Antimicrobial properties and membrane-active mechanism of a potential α-helical antimicrobial derived from cathelicidin PMAP-36

Interaction with lipopolysaccharide is key to efficacy of tryptophan- and arginine-rich α-melanocyte-stimulating hormone analogs against Gram-negative bacteria

Aim: In order to search for novel antibacterial therapeutics against Gram-negative bacteria, the antibacterial efficacies and mechanism of action of tryptophan- and arginine-rich α-melanocyte-stimulating hormone analogs were investigated. Materials & methods: We performed a killing assay to determine their efficacy; fluorescence, microscopic studies were used to understand their mechanism and peptide-lipopolysaccharide interaction. A checkerboard assay was used to find the effective combination of peptide and antibiotics. Results: Ana-peptides displayed good killing activity against Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. Their strong interaction with lipopolysaccharide damaged the bacterial membranes and led to their subsequent death. Ana-5, the highest cationic and hydrophobic analog, emerged as the most potent peptide, showing synergistic action with rifampicin and erythromycin. Conclusion: Ana-5 can be presented as an important therapeutic candidate against bacterial infections.

Studies on a new antimicrobial peptide from Vibrio proteolyticus MT110

The marine environment is known for its vast diversity of the microbial population; however, less explored for bioactive compounds. In this study, an AMP produced by a new marine isolate, Vibrio proteolyticus MT110, showed broad-spectrum antimicrobial activity against Gram-positive and Gram-negative bacteria. The AMP was purified to homogeneity using ethyl acetate extraction followed by RP-HPLC, and LC-MS analysis showed its molecular weight as 980 Da. The MIC of AMP (peptide-MT110) was obtained in the 7.81-31.25 µg/mL range against different indicator strains. Peptide-MT110 showed stability of its antimicrobial activity at 15-121 °C and pH 4-10 and in the presence of various hydrolytic enzymes. The peaks at 1536 cm-1 and 1712 cm-1 wavenumbers in FTIR spectra confirmed the peptidic nature of AMP, and its amino acid analysis confirmed the presence of tyrosine and isoleucine. The antibacterial activity of peptide-MT110 is confirmed by PI assay and TEM. The optimization of peptide-MT110 production using statistical methods resulted in a 2.64-fold higher production. The physicochemical properties and stability in wide pH and temperature ranges showed the potential of peptide-MT110 for its development as a drug candidate. This is believed to be the first report on an AMP from Vibrio proteolyticus.

Therapeutic effect of iturin A on Candida albicans oral infection and its pathogenic factors

Candida albicans is responsible for conditions ranging from superficial infections such as oral or vaginal candidiasis to potentially fatal systemic infections. It produces pathogenic factors contributing to its virulence. Iturin A, a lipopeptide derived from Bacillus sp., exhibits a significant inhibitory effect against C. albicans. However, its exact mechanism in mitigating the pathogenic factors of C. albicans remains to be elucidated. This study aimed to explore the influence of iturin A on several pathogenic attributes of C. albicans, including hypha formation, cell membrane permeability, cell adhesion, biofilm formation, and therapeutic efficacy in an oral C. albicans infection model in mice. The minimal inhibitory concentration of iturin A against C. albicans was determined to be 25 µg/mL in both YEPD and RPMI-1640 media. Iturin A effectively inhibited C. albicans hyphal formation, decreased cell viability within biofilms, enhanced cell membrane permeability, and disrupted cell adhesion in vitro. Nonetheless, iturin A did not significantly affect the phospholipase activity or hydrophobicity of C. albicans. A comparative study with nystatin demonstrated the superior therapeutic efficacy of iturin A in a mouse model of oral C. albicans infection, significantly decreasing C. albicans count and inhibiting both fungal hypha formation and tongue surface adhesion. High-dose iturin A treatment (25 µg/mL) in mice had no significant effects on blood indices, tongue condition, or body weight, indicating the potential for iturin A in managing oral infections. This study confirmed the therapeutic potential of iturin A and provided valuable insights for developing effective antifungal therapies targeting C. albicans pathogenic factors.

