## 3 Bacteriophage Depolymerases as an Alternative to Antibiotics  
Phage depolymerases have gained the interest of the scientific world due to their participation in phage adsorption and digestion of bacterial capsules. According to [PERSON] et al., most of these proteins are encoded in the region of structural genes in a phage genome (i.e., tail fibers and base plates) or next to it [98]. These phage-encoded enzymes recognize, bind, and digest the polysaccharide compounds of bacterial cell walls [80]. Degradation of these structures allows exposing the phage receptor, which is crucial for efficient phage infection of the host [99]. Bearing in mind the mechanism of action of phage depolymerases, we can divide them into two classes, hydrolases and lysates. Both groups are focused on the process of degradation of polysaccharides, including capsular polysaccharides (CPSs), lipopolysaccharidesachrides (LPSs), \(O\)-polysaccharides, or exopolysaccharide (EPSs), produced in the biofilm [81]. Some of them can also degrade polypeptides or lipids [100]. Interestingly, phage hydrolases mainly have the activities of sialidases, xylosidases, levanases, rhamosidases, dextranases, and peptidases. This group of enzymes belongs to the \(O\)-glycos hydrolases that catalyze the hydrolysis of glycosidic bonds [98; 99]. Moreover, another class of depolymerases is represented by hyalurone, alginate, and pectin/pectate lyase. Their mechanisms of depolymerization do not include water usage and are based on \(\beta\)-elimination to form a new double bond [101]. Since the discovery of activities of phage depolymerases, they have become an alternative to antibiotics, especially in the treatment of multidrug-resistant bacteria. Unlike antibiotics, depolymerases can be specific to the host and allow for the natural bacterial flora to remain untouched [102; 103]. They can not only be engineered to increase their degrading activity, but also used as tools for the detection of bacteria [104]. Depolymerases may be used in two alternative forms, (i) as tail-spike proteins (TSP) which have depolymerase domains, being components of virions, or (ii) as free enzymes [101]. When whole phages are used, they can multiply in sensitive host cells, producing significantly more virions with TSPs. This option might be beneficial if there are difficulties in supplying materials directly to biofilms, e.g., during treatment of clinical biofilm infections, as phages may allow for more effective delivery of these enzymes to the target place [105]. On the other hand, phages can transfer genes coding for toxins and antibiotic resistance proteins between bacterial cells, which causes safety issues [106; 107; 108]. The development of bacteriophage resistance among bacteria is another problem [109; 110]. In contrast, free depolymerases are genetically stable agents, active in harsh external conditions. Development of resistance to these enzymes is unlikely, which is another advantage of free proteins (obtained biotechnologically as products of recombinant genes) over the use of whole phage particles [99; 111; 112]. In addition, the diffusion of free enzymes seems to be more rapid and effective than that of virion-associated depolymerases [105]. As described above, bacterial biofilms are formed mainly with EPSs [11; 113]. Both CPS and EPS layers present large diversity in their polysaccharide content. Differences can be observed at the species and strain levels. In addition, the EPS matrix may present structural heterogeneity across individual biofilms as the physiological state of the host cells, their growth phase, their co-aggregation, and the environmental conditions also affect its composition [114]. Therefore, in some cases, a depolymerase that is able to degrade the CPS layer of a particular bacterial strain may not be capable of digesting its EPS layer. In response to the existing tremendous variation in bacterial polysaccharides, huge diversity in phage-derived depolymerases is observed. They present high specificity to a narrow range of target polysaccharides [99].