## 4 Discussion  
In this study, 86 peptides from the APD were tested, and 35 of these were found to be effective against G\({}^{+}\). Thirteen structurally specific peptides of different Brevinin-2 subfamilies were screened in the experiments using multiple sequence alignment. The effects of MSSA and MRSA on nematode movement and mortality had been shown previously. Both were found to have a significant influence on nematode behavior and survival rate. A model of methicillin resistant _S. aureus_ infection was designed, and 13 structurally specific peptides from varying Brevinin-2 subfamilies were used for screening. After 96 h of observation, the results showed that all of the 13 antimicrobial peptides had a therapeutic effect on the methicillin-resistant _S. aureus_ infected group in comparison with the control group. Brevinin-2, Brevinin-2-OA3, Brevinin-2 ISb, and Brevinin-2 TSa demonstrated an especially high curative effect on _C. elegans_. Brevinin-2 ISb and Brevinin-2 TSa were the most effective. Besides the protective effect, all peptides showed bacteriostatic activity. It is possible that nematode survival was--at least in part--improved by the exposure to a reduced number of bacteria or impaired bacteria. Brevinin-2, Brevinin-2-OA3, Brevinin-2 ISb, and Brevinin-2 TSa also stimulated a markedly increased survival rate after 96 h of treatment compared to other peptides (>60% survival). Our data indicate that these four antimicrobial peptides regulate the innate immunity pathway of nematodes. Brevinin-2 ISb has an especially strong therapeutic effect in 96 h, with a survival rate of nearly 80%. A four-day continuous determination of gene expression was then conducted in order to further study the effect of the peptides on MRSA inhibition and the effect of DAF-2/DAF-16 on the immune system of _C. elegans_. Brevinin-2-OA3, Brevinin-2 ISb, and Brevinin-2 TSa significantly up-regulated both the mRNA and protein levels of the key antimicrobial gene _lys-7_ in the DAF-2/DAF-16 pathway during 24-96 h.  
The damage mechanism of antimicrobial peptides to bacterial cell membranes can be roughly divided into two steps. Firstly, antimicrobial peptides selectively adsorb on the surface of a negatively charged bacterial cell membrane, and then sterilize through perforation or non-perforation mode [47]. Some studies have also shown that some antimicrobial peptides can penetrate the bacterial cell membrane into the cytoplasm, affecting the biochemical process of cells, thus playing a bactericidal role in cells [48]. Through the research, we found that the typical antimicrobial peptides of the Brevinin-2 family are more important as immune effector molecules, e.g., host immune defense system regulating, communication between innate immune defense, and acquired immune defense system. The promotion of immune function will eliminate infection and protect the host from pathogenic microorganisms. Previous studies [49; 50] have shown that antimicrobial peptides can regulate the production of host cytokines and the antigen-specific immune responses of B cell and T cell, thus initiating innate and acquired immunity. When considering DAF-2/DAF-16, the main innate immune pathway of _C. elegans_, we speculated that the activation mechanism of the four antimicrobial peptides in this study on the innate immune system of _C. elegans_ was also induced by regulating the production of host cytokines. Other studies have shown that some antimicrobial peptides can selectively inhibit the physiological activities of cancer cells without affecting normal cells. For example, the lactoferrin peptide can inhibit the growth of breast cancer cells and gastric cancer cells [51]. Polypeptide drugs not only have a simple structure-activity relationship and can be easily modified, but they also have a similar activity to protein drugs with remarkable curative effects. Therefore, they have great advantages in the process of research, development, production, and use for new drugs. Moreover, many antimicrobial peptide drugs have been listed.