Investigating antimicrobial peptide RI12 (K3W) as an effective bio-preservative against Listeria monocytogenes: a major foodborne pathogen

Due to public apprehension regarding the use of chemical preservatives to prevent food spoilage and food-borne diseases, it is imperative to identify natural alternatives such as antimicrobial peptides as a potential solution. The study aimed at evaluating the effectiveness of the antimicrobial peptide RI12 (K3W) against Listeria monocytogenes. RI12 (K3W) exhibited potent antimicrobial properties, with a minimum inhibitory concentration and minimum bactericidal concentration of 16 µM and 32 µM, respectively. The time-kill assay revealed a consistent reduction in bacterial viability at 8, 16, and 24 h of study. Cytotoxicity testing on mammalian cells demonstrated no apparent change in morphology or cell count. Investigating how well it worked in a food matrix to replicate real-world conditions showed a significant decrease in the bacterial count. The study underscores the potential of RI12 (K3W) as a safe and effective antimicrobial against L. monocytogenes that might also serve as an alternative to chemical preservatives.

Detection and characterization of potential virulence determinants in Staphylococcus pseudintermedius and S. schleiferi strains isolated from canine otitis externa in Korea

BACKGROUND

A recent increase in the occurrence of canine skin and soft tissue infections, including otitis externa and pyoderma, caused by antimicrobial-resistant Staphylococcus pseudintermedius and S. schleiferi has become a significant public and veterinary health issues.

OBJECTIVE

We investigated the virulence potentials associated with the occurrence of canine otitis externa in S. pseudintermedius and S. schleiferi.

METHODS

In this study, the prevalence of genes encoding leukocidins, exfoliative toxins, and staphylococcal enterotoxins (SEs) was investigated using previously characterized S. pseudintermedius (n = 26) and S. schleiferi (n = 19) isolates derived from canine otitis externa. Susceptibility to cathelicidins (K9CATH and PMAP-36) and hydrogen peroxide (H2O2) was also examined in both staphylococcal species.

RESULTS

A high prevalence of genes encoding leukocidins (lukS/F-I, lukS1/F1-S, and lukS2/F2-S), exfoliative toxins (siet, expB, and sset), and SEs was identified in both S. pseudintermedius and S. schleiferi isolates. Notably, S. pseudintermedius isolates possessed higher number of SE genes, especially newer SE genes, than S. schleiferi isolates harboring egc clusters. Although no significant differences in susceptibility to K9CATH and H2O2 were observed between the two isolate groups, S. pseudintermedius isolates exhibited enhanced resistance to PMAP-36 compared to S. schleiferi isolates.

CONCLUSIONS

These findings suggest that high a prevalence of various toxin genes together with enhanced resistance to cathelicidins may contribute to the pathogenicity of S. pseudintermedius and S. schleiferi in canine cutaneous infections.

Development of a Novel Peptide with Antimicrobial and Mineralising Properties for Caries Management

The purpose of the study is to develop a novel peptide for caries management. Gallic-Acid-Polyphemusin-I (GAPI) was synthesised by grafting Polyphemusin I (PI) and gallic acid (GA). Biocompatibility was evaluated using a Cell Counting Kit-8 Assay. Antimicrobial properties were assessed using minimum inhibitory concentration (MIC) and minimum bactericidal/fungicidal concentration (MBC/MFC). The bacterial and fungal morphology after GAPI treatment was investigated using transmission electron microscopy (TEM). The architecture of a consortium biofilm consisting of Streptococcus mutans, Lacticaseibacillus casei and Candida albicans was evaluated using scanning electron microscopy (SEM) and confocal laser scanning microscopy. The growth kinetics of the biofilm was examined using a propidium monoazide-quantitative polymerase chain reaction. The surface and calcium-to-phosphorus molar ratio of GAPI-treated enamel after pH cycling were examined with SEM and energy-dispersive X-ray spectroscopy. Enamel crystal characteristics were analysed using X-ray diffraction. Lesion depths representing the enamel's mineral loss were assessed using micro-computed tomography. The MIC of GAPI against S. mutans, L. casei and C. albicans were 40 μM, 40 μM and 20 μM, respectively. GAPI destroyed the biofilm's three-dimensional structure and inhibited the growth of the biofilm. SEM showed that enamel treated with GAPI had a relatively smooth surface compared to that treated with water. The calcium-to-phosphorus molar ratio of enamel treated with GAPI was higher than that of the control. The lesion depths and mineral loss of the GAPI-treated enamel were less than the control. The crystallinity of the GAPI-treated enamel was higher than the control. This study developed a biocompatible, mineralising and antimicrobial peptide GAPI, which may have potential as an anti-caries agent.

