# Phase Therapy: Consider the Past, Embrace the Future
## 4 Barriers and Limitations of Phage Therapy  
Although phage therapy has obvious advantages over antibiotic therapy, phage therapy as well as the related problems remain to be further investigated. There are still some obstacles to be overcome before the large-scale introduction of phage therapy to the clinic. Firstly, the strict specificity of phages to the host bacteria is a very good advantage over antibiotics. At the same time, the narrow lysis spectrum limits the effectiveness of bacteriophages in killing a variety of bacteria. This limitation can be partly overcome by phage cocktail therapy, utilizing phage lyases, and engineering a phage for the broader host spectrum. In the process of phage therapy, it is necessary to identify the pathogen to select the effective phage, and a bacterial disease is usually caused by a variety of pathogens. In addition, the development of phage resources still needs a lot of work, and some studies have shown that some bacteriophages are difficult to isolate [12; 30]. Secondly, bacteria have evolved a series of defense mechanisms to press the stop button in different stages of the phage infection cycle, including an intracellular restriction modification system, abortive infection, phage exclusion, superinfection exclusion, a CRISPR-Cas system, Acr protein, etc. [90; 92]. In addition, bacteria also become resistant to phages by modifying the receptor on the cell surface, blocking effect of outer membrane vesicles, and quorum sensing effect during the co-evolution between phage and bacteria [92; 93; 27; 94]. Moreover, as a bacterial virus, bacteriophage cannot directly penetrate eukaryotic cells unless they use bacteria as carriers, which limits the treatment of intracellular infection [12; 92]. Drug therapy delivery usually uses different porous nanoparticles to carry therapeutic cargoes. However, the phage particles are nonsymmetrical and larger than the pore sizes of nanoparticles used to load them [84]. Bacterial infections can occur in any place in the body; therefore, the reasonable way of encapsulation and delivery helping phage or its derivatives to reach the site of infection is needed. Different strategies of encapsulating and stabilizing bacteriophages have been introduced to overcome the limitation of phage delivery. For example, the encapsulation of a phage in liposomes has been found to promote phage delivery to intracellular infection sites and to increase the retention time of phage in the host [95; 96]. As is well known, safety and effectiveness are the two most important standards of medical treatment. Due to the lack of understanding of the functional genomics of phages, phage-carried genes with unknown functions may be resistance genes and virulence factors [26; 97]. Bacteria lysed by a phage may release endotoxins and super antigens that cause inflammation, which may potentially cause serious complications [98; 99]. For example, the phages of _S. aureus_ and _P. aeruginosa_ are able to induce endotoxin-independent anti-inflammatory response [100]. In addition, most data of the activity and growth of phages come from in vitro experiments, which may be greatly different from complex in vivo environments. On the other hand, the pharmacokinetics of the phage during the treatment process are not well-known, especially those in the multiple species used for phage cocktails. Phages mainly eliminate bacteria through active treatment and passive treatment. Phages at high concentrations can directly kill bacteria (known as passive treatment), which is consistent with traditional antibiotic treatment. However, the treatment outcome is highly dependent on the quantity ratio between phage and host bacteria in active treatment. When it infects its host bacteria, a phage will be resisted by the defense systems of the host bacteria. Only when the host bacteria reach a certain biomass can the phage replicate and proliferate [93; 101]. All these suggest that it is very important to optimize the initial time of phage therapy and the dose of phage.