## 1 Introduction  
Bacteriophages or phages are viruses that specifically infect bacteria. They are extremely diverse regarding their morphology, genomic material, and lifestyle [1]. The most common phage lifestyles encountered are the lysogenic and lytic cycles. In a lysogenic cycle, after receptor recognition and DNA injection, temperate phages coexist within their host, either integrated in the chromosome or as independent plasmid-like molecules, in a state called prophage [2, 3]. During a lytic cycle, after the phage multiplication using the host replication and translation machineries, the virion progeny is released in a lytic process causing the host death. Phage-encoded enzymes, called endolysins, mediate bacterial lysis [4] via peptidoglycan (PG) degrading activity [5]. In general, endolysins do not possess a signal sequence to be transported through the plasmidic membrane. They rather cooperate with another protein, the holin, to achieve cell lysis [6]. At the end of the lytic cycle, endolysins accumulate in the cytoplasm until holins have oligomerized and formed pores in the membrane, allowing endolysins to reach the PG layer, in a tight time regulated process [7, 8]. Endolysins encoded by Gram-positive infecting phages display a modular structure with an N-terminal enzymatically-active domain(s) (EAD) and a C-terminal cell wall-binding domain(s) (CBD), both separated by a linker sequence [5]. The EAD contains one or several domains involved in the cleavage of specific bonds in the PG meshwork. These enzymatic activities can be divided into four main classes based on the targeted link or cleavage mechanism: glycosidases (including \(N\)-acetylglucosamineaminidase, \(N\)-acetylmuramidase, and transglycos), amidases, endopeptidase, and cysteine, histidine-dependent amidohydrolases/peptidases (CHAP). Less conserved than the EAD, the CBD is responsible for recognition of the host PG and allows keeping the endolysin in close proximity with the cell wall, thus avoiding the release of the enzyme which could lead to unwanted cleavages of neighboring hosts [9]. CBD usually display structural repeats and have been broadly classified into four classes: SH3 domains, choline-binding modules, three-helix bundles, and \(\alpha\)/\(\beta\) multimers [5]. Endolysins can be used as antimicrobial compounds in the food and medical sectors instead of their parental phages [10]. Indeed, phages sometimes have such a narrow host spectrum that they cannot target all the pathogenic strains of a certain group whereas their corresponding endolysins usually display a broader spectrum [11]. Moreover, no bacterial resistance to endolysins has been identified so far. This is mainly due to the fact that the rise of such resistant strains would involve important changes in highly conserved PG bonds that are essential for bacterial survival [12]. Endolysin CBDs have also been fused with fluorescent markers or paramagnetic beads and used as specific detection tools [13]. Additionally, endolysins and their CBDs can be engineered (e.g., domain swapping or mutagenesis) to modulate their activity, specificity, or stability [14]. The _Bacillus cereus_ group is a highly debated cluster of closely related species. _B. cereus sensu stricto_ (s.s), _Bacillus thuringiensis_, _Bacillus anthracis_, _Bacillus weihenstephanensis_, _Bacillus mycoides_, _Bacillus pseudomycoides_, and _Bacillus cytotoxicus_ are the seven species most commonly recognized as members of this group, but other species have been suggested as new members [15; 16]. In the last few years, several endolysins of phages infecting diverse _B. cereus_ group members have been identified and characterized [11; 17; 18; 19; 20; 21]. Deep-Blue [22; 23] and Deep-Purple [24], two phages infecting the _B. cereus_ group, belong to the families _Herelleviridae_ and _Siphoviridae_, respectively. The present work focuses on the identification and detailed characterization of the endolysins PlyB221 and PlyP32, encoded by the Deep-Blue and Deep-Purple phages, respectively.