Structural and biological characterization of shortened derivatives of the cathelicidin PMAP-36

Cathelicidins, a family of host defence peptides in vertebrates, play an important role in the innate immune response, exhibiting antimicrobial activity against many bacteria, as well as viruses and fungi. This work describes the design and synthesis of shortened analogues of porcine cathelicidin PMAP-36, which contain structural changes to improve the pharmacokinetic properties. In particular, 20-mers based on PMAP-36 (residues 12-31) and 13-mers (residues 12-24) with modification of amino acid residues at critical positions and introduction of lipid moieties of different lengths were studied to identify the physical parameters, including hydrophobicity, charge, and helical structure, required to optimise their antibacterial activity. Extensive conformational analysis, performed by CD and NMR, revealed that the substitution of Pro25-Pro26 with Ala25-Lys26 increased the α-helix content of the 20-mer peptides, resulting in broad-spectrum antibacterial activity against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus epidermidis strains. Interestingly, shortening to just 13 residues resulted in only a slight decrease in antibacterial activity. Furthermore, two sequences, a 13-mer and a 20-mer, did not show cytotoxicity against HaCat cells up to 64 µM, indicating that both derivatives are not only effective but also selective antimicrobial peptides. In the short peptide, the introduction of the helicogenic α-aminoisobutyric acid forced the helix toward a prevailing 310 structure, allowing the antimicrobial activity to be maintained. Preliminary tests of resistance to Ser protease chymotrypsin indicated that this modification resulted in a peptide with an increased in vivo lifespan. Thus, some of the PMAP-36 derivatives studied in this work show a good balance between chain length, antibacterial activity, and selectivity, so they represent a good starting point for the development of even more effective and proteolysis-resistant active peptides.

The protective role of chicken cathelicidin-1 against Streptococcus suis serotype 2 in vitro and in vivo

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen with the characteristics of high mortality and morbidity, which brings great challenges to prevent and control epidemic disease in the swine industry. Cathelicidins (CATH) are antimicrobial peptides with antimicrobial and immunomodulatory activities. In this study, bactericidal and anti-inflammatory effects of chicken cathelicidin-1 (CATH-1) were investigated in vitro and in vivo against SS2 infection. The results show that CATH-1 exhibited a better bactericidal effect compared to other species' cathelicidins including chickens (CATH-2, -3, and -B1), mice (CRAMP) and pigs (PMAP-36 and PR-39), which rapidly killed bacteria in 20 min by a time-killing curve assay. Furthermore, CATH-1 destroyed the bacterial morphology and affected bacterial ultrastructure as observed under electron microscopy. Moreover, CATH-1 antibacterial activity in vivo shows that CATH-1 increased survival rate of SS2-infected mice by 60% and significantly reduced the bacterial load in the lungs, liver, spleen, blood, and peritoneal lavage as well as the release of SS2-induced inflammatory cytokines including IL-1α, IL-1β, IL-12, and IL-18. Importantly, CATH-1 did not show severe histopathological changes in mice. Further studies on the mechanism of anti-inflammatory activity show that CATH-1 not only reduced the inflammatory response through direct neutralization, but also by regulating the TLR2/4/NF-κB/ERK pathway. This study provides a scientific basis for the research and development of antimicrobial peptides as new antimicrobial agents.

Real-Time Monitoring of Multitarget Antimicrobial Mechanisms of Peptoids Using Label-Free Imaging with Optical Diffraction Tomography

Antimicrobial peptides (AMPs) are promising therapeutics in the fight against multidrug-resistant bacteria. As a mimic of AMPs, peptoids with N-substituted glycine backbone have been utilized for antimicrobials with resistance against proteolytic degradation. Antimicrobial peptoids are known to kill bacteria by membrane disruption; however, the nonspecific aggregation of intracellular contents is also suggested as an important bactericidal mechanism. Here,structure-activity relationship (SAR) of a library of indole side chain-containing peptoids resulting in peptoid 29 as a hit compound is investigated. Then, quantitative morphological analyses of live bacteria treated with AMPs and peptoid 29 in a label-free manner using optical diffraction tomography (ODT) are performed. It is unambiguously demonstrated that both membrane disruption and intracellular biomass flocculation are primary mechanisms of bacterial killing by monitoring real-time morphological changes of bacteria. These multitarget mechanisms and rapid action can be a merit for the discovery of a resistance-breaking novel antibiotic drug.

