also generally reduce the activities of the antimicrobial peptides, due either to stabilization of the lipid bilayer or to interactions between cholesterol and the peptide. so the cholesterol in mammalian cells will protect the cells from attack by the antimicrobial peptides. besides, the transmembrane potential is well known to affect peptide-lipid interactions. there's an inside-negative transmembrane potential existing from the outer leaflet to the inner leaflet of the cell membranes and this inside-negative transmembrane potential will facilitate membrane permeabilization probably by facilitating the insertion of positively charged peptides into membranes. by comparison, the transmembrane potential of bacterial cells is more negative than that of normal mammalian cells, so bacterial membrane will be prone to be attacked by the positively charged antimicrobial peptides. similarly, it is also believed that increasing ionic strength, which in general reduces the activity of most antimicrobial peptides, contributes partially to the selectivity of the antimicrobial peptides by weakening the electrostatic interactions required for the initial interaction. molecular basis of cell selectivity of antimicrobial peptides mechanism the cell membranes of bacteria are rich in acidic phospholipids, such as phosphatidylglycerol and cardiolipin. in contrast, the outer part of the membranes of plants and mammals is mainly composed of lipids without any net charges since most of the lipids with negatively charged headgroups are principally sequestered into the inner leaflet of the plasma membranes. thus in the case of mammalian cells, the outer surfaces of the membranes are usually made of zwitterionic phosphatidylcholine and sphingomyelin, even though a small portion of the membrane's outer surfaces contain some negatively charged gangliosides. therefore, the hydrophobic interaction between the hydrophobic face of amphipathic antimicrobial peptides and the zwitterionic phospholipids on the cell surface of mammalian cell membranes plays a major role in the formation of peptide-cell binding. dual polarisation interferometry has been used in vitro to study and quantify the association to headgroup, insertion into the bilayer, pore formation and eventual disruption of the membrane. control a lot of effort has been put into controlling cell selectivity. for example, attempts have been made to modify and optimize the physicochemical parameters of the peptides to control the selectivities, including net charge, helicity, hydrophobicity per residue (h), hydrophobic moment (μ) and the angle subtended by the positively charged polar helix face (φ). other mechanisms like the introduction of d-amino acids and fluorinated amino acids in the hydrophobic phase are believed to break the secondary structure and thus reduce hydrophobic interaction with mammalian cells. it has also been found that pro→nlys substitution in pro-containing β-turn antimicrobial peptides was a promising strategy for the design of new small bacterial cell-selective antimicrobial peptides with intracellular mechanisms of action. it has been suggested that direct attachment of magainin to the substrate surface decreased nonspecific cell binding and led to improved detection limit for bacterial cells such as salmonella and e. coli. bacterial resistance bacteria use various resistance strategies to avoid antimicrobial peptide killing. some microorganisms alter net surface charges. staphylococcus aureus transports d-alanine from the cytoplasm to the surface teichoic acid which reduces the net negative charge by introducing basic amino groups. s. aureus also modifies its anionic membranes via mprf with l-lysine, increasing the positive net charge. the interaction of antimicrobial peptides with membrane targets can be limited by capsule polysaccharide of klebsiella pneumoniae. salmonella species reduce the fluidity of their outer membrane by increasing hydrophobic interactions between an increased number of lipid a acyl tails by adding myristate to lipid a with 2-hydroxymyristate and forming hepta-acylated lipid a by adding palmitate. the increased hydrophobic moment is thought to retard or abolish antimicrobial peptide insertion and pore formation. the residues undergo alteration in membrane proteins. in some gram-negative bacteria, alteration in the production of outer membrane proteins correlates with resistance to killing by antimicrobial peptides. non-typeable hemophilus influenzae transports amps into the interior of the cell, where they are degraded. furthermore, h. influenzae remodels its membranes to make it appear as if the bacterium has already been successfully attacked by amps, protecting it from being attacked by more amps. atp-binding cassette transporters import antimicrobial peptides and the resistance-nodulation cell-division efflux pump exports antimicrobial peptides. both transporters have