for the identification and genomic characterization of uncommon bacterial pathogens [4, 5]. We report a case of W. viridescens bacteremia in a patient with acute cholangitis, confirmed by whole-genome analysis. The Institutional Review Board of Seoul National University Hospital, Seoul, Korea, approved this study (2510-124-1686) and waived the requirement for informed consent.

In January 2022, a man in his late 50s with hepatocellular carcinoma who was receiving locoregional therapy presented to the emergency department with a two-day history of abdominal pain. On admission (day 1), his body temperature was 37.8°C, leukocyte count was 5.45×10⁹/L (reference interval [RI]: 4.0–10.0×10⁹/L), and neutrophil count was 4.622×10⁹/L. C-reactive protein (CRP) level was elevated at 62.8 mg/L (RI: <5 mg/L). Gamma-glutamyl transferase (GGT), AST, and ALT levels were elevated to 136 IU/L (RI: <63 IU/L), 54 IU/L (RI: <40 IU/L), and 55 IU/L (RI: <40 IU/L), respectively. Computed tomography of the abdomen and pelvis suggested common bile duct obstruction.

After collecting two pairs of peripheral blood cultures, the patient was initiated on empirical antibiotic therapy with ceftriaxone and metronidazole. Blood was inoculated into BACTEC Plus Aerobic/F and Anaerobic/F culture vials (Becton Dickinson, Sparks, MD, USA) and incubated in a BACTEC FX System (Becton Dickinson). After 6 hr of incubation, gram-positive cocci were detected in two aerobic culture bottles.

Endoscopic retrograde cholangiopancreatography (ERCP) was performed on day 2, and vancomycin was added to the antibiotic regimen based on the Gram staining results. Bile cultures were not obtained during ERCP. On day 3, the patient was transferred to another hospital while continuing intravenous antibiotics. When the patient visited the outpatient clinic 18 days later, he had no infection-related symptoms, and his laboratory test results, including CRP, GGT, AST, and ALT levels, were within the RIs.

After transfer, the gram-positive cocci were identified as W. viridescens DSM 20248 (score 2.31) via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) using the Bruker Biotyper (Bruker Daltonics, Bremen, Germany), with W. viridescens DSM 20410^T (score 2.15) as the second-best match. 16S rRNA sequencing showed 100.00% (1479/1479) identity to W. viridescens (NR_040813.1), and 98.71% (1459/1478) to W. minor (NR_040809.1). Phylogenetic analysis supported the identification of W. viridescens (Fig. 1A).

Fig. 1. Phylogenetic analysis of W. viridescens based on the 16S rRNA gene and whole-genome sequences. (A) Phylogenetic tree based on 16S rRNA gene sequences. The tree was constructed using the maximum likelihood method, with the GTR+I+G model and Enterococcus plantarum CCM 7889^T (MT758013) as the outgroup. Bootstrap values are expressed as percentages of 1,000 replications, and the scale bar represents the number of substitutions per site. GenBank accession numbers are presented in parentheses. (B) Phylogenetic tree based on whole-genome sequences. The tree was constructed using the Type Strain Genome Server with E. plantarum CCM 7889^T as the outgroup. The genome Basic Local Alignment Search Tool distance phylogeny (GBDP) method with distance formula d5 was used to scale the branch lengths, and GBDP pseudo-bootstrap support values from 100 replications are indicated above the branches.

For further identification, whole-genome sequencing of the isolate was performed using the NovaSeq platform (Illumina, San Diego, CA, USA). The assembled genome had a total length of 1,650,794 bp and a GC content of 40.95%. A comparison using fastANI against NCBI type-strain genomes revealed the highest average nucleotide identity to W. viridescens (98.28%), followed by W. minor (80.30%) [6]. Genome-based phylogenetic analysis using the Type Strain Genome Server supported the species-level identification of W. viridescens (Fig. 1B), with a digital DNA–DNA hybridization (dDDH; d4) value of 87.8% with W. viridescens NCDO 1655 [7].

Antimicrobial susceptibility was tested using the bioMérieux Etest (Marcy l’Étoile, France; Table 1). As no interpretive criteria exist for this genus, minimum inhibitory concentrations (MICs) are reported as measured: 0.125–1.5 μg/mL for penicillin, clindamycin, and imipenem; ≥32 μg/mL for ceftriaxone; and 48 μg/mL for vancomycin.

Table 1. Antimicrobial susceptibility of the W. viridescens clinical blood culture isolate as determined using the bioMérieux Etest.

Antimicrobial agent	MIC (μg/mL)	Interpretation*
Ceftriaxone	≥32	NA
Vancomycin	48	R
Penicillin	1.5	S
Ciprofloxacin	1	NA
Clindamycin	0.125	S
Imipenem	0.19	S
Erythromycin	1.5	I
Piperacillin/tazobactam	2	NA

*Interpretation according to the criteria for the taxonomically related genus Lactobacillus [11, 12].

Abbreviations: MIC, minimum inhibitory concentration; R, resistant; I, intermediate; S, susceptible; NA, not available.

Weissella spp. have been isolated from feces, milk, fermented foods, vegetables, and soil [2]. Human infections caused by Weissella spp. usually occur in immunocompromised patients [1]. Phenotypic similarities to Leuconostoc and heterofermentative lactobacilli may lead to misidentification [2]. Currently, 16S rRNA sequencing is used as a reference for identification [1].

Bacteremia occurs in approximately 50% of patients with acute cholangitis and is associated with increased 30-day mortality [8]. Standard treatment involves therapeutic ERCP and empirical antibiotics, such as ceftriaxone plus metronidazole, targeting gram-negative microorganisms that commonly cause bacteremia in acute cholangitis [9]. However, Weissella spp. are resistant to metronidazole [1], and the clinical isolate in this case had a ceftriaxone MIC of ≥32 μg/mL. Infection resolution via ERCP, despite the use of suboptimal empiric antibiotics, highlights the importance of source control for this opportunistic pathogen. Although this may indicate low virulence of W. viridescens, the high mortality rate of W. confusa infection necessitates further investigation [3]. Furthermore, early identification of Weissella spp. is important because this genus is intrinsically resistant to vancomycin, the main empirical antibiotic used for gram-positive infections [10].

MALDI-TOF MS or 16S rRNA sequencing can accurately identify Weissella spp., and penicillin, clindamycin, or imipenem may be appropriate for treating Weissella infections based on susceptibility testing.

The draft genome assembly and raw reads are publicly available at the NCBI (BioProject PRJNA1393268; BioSample SAMN54286478).