Hylin-a1: A Host Defense Peptide with Antibacterial Potential against Staphylococcus aureus Multi-Resistant Strains

In recent years, the resistance of pathogenic microorganisms to common antimicrobial agents has raised to a severe public health problem. The moderate and wise use of antimicrobials and the prevention of infections are the most effective strategies for decreasing the spread and development of resistance. Therefore, the World Health Organization (WHO) has intensified the search for new drugs to fight emerging pathogens. Antimicrobial peptides (AMPs), also known as host defense peptides (HDPs), play a crucial role in innate immunity, representing one of the first line of defense against microbial attacks. In this study, we evaluated the antibacterial activity of the AMP named Hylin-a1 (derived from the skin of the frog Heleioporus albopunctatus) against Staphylococcus aureus strains. S. aureus represents a commensal bacterium but also the principal causative agent of several human infections, including bacteremia, endocarditis, skin and device-related infections. Hylin-a1 toxicity was evaluated on human keratinocytes; once the non-cytotoxic concentration range was determined, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were analyzed, and time-killing assays were performed to verify the bacteriostatic and/or bactericidal activity of the peptide. We found that Hylin-a1 exerted a bacteriostatic action against most of the tested strains, with 90% inhibition at the concentration of 6.25 μM. Noteworthy, the peptide at a very low concentration (~3 μM) significantly blocked the growth of β-lactam- and methicillin-resistant S. aureus. The levels of interleukin (IL)-1β, IL-6 and IL-8 were quantified through a molecular assay, indicating that the peptide was able also to regulate the inflammatory response following bacterial infection. The effect of Hylin-a1 on S. aureus cell morphology was also evaluated. Altogether, these results indicate the high therapeutic potential of Hylin-a1 against a wide variety of clinical manifestations caused by S. aureus.

Membrane-Active Cyclic Amphiphilic Peptides: Broad-Spectrum Antibacterial Activity Alone and in Combination with Antibiotics

We designed a library of 24 cyclic peptides containing arginine (R) and tryptophan (W) residues in a sequential manner [R_nW_n] (n = 2-7) to study the impact of the hydrophilic/hydrophobic ratio, charge, and ring size on the antibacterial activity against Gram-positive and Gram-negative strains. Among peptides, 5a and 6a demonstrated the highest antimicrobial activity. In combination with 11 commercially available antibiotics, 5a and 6a showed remarkable synergism against a large panel of resistant pathogens. Hemolysis (HC₅₀ = 340 μg/mL) and cell viability against mammalian cells demonstrated the selective lethal action of 5a against bacteria over mammalian cells. Calcein dye leakage and scanning electron microscopy studies revealed the membranolytic effect of 5a. Moreover, the stability in human plasma (t_1/2 = 3 h) and the negligible ability of pathogens to develop resistance further reflect the potential of 5a for further development as a peptide-based antibiotic.

Distinct mode of membrane interaction and disintegration by diverse class of antimicrobial peptides

The exploitation of conventional antibiotics in conjunction with the adeptness of microbes has led to the emergence of multi-drug-resistant pathogens. This has posed a severe threat to combating life-threatening infectious diseases. Antimicrobial peptides (AMP), which are considered to be the first line of defense in all living organisms, are being developed for therapeutic use. Herein, we determined the NMR solution structure of Rhesus macaque Myeloid Alpha Defensin-4 (RMAD4), a defensin AMP. Additionally, the distinct modes of membrane perturbation for two structurally dissimilar classes of AMPs was studied using biophysical methods namely, Solid-state ³¹P NMR, DSC and cryo-TEM. The cathelicidin - Bovine myeloid antimicrobial peptide (BMAP-28 (1-18)), which adopts a helical conformation, and the defensin RMAD4 peptide that natively folds to form β-sheets appeared to engage differently with the bacterial membrane. The helical BMAP-28 (1-18) peptide initiates lipid segregation and membrane thinning followed by pore formation, while the β-stranded RMAD4 peptide demonstrates fragmentation of the bilayer by the carpet or detergent-like mechanism of action. Molecular dynamics studies sufficiently corroborated these findings. The structure and mechanism of action of the AMPs studied using experimental and computational approaches are believed to help in providing a platform for the rational design of new competent and cost-effective antimicrobial peptides for therapeutic applications.

Matching amino acids membrane preference profile to improve activity of antimicrobial peptides

Antimicrobial peptides (AMPs) are cationic antibiotics that can kill multidrug-resistant bacteria via membrane insertion. However, their weak activity limits their clinical use. Ironically, the cationic charge of AMPs is essential for membrane binding, but it obstructs membrane insertion. In this study, we postulate that this problem can be overcome by locating cationic amino acids at the energetically preferred membrane surface. All amino acids have an energetically preferred or less preferred membrane position profile, and this profile is strongly related to membrane insertion. However, most AMPs do not follow this profile. One exception is protegrin-1, a powerful but neglected AMP. In the present study, we found that a potent AMP, WCopW5, strongly resembles protegrin-1 and that the match between its sequence and the preferred position profile closely correlates with its antimicrobial activity. One of its derivatives, WCopW43, has antimicrobial activity comparable to that of the most effective AMPs in clinical use.

