Clinical characteristics of Raoultella ornithinolytica bacteremia and antimicrobial susceptibility of Raoultella ornithinolytica

Raoultella ornithinolytica (R. ornithinolytica) is a gram-negative rod that was considered related to Klebsiella oxytoca and was classified as R. ornithinolytica in 2001. R. ornithinolytica is known as a histamine-producing bacterium that causes mackerel poisoning. Although only few clinical cases of R. ornithinolytica infection in humans have been reported, the number of diagnosed cases is expected to increase owing to the advancements in identification methods. In the present study, we performed a retrospective analysis of cases of R. ornithinolytica infection detected at our hospital. From September 2019 to July 2021, 62 specimens positive for R. ornithinolytica were obtained after removing duplicates. The clinical courses of these cases were investigated retrospectively based on electronic medical records. Of the 62 specimens, 24 were sputum, 19 were urine, three were stool, six were blood, four were bile, and six were other specimens. All the six blood culture specimens in which R. ornithinolytica was detected were from male patients, and the causative diseases were cholangitis in four cases and complicated pyelonephritis in two cases. Of these, two patients with cholangitis succumbed to death due to the worsening of underlying cancer. Identification of R. ornithinolytica is reportedly difficult, and some instruments may misidentify it as Klebsiella oxytoca. The prognosis of R. ornithinolytica infection has been reported to be good when susceptible drugs are used. However, high mortality rates were also reported despite the use of these drugs, suggesting the need for further investigation of clinical features of R. ornithinolytica infection.