Rumyana T. Eneva*, Stephan A. Engibarov, Yana G. Gocheva, Simona L. Mitova and Penka M. Petrova Novel sialidase from non-pathogenic bacterium Oerskovia paurometabola strain O129 https://doi.org/10.1515/znc-2022-0051 Received March 10, 2022; accepted October 20, 2022; published online November 10, 2022 Abstract: Bacterial sialidases are enzymes that are involved in a number of vital processes in microorganisms and in their interaction with the host or the environment. Their wide application for scientific and applied purposes requires the search for highly effective and non-pathogenic producers. Here, we report the first description of sialidase from Oer- skovia paurometabola. The extracellular enzyme preparation was partially purified. The presence of sialidase was confirmed in native PAGE treated with the fluorogenic sub- strate 4MU-Neu5Ac. Maximum enzyme activity was regis- tered at 37 °C and in the pH range of 4.0–5.5. The influence of metal ions and EDTA was examined. It was demonstrated that EDTA, Mn 2+ and Ba 2+ ions inhibit the sialidase activity to different extent, while Cd 2+ , Fe 2+ and Fe 3+ have stimulating effect on it. These features are studied for the first time concerning sialidase of Oerskovia representative. Cell bound sialidase and sialate aldolase were also established. Keywords: Oerskovia; pH optimum; sialate aldolase; siali- dase; temperature optimum. 1 Introduction Sialidases (neuraminidases, exo-α-sialidases, EC 3.2.1.18) are enzymes that cleave terminal sialic acid residues from sialoconjugates such as glycoproteins, glycolipids, oligo- saccharides and polysialic acid compounds. Sialidase ac- tion is the first step in the catabolism of sialoconjugates. These substances and enzymes of their metabolic pathway are found mainly in animals of the Deuterostomata lineage and in rare cases in microorganisms. Bacteria that possess sialidases are usually pathogens or commensals in these animals. Saprophytic bacterial species producing siali- dases are rare. Interestingly, the well-studied saprophytic producers of microbial sialidases are of the phylum Acti- nobacteria – Arthrobacter ureafaciens, Micromonospora viridifaciens, Arthrobacter nicotianae and Actinomyces species [1–4]. Some well-known pathogens producing sialidase also belong to the actinobacteria – Arcanobacte- rium pyogenes, Corynebacterium diphtheriae, Corynebac- terium ulcerans and Gardnerella vaginalis [5–7]. The next step in the catabolism of sialoconjugates is the degradation of sialic acid (Neu5Ac) to N-acetyl-D- mannosamine and pyruvate catalyzed by the enzyme sialate aldolase (N-acetylneuraminate pyruvate lyase, EC 4.1.3.3). The presence of this enzyme in a sialidase producer indicates that it can utilize sialic acid as a carbon source. Sialate aldolase in actinobacteria is scarcely studied, and the first report of its presence in Oerskovia was published by us in 2016 [8]. According to recent taxonomic studies, the genus Oerskovia belongs to the eponymous family Oerskoviaceae, part of the Actinobacteria phylum [9]. The genus includes aerobic to facultative aerobic Gram-positive bacilli forming substrate mycelium on solid nutrient media that breaks down into motile or non-motile rods. No aerial mycelium is formed. So far, five species have been described. They are widely distributed in the environment and have been iso- lated mainly from soil, water, grass cuttings and occa- sionally from clinical specimens [10]. However, isolates from blood, cerebrospinal fluid, urine, heart tissues and wounds have been identified as Oerskovia turbata or Oer- skovia xanthineolytica. Oerskovia species have been also implicated to cause various infections and bacteremia [10]. According to the international classification of microor- ganisms in risk groups, the species of the genus Oerskovia belong to Risk Group 1, which include microorganisms with low individual and low community risk for humans [11]. These microorganisms are unlikely to cause disease. The presence of sialidase in a member of this genus, O. turbata, was first reported by Müller [12]. Zhang et al. present a new sialidase-producing strain of O. xanthyneolytica, isolated from soil, and describe the establishment of a simple and large-scale biotechnological process for GM1 production using O. xanthyneolytica YZ-2 [13]. In 2016, our team published data *Corresponding author: Rumyana T. Eneva, The Stephan Angeloff Institute of Microbiology, 1113 Sofia, Bulgaria, E-mail: rum_eneva@abv.bg. https://orcid.org/0000-0001-6905- 6190 Stephan A. Engibarov, Yana G. Gocheva, Simona L. Mitova and Penka M. Petrova, The Stephan Angeloff Institute of Microbiology, 1113 Sofia, Bulgaria. https://orcid.org/0000-0002-6680-922X (Y.G. Gocheva) Z. Naturforsch. 2022; aop