Novel D-form of hybrid peptide (D-AP19) rapidly kills Acinetobacter baumannii while tolerating proteolytic enzymes

Antimicrobial peptides (AMPs) are being developed as potent alternative treatments to conventional antibiotics which are unlikely to induce bacterial resistance. They can be designed and modified to possess several druggable properties. We report herein a novel hybrid peptide of modified aurein (A3) and cathelicidin (P7), or A3P7, by a flipping technique. It exhibited potent antibacterial activity against both Gram-negative and -positive pathogenic bacteria but had moderate hemolytic activity. To reduce the sequence length and toxicity, C-terminal truncation was serially performed and eight truncated derivatives (AP12-AP19) were obtained. They had significantly less hemolytic activity while preserving antibacterial activity. Secondary structures of the candidate peptides in environments simulating bacterial membranes (30 mM SDS and 50% TFE), determined by CD spectroscopy, showed α-helical structures consistent with predicted in silico 3D structural models. Among the peptides, AP19 demonstrated the best combination of broad-spectrum antibacterial activity (including toward Acinetobacter baumannii) and minimal hemolytic and cytotoxic activities. A D-form peptide (D-AP19), in which all L-enantiomers were substituted with the D-enantiomers, maintained antibacterial activity in the presence of pepsin, trypsin, proteinase K and human plasma. Both isomers exhibited potent antibacterial activity against multi-drug (MDR) and extensively-drug resistant (XDR) clinical isolates of A. baumannii comparable to the traditional antibiotic, meropenem. D-AP19 displayed rapid killing via membrane disruption and leakage of intracellular contents. Additionally, it showed a low tendency to induce bacterial resistance. Our work suggested that D-AP19 could be further optimized and developed as a novel compound potentially for fighting against MDR or XDR A. baumannii.

The design of cell-selective tryptophan and arginine-rich antimicrobial peptides by introducing hydrophilic uncharged residues

Antimicrobial peptides (AMPs) are considered to be powerful weapons in the fight against traditional antibiotic resistance due to their unique membrane-disruptive mechanism. The combination of traditional and classical hydrophobic tryptophan (W) residues and hydrophilic charged arginine (R) residues is considered as the first choice for the minimalist design of AMPs due to its potent performance in antibacterial activity. However, some W- and R-rich AMPs that are not rationally designed and contain excessive repeats of W and R residues may cause severe cytotoxicity and hemolysis. To address this issue, we designed the (WRX)_n (where X = hydrophilic uncharged amino residues; n = number of repeat units) series engineered peptides with high cell selectivity by introducing hydrophilic uncharged threonine (T), serine (S), glutamine (Q) or asparagine (N) residues into the minimalist design of W- and R-rich AMPs. The results showed that the introduction of these hydrophilic uncharged amino residues, especially T residues, significantly improved the cell selectivity of the W- and R-rich engineered peptides. Among (WRX)_n series engineered peptides, T6 presents a mixture structure of β-turn and α-helix. It has broad spectrum and potent antibacterial activity (no activity against probiotics), good biocompatibility, high selectivity index, strong tolerance (physiological salts, serum acid, alkali, and heat conditions), rapid and efficient time-kill kinetics, and no tendency of resistance. Studies on antibacterial mechanism show that T6 exert antibacterial activity mainly by disrupting bacterial cell membrane and inducing the accumulation of reactive oxygen species in bacterial cells. Furthermore, T6 exhibited potent antibacterial and anti-inflammatory capabilities in vivo in a mouse peritonitis-sepsis model infected with Escherichia coli. In conclusion, our study confirms an effective strategy for the minimalist design of highly cell selective W- and R-rich AMPs by introducing hydrophilic uncharged T residues, which may trigger widespread attention to hydrophilic uncharged amino acid residues, including T residues, and provide new insights into the design of peptide-based antibacterial biomaterials. STATEMENT OF SIGNIFICANCE: We have introduced hydrophilic uncharged T, S, Q or N residues into the minimalist design of W- and R-rich engineered peptides and found that the introduction of these hydrophilic uncharged amino residues, especially the T residues, can significantly improve the cell selectivity of W- and R-rich engineered peptides. The target compound T6 showed potent antibacterial activity, high cell selectivity, strong tolerance, good in vivo efficacy and killed bacteria through multiple mechanisms mainly membrane-disruptive. These findings may spark widespread interest in hydrophilic uncharged amino acid residues, and provide new insights into the design of peptide-based antimicrobial biomaterials.

