# A Novel Singleton Giant Phage Yong-XC31 Lytic to the _Pyropia_ Pathogen _Vibrio mediterranean_
## 4 Discussion  
_Pyropia_ is an important cultivated seaweed in Northeast Asia [7; 47]. In recent years, increase in the prevalence of diseases and pests has caused a subsequent reduction in their quantity and commercial value [47; 48]. Among these diseases, yellow spot disease (YSD) is a destructive disease of the conchocelis sporegling culture of _Pyropia_[7; 47; 48]. _V. mediteranei_ 117-T6 (CGMCC1.16311) was pathogenic to the YSD of conchocelis of several  
Figure 8: The proteomic tree is generated using ViPTree online based on genome-wide similarities determined by tBLASTx. 56 type species of nine families belonging to the order Caudovirales, _Vibrio_ phage Yong-XC31, and 10 giant phages in Table 2 were included in the analysis. Bacteriophage family assignments according to the official ICTV classification are provided with different color bars. No color bar is marked to unclassified giant phages. Red star refers to Yong-XC31. The sizes or size ranges of the giant phage genomes are shown in red font on the right. Pyropia_ species [8]. Traditional antibiotic therapy has no stable effect on YSD [49]. In this study, antibiotic susceptibility test demonstrated that _V. mediterranei_ 117-T6 was multidrug resistant, which accord with the ineffective antibiotic treatment. Therefore, the development of new tools and strategies to control pathogens and treat diseased _Pyropia_ is becoming a most important issue. Phages are currently emerging as potential treatments for multidrug resistant bacterial infections. Yet, literature search yielded no information about _V. mediterranei_ phage. In this study, the first _V. mediterranei_ phage was isolated and identified as a novel giant phage representing a novel phage genus. Proteomic tree indicates that Yong-XC31 together with giant _Vibrio_ phage BONAISHII, _Vibrio_ phage 2 TSL-2019 and _Pseudomonas_ phage Phabio represent a new evolutionary lineage independently between the family Drexterviridae and Herelleviridae. The four giant phages all infect Gram-negative bacteria, contain genome of 242,446-309,157 bp with only an average 1/4 of the predicted genes being assigned functions. A new family is proposed here in Caudovirales. In the novel proposed family, Yong-XC31 presents a monophyletic genus in consideration of the pair-wise ANI, isDDH, POCP and PASC values. The therapeutic value of a candidate bacteriophage relies on the characterization of viral properties such as stability, growth kinetics, host range and viral yield [50]. Results of this study indicated that the temperature and pH stability of Yong-XC31 was good, which is conducive to its application to control _V. mediterranei_ infections in complex environment. Results also demonstrated other good properties of Yong-XC31 including a very low optimal MOI (0.001), efficient adsorption to host (reach 93.8% adsorption rate in only 2 min), a short latent period (30 min), a high burst size (64,227) and strict host specificity. Additionally, our small-scale laboratory experiments proved that Yong-XC31 could protect both the free-living conchocelis (FLC) and the shell-borne conchocelis (SBC) of _Pyropia_ from harm caused by _V. mediterranei_[49]. These results suggest significant application potential of Yong-XC31 in _Pyropia_ production. Thus, far, only fourteen jumbo phages infecting _Vibrio_ bacteria have been characterized. Among them, Yong-XC31 is most related to the unclassified phage BONAISHI, which is strictly lytic for several strains of _V. coralliilyticus_ pathogenic to coral [50]. Yong-XC31 and BONAISHI share several special characteristics. Their genomes are both about of 290 kb, contain more than one paralogous gene encoding RNAPs and do not harbour identified tRNA gene. Most phages rely on RNAPs of a bacterial host to transcribe their genes [51]. A strategy used by some phages is to depend on their own single-subunit RNAPs to transcribe a subset of viral genes [52].