been associated with antimicrobial peptide resistance bacteria produce proteolytic enzymes, which may degrade antimicrobial peptides leading to their resistance. outer membrane vesicles produced by gram-negative bacteria bind the antimicrobial peptides and sequester them away from the cells, thereby protecting the cells. the outer membrane vesicles are also known to contain various proteases, peptidases and other lytic enzymes, which may have a role in degrading the extracellular peptide and nucleic acid molecules, which if allowed to reach to the bacterial cells may be dangerous for the cells. cyclic-di-gmp signaling had also been involved in the regulation of antimicrobial peptide resistance in pseudomonas aeruginosa while these examples show that resistance can evolve naturally, there is increasing concern that using pharmaceutical copies of antimicrobial peptides can make resistance happen more often and faster. in some cases, resistance to these peptides used as a pharmaceutical to treat medical problems can lead to resistance, not only to the medical application of the peptides, but to the physiological function of those peptides. the ‘trojan horse’ approach to solving this problem capitalizes on the innate need for iron by pathogens. “smuggling” antimicrobials into the pathogen is accomplished by linking them to siderophores for transport. while simple in concept, it has taken many decades of work to accomplish the difficult hurdle of transporting antimicrobials across the cell membranes of pathogens. lessons learned from the successes and failures of siderophore-conjugate drugs evaluated during the development of novel agents using the ‘trojan horse’ approach have been reviewed. examples fruit flies infected by gfp-producing bacteria. red-eyed flies lacking antimicrobial peptide genes are susceptible to infection, while white-eyed flies have a wild-type immune response. antimicrobial peptides are produced by species across the tree of life, including: bacteria (e.g. bacteriocin, and many others) fungi (e.g. peptaibols, plectasin, and many others) cnidaria (e.g. hydramacin, aurelin) many from insects and arthropods (e.g. cecropin, attacin, melittin, mastoparan, drosomycin) amphibia, frogs (magainin, dermaseptin, aurein, and others) birds (e.g. avian defensins) and mammals (e.g. cathelicidins, alpha- and beta-defensins, regiii peptides) research has increased in recent years to develop artificially-engineered mimics of antimicrobial peptides such as snapps, in part due to the prohibitive cost of producing naturally-derived amps. an example of this is the facially cationic peptide c18g, which was designed from the c-terminal domain of human platelet factor iv. currently, the most widely used antimicrobial peptide is nisin; being the only fda approved antimicrobial peptide, it is commonly used as an artificial preservative. bioinformatics several bioinformatic databases exist to catalogue antimicrobial peptides. the apd (the antimicrobial peptide database) is the original and model database for antimicrobial peptides (https://aps.unmc.edu). based on the apd, other databases have also been built, including adam (a database of anti-microbial peptides), biopd (biologically active peptide database), camp (collection of sequences and structures of antimicrobial peptides), dbaasp (database of antimicrobial activity and structure of peptides), dramp(data repository of antimicrobial peptides)welcome to dramp database, and lamp (linking amps). the antimicrobial peptide databases may be divided into two categories on the basis of the source of peptides it contains, as specific databases and general databases. these databases have various tools for antimicrobial peptides analysis and prediction. for example, the apd has a widely used calculation interface. it also provides links to many other tools. camp contains amp prediction, feature calculator, blast search, clustalw, vast, pratt, helical wheel etc. in addition, adam allows users to search or browse through amp sequence-structure relationships. antimicrobial peptides often encompass a wide range of categories such as antifungal, antibacterial, and antituberculosis peptides. dbamp: provides an online platform for exploring antimicrobial peptides with functional activities and physicochemical properties on transcriptome and proteome data. dbamp is an online resource that addresses various topics such as annotations of antimicrobial peptides (amps) including sequence information, antimicrobial activities, post-translational modifications (ptms), structural visualization, antimicrobial potency, target species with minimum inhibitory concentration (mic), physicochemical properties, or amp–protein interactions. tools such as peptideranker, peptidelocator, and antimpmod allow for the prediction of antimicrobial peptides while others have been developed to predict antifungal and anti-tuberculosis activities.