Functional Characterization, Antimicrobial Effects, and Potential Antibacterial Mechanisms of NpHM4, a Derived Peptide of Nautilus pompilius Hemocyanin

Hemocyanins present in the hemolymph of invertebrates are multifunctional proteins that are responsible for oxygen transport and play crucial roles in the immune system. They have also been identified as a source of antimicrobial peptides during infection in mollusks. Hemocyanin has also been identified in the cephalopod ancestor Nautilus, but antimicrobial peptides derived from the hemocyanin of Nautilus pompilius have not been reported. Here, the bactericidal activity of six predicted peptides from N. pompilius hemocyanin and seven mutant peptides was analyzed. Among those peptides, a mutant peptide with 15 amino acids (1RVFAGFLRHGIKRSR15), NpHM4, showed relatively high antibacterial activity. NpHM4 was determined to have typical antimicrobial peptide characteristics, including a positive charge (+5.25) and a high hydrophobic residue ratio (40%), and it was predicted to form an alpha-helical structure. In addition, NpHM4 exhibited significant antibacterial activity against Gram-negative bacteria (MBC = 30 μM for Vibrio alginolyticus), with no cytotoxicity to mammalian cells even at a high concentration of 180 µM. Upon contact with V. alginolyticus cells, we confirmed that the bactericidal activity of NpHM4 was coupled with membrane permeabilization, which was further confirmed via ultrastructural images using a scanning electron microscope. Therefore, our study provides a rationalization for the development and optimization of antimicrobial peptide from the cephalopod ancestor Nautilus, paving the way for future novel AMP development with broad applications.

Synergistic Antimicrobial Effect of Antimicrobial Peptides CATH-1, CATH-3, and PMAP-36 With Erythromycin Against Bacterial Pathogens

With the increasing bacterial resistance to traditional antibiotics, there is an urgent need for the development of alternative drugs or adjuvants of antibiotics to enhance antibacterial efficiency. The combination of antimicrobial peptides (AMPs) and traditional antibiotics is a potential alternative to enhance antibacterial efficiency. In this study, we investigated the synergistic bactericidal effect of AMPs, including chicken (CATH-1,−2,−3, and -B1), mice (CRAMP), and porcine (PMAP-36 and PR-39) in combination with conventional antibiotics containing ampicillin, tetracycline, gentamicin, and erythromycin against Staphylococcus aureus, Salmonella enteritidis, and Escherichia coli. The results showed that the minimum bactericidal concentration (MBC) of CATH-1,−3 and PMAP-36 was lower than 10 μM, indicating that these three AMPs had good bacterial activity against S. aureus, S. enteritidis, and E. coli. Then, the synergistic antibacterial activity of AMPs and antibiotics combination was determined by the fractional bactericidal concentration index (FBCI). The results showed that the FBCI of AMPs (CATH-1,−3 and PMAP-36) and erythromycin was lower than 0.5 against bacterial pathogens, demonstrating that they had a synergistic bactericidal effect. Furthermore, the time-killing kinetics of AMPs (CATH-1,−3 and PMAP-36) in combination with erythromycin showed that they had a continuous killing effect on bacteria within 3 h. Notably, the combination showed lower hemolytic activity and cytotoxicity to mammal cells compared to erythromycin and peptide alone treatment. In addition, the antibacterial mechanism of CATH-1 and erythromycin combination against E. coli was studied. The results of the scanning electron microscope showed that CATH-1 enhanced the antibacterial activity of erythromycin by increasing the permeability of bacterial cell membrane. Moreover, the results of bacterial migration movement showed that the combination of CATH-1 and erythromycin significantly inhibits the migration of E. coli. Finally, drug resistance analysis was performed and the results showed that CATH-1 delayed the emergence of E. coli resistance to erythromycin. In conclusion, the combination of CATH-1 and erythromycin has synergistic antibacterial activity and reduces the emergence of bacterial drug resistance. Our study provides valuable information to develop AMPs as potential substitutes or adjuvants for traditional antibiotics.

