Investigation of a Pseudo-outbreak of Myroides odoratimimus in High-Dependency Units Reveals an Environmental Reservoir.

Abstract

Background: Unusual clustering of opportunistic pathogens in high-dependency units (HDUs) may raise concern for healthcare-associated outbreaks and requires careful investigation to distinguish true transmission from pseudo-outbreaks. In mid-2025, a sudden increase in urinary isolates of Myroides spp., intrinsically multidrug-resistant non-fermentative Gram-negative bacilli, was observed in HDUs of a tertiary-care hospital. This study aimed to determine whether the clustering represented sustained transmission or a pseudo-outbreak linked to a common source.

Methods: A combined retrospective-prospective investigation was conducted in HDUs comprising eight intensive care units and one palliative care unit (total capacity:89 beds). Seventy-five unique patients with Myroides spp. isolated from urine cultures between July 2024-August 2025 were included. Prospective isolates underwent species identification, antimicrobial susceptibility testing, biofilm production assessment, rep-PCR genotyping, selected 16S rRNA gene sequencing. Targeted environmental sampling was performed in units with case clustering.

Results: A temporal peak occurred in June 2025 (n:16). All prospectively analysed isolates were identified as Myroides odoratimimus. Environmental sampling recovered the organism from urine collection containers. rep-PCR analysis of 33 isolates demonstrated a dominant molecular profile shared by 32 isolates, including 29 of 30 clinical isolates and all 3 environmental isolates. Most isolates showed strong biofilm production. After the introduction of single-use urine collection bags, no further isolates were detected.

Conclusions: These findings indicate a healthcare-associated pseudo-outbreak linked to an environmental source rather than sustained patient-to-patient transmission.