## 1 Introduction  
Innate immunity plays an important role in resistance against pathogens, consisting of cellular and humoral arms [1]. Soluble pattern recognition molecules (PRMs) are parts of the humoral arm that recognize pathogen-associated molecular patterns (PAMPs), and regulate and sustain the immune response in coordination with the cellular arm [2, 3]. Collections, ficolis and pentraxins, as the PRMs in immune system, are able to recognize pathogens, activate the complement system, regulate inflammation, and so on [4]. Pentraxins, including short-chain pentraxins such as C-reactive protein (CRP/PTX1), serum amyloid P component (SAP/PTX2) and long-chain pentraxins, are PRMs of the immune system, constituting a conservative protein superfamily characterized by a cyclic multimeric structure [5, 6]. Protein motifs of pentraxins are characterized by the presence of a 200-amino acid PTX domain in the C-terminal and an intramolecular conserved amino acid sequence named pentraxin signature (_HxCxs/TWxS_, x representing any amino acid). As for long-chain pentraxins, for example, pentraxin member 3 (PTX3), has an additional N-terminal domain preceding the PTX domain [7], whereas others have only a PTX domain in short-chain pentraxin, including CRP and SAP [8]. SAP is a key member of pentraxins with a PTX domain and an intramolecular conserved pentraxin signature, which could form an oligomer by the interaction of five identical units, playing a critical role in innate immune defense [9]. As a Ca\({}^{2+}\)-dependent protein, SAP is mainly secreted from the hepatocytes and macrophages into the circulation [10]. SAP is able to bind not only viruses (e.g., influenza), but also bacteria (e.g., _Streptococcus pneumoniae, Staphylococcus aureus, Escherichia coli, Streptococcus pyogenes_ and _Neisseria meningitidis_) and parasites such as malaria [11; 1]. Besides, SAP makes leukocyte-mediated phagocytosis by interacting with the D1 and D2 domains of FcyRIIa contact the ridge helices from two neighboring SAP subunits, which could be inhibited by competitive inhibition by IgG due to a shared binding site, resulting in the inhibition of immune complex-mediated phagocytosis [12; 13]. Moreover, SAP could activate the complement system by interacting with C1q, C4-binding protein (C4 BP) and MBL, contributing to the enhancing host defense [6; 11; 14; 15]. In teleost fish, the SAP has been reported in a few species, including rainbow trout (_Oncorhynchus mykiss_), Atlantic salmon (_Salmo salar_), half-smooth tongue sole (_Cynoglosus semilaevis_) and rock bream (_Oplegnathus fasciatus_) [2; 16; 17; 18]. In trout, SAP native protein was isolated and identified from the serum [16]. In addition, in response to _Escherichia coli_ LPS and _Aeromonas salmonicida_, the dynamics of trout SAP level were investigated [17]. Besides, expression of SAP in half-smooth tongue sole and rock bream was found increased after bacterial and viral pathogen infection [2; 18]. Existing in species such as horseshoe crabs, reptiles, lizards and mammals, SAP acts as an ancient immune mediator. However, the distribution and function of SAP might be different due to different species living in different environments [7]. Although some studies focusing on SAP have been reported, research on SAP related to bacterial infection such as phagocytosis, regulation of complement system and so on in teleost fish remains unclear. Thus, a more comprehensive study is urgently needed. In this study, the functional characterization of SAP was examined and identified in Nile tilapia (_Oreochromis niloticus_), a primary vertebrate. The ORF of _O. niloticus_ SAP (_OnSAP_) gene was identified and analyzed. The mRNA expression of OnSAP in different tissues of healthy or stressed fish has been explored. Further, functions of recombinant OnSAP protein towards bacteria binding, agglutination and promotion of phagocytosis were characterized. Moreover, we also found that the OnSAP could facilitate complement-mediated cell lysis.