Truncated analog Brevinin2-CE-N26V5K: Revelation the Augmentation of Antimicrobial Activity

Brevinin2-CE (B2CE), a natural peptide containing 37 amino acids, was first isolated from the skin secretions of the Chinese forest frog Rana chensinensis. B2CE shows good antibacterial activity. In this study, a series of B2CE analogs with differences in cationicity, α-helicity, hydrophobicity and amphipathic properties were designed through chain-length deletion and amino acid substitution. The most potent, nontoxic analog, B2CE-N26V5K, was identified by examination of its antibacterial activity, hemolytic activity, and stability under physiological conditions. The increased cationicity, hydrophobicity and more obvious hydrophilic and hydrophobic surface of B2CE-N26-N16WA18KG23K did not improve the antibacterial activity but increased the hemolytic activity of this modified peptide. The helicity might promote antibacterial activity for brevinin-2 peptides, as the 15-aa analogs with lower helicity show decreased potency against different test bacteria (approximately 2- to 72-fold) compared to B2CE-N26V5K. Additionally, the results indicated that the "Rana box" does not affect the antimicrobial activity of brevinin-2 peptides, as B2CE, B2CE-nonDS and B2CE-C31-37 S have similar strong inhibitory effects on both gram-positive and gram-negative bacteria. However, the "Rana box" does affect the hemolytic activity, as the HC₅₀ values of the 3 peptides range from 25 ~ 130 µM. Furthermore, B2CE-N26V5K caused obvious morphological alterations of the bacterial surfaces, as shown by atomic force microscopy. Additionally, B2CE-N26V5K exhibited strong membrane-disrupting activity when examined using the LIVE/DEAD Bac Light Bacterial Viability Kit. Thus, the antibacterial effect of B2CE-N26V5K on gram-negative and gram-positive bacteria may be caused by cell membrane attack. In conclusion, the excellent candidate B2CE-N26V5K was obtained and has application prospects as a novel anti-infective agent.

Deficiency of GntR Family Regulator MSMEG_5174 Promotes Mycobacterium smegmatis Resistance to Aminoglycosides via Manipulating Purine Metabolism

The increasing incidence of drug-resistant tuberculosis is still an emergency for global public health and a major obstacle to tuberculosis treatment. Therefore, deciphering the novel mechanisms of mycobacterial antibiotic resistance is crucial for combatting the rapid emergence of drug-resistant strains. In this study, we identified an unexpected role of Mycobacterium smegmatis GntR family transcriptional regulator MSMEG_5174 and its homologous gene Mycobacterium tuberculosis Rv1152 in aminoglycoside antibiotic resistance. Deficiency of MSMEG_5174 rendered Mycobacterium smegmatis highly resistant to aminoglycoside antibiotic treatment, and ectopic expression of Rv1152 in MSMEG_5174 mutants restored antibiotic-induced bacterial killing. We further demonstrated that MSMEG_5174 negatively regulates the expression of purine metabolism-related genes and the accumulation of purine metabolites. Moreover, overexpression of xanthine dehydrogenase MSMEG_0871 or xanthine treatment elicited a significant decrease in aminoglycoside antibiotic lethality for Mycobacterium smegmatis. Together, our findings revealed MSMEG_5174 as a metabolic regulator and hint toward unexplored crosstalk between purine metabolism and antibiotic resistance.

Optimization of Antibacterial Activity in Tibetan Swine α-Helix Peptide TP by Site-Directed Mutagenesis

Antimicrobial peptides (AMPs) have attracted extensive attention because of their broad-spectrum antibacterial activity and low level of induced bacterial resistance. However, the development of some natural AMPs does not consider the perfect balance of structural characteristics, resulting in some empirical and controversial practices still existing. To further explore and complete the relationship between parameters and function of α-helix peptide, in this study, the natural antimicrobial peptide TP secreted from Bacillus strain of Tibetan pigs was selected as a template to investigate the effect of systematic mutations in the hydrogen bond formation site of the α-helical antimicrobial peptide on the activity and cell selectivity of the antimicrobial peptide. The target peptide TP(i+4) 1&2&5 with modification of two pairs of positively charged amino acids and a pair of hydrophobic amino acids showed excellent antibacterial ability and the best selectivity index (SI = 64) in vitro. At the same time, TP(i+4) 1&2&5 remained active in the presence of physiological salts and serum. The results of fluorescence, flow cytometry, and electron microscopy showed that the optimized sequences showed good antibacterial activity by membrane infiltration and membrane destruction. The potential of TP(i+4) 1&2&5 in vivo was tested in a mouse peritonitis model. Organ bacterial loads in the liver, kidney, spleen, and lungs of mice treated with TP(i+4) 1&2&5 were significantly lower compared to the infected group (p < 0.05). Overall, these findings contribute to the design and optimization of antimicrobial peptides with high activity and low toxicity and may accelerate the clinical application of antimicrobial peptides.

