In vitro aggregate formation and unusual colony morphology impairing identification of Roseomonas sp. from a septic patient

The Gram-stained morphology of organisms from positive blood culture bottles often suggests diagnoses and therapeutic options. In vitro aggregate formation by the organism in this case led to initial confusion in the diagnosis.

A 57-year-old woman with a recent stroke returned to the emergency department with acute exacerbation of her neurological deficit. Although afebrile, she had an elevated white blood cell count. Urine analysis revealed 4+ blood, nitrates, leukocyte esterase and bacteria. Blood and urine cultures were collected and intravenous piperacillin/tazobactam treatment was begun. The urine culture grew >100 000 colonies Escherichia coli ml− 1; the antibiotic was changed to ceftriaxone when susceptibilities became available at 48 h. At 61 h, the aerobic blood culture bottle was flagged as positive. A Gram stain revealed spherical structures, which stained predominantly Gram-negative but were Gram-positive in some areas of the smear. Subcultures grew overnight on blood, chocolate and MacConkey agars and on brain–heart infusion (BHI) broth. Growth on MacConkey agar was sparse. A Gram stain of the snow-white, mucoid colonies revealed Gram-negative rods, spherical aggregates and thick-walled tetrads. Subsequent cultures on solid medium grew only Gram-negative rods, but inoculation of isolated colonies from these cultures into BHI broth or negative blood culture bottles caused the thick-walled spheres and tetrads to reappear.

MicroScan and matrix-assisted laser desorption/ionization time of flight failed to identify the organism, but 16S rRNA gene sequencing identified it as Roseomonas genomospecies 5 (100 % match). To the best of our knowledge, there are no reports in the literature of aggregate-forming Roseomonas.