Potential Antimicrobial Properties of Coffee Beans and Coffee By-Products Against Drug-Resistant Vibrio cholerae

Vibrio cholerae is the causative organism of the cholera epidemic, and it remains a serious global health problem, particularly the multidrug-resistant strain, despite the development of several generic drugs and vaccines over time. Natural products have long been exploited for the treatment of various diseases, and this study aimed to evaluate the in vitro antibacterial activity of coffee beans and coffee by-products against V. cholerae antimicrobial resistant strains. A total of 9 aqueous extracts were investigated, including light coffee (LC), medium coffee (MC), dark coffee (DC), dried green coffee (DGC), dried red coffee (DRC), fresh red coffee (FRC), Arabica leaf (AL), Robusta leaf (RL), and coffee pulp (CP). The influential coffee phytochemicals, i.e., chlorogenic acid (CGA), caffeic acid (CA), and caffeine, were determined using HPLC. The antibacterial properties were tested by agar well-diffusion techniques, and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were further determined against 20 V. cholerae isolates. The results revealed that all tested strains were sensitive to coffee extracts, with MIC and MBC values in the range of 3.125-25.0 mg/mL and 12.5-50.0 mg/mL, respectively. With a MIC of 6.25 mg/mL, DGC, DRC, and CP appeared to be the most effective compounds against 65, 60, and 55% of clinical strains, respectively. The checkerboard assay revealed that the combination of coffee extract and tetracycline was greater than either treatment alone, with the fractional inhibitory concentration index (FICI) ranging from 0.005 to 0.258. It is important to note that CP had the lowest FICI (0.005) when combined with tetracycline at 60 ng/mL, which is the most effective dose against V. cholerae six-drug resistance strains (azithromycin, colistin, nalidixic acid, sulfamethoxazole, tetracycline, and trimethoprim), with a MIC of 47.5 μg/mL (MIC alone = 12.5 mg/mL). Time killing kinetics analysis suggested that CA might be the most effective treatment for drug-resistant V. cholerae as it reduced bacterial growth by 3 log10 CFU/mL at a concentration of 8 mg/mL within 1 h, via disrupting membrane permeability, as confirmed by scanning electron microscopy (SEM). This is the first report showing that coffee beans and coffee by-product extracts are an alternative for multidrug-resistant V. cholerae treatment.

A Cecropin-4 Derived Peptide C18 Inhibits Candida albicans by Disturbing Mitochondrial Function

Global burden of fungal infections and related health risk has accelerated at an incredible pace, and multidrug resistance emergency aggravates the need for the development of new effective strategies. Candida albicans is clinically the most ubiquitous pathogenic fungus that leads to high incidence and mortality in immunocompromised patients. Antimicrobial peptides (AMPs), in this context, represent promising alternatives having potential to be exploited for improving human health. In our previous studies, a Cecropin-4-derived peptide named C18 was found to possess a broader antibacterial spectrum after modification and exhibit significant antifungal activity against C. albicans. In this study, C18 shows antifungal activity against C. albicans or non-albicans Candida species with a minimum inhibitory concentration (MIC) at 4∼32 μg/ml, and clinical isolates of fluconazole (FLZ)-resistance C. tropicalis were highly susceptible to C18 with MIC value of 8 or 16 μg/ml. Additionally, C18 is superior to FLZ for killing planktonic C. albicans from inhibitory and killing kinetic curves. Moreover, C18 could attenuate the virulence of C. albicans, which includes damaging the cell structure, retarding hyphae transition, and inhibiting biofilm formation. Intriguingly, in the Galleria mellonella model with C. albicans infection, C18 could improve the survival rate of G. mellonella larvae to 70% and reduce C. albicans load from 5.01 × 107 to 5.62 × 104 CFU. For mechanistic action of C18, the level of reactive oxygen species (ROS) generation and cytosolic Ca2 + increased in the presence of C18, which is closely associated with mitochondrial dysfunction. Meanwhile, mitochondrial membrane potential (△Ψm) loss and ATP depletion of C. albicans occurred with the treatment of C18. We hypothesized that C18 might inhibit C. albicans via triggering mitochondrial dysfunction driven by ROS generation and Ca2 + accumulation. Our observation provides a basis for future research to explore the antifungal strategies and presents C18 as an attractive therapeutic candidate to be developed to treat candidiasis.

Antimicrobial Activity of the Green Tea Polyphenol (−)-Epigallocatechin-3-Gallate (EGCG) against Clinical Isolates of Multidrug-Resistant Vibrio cholerae

The spread of multidrug-resistant (MDR) Vibrio cholerae necessitates the development of novel prevention and treatment strategies. This study aims to evaluate the in vitro antibacterial activity of green tea polyphenol (−)-epigallocatechin-3-gallate (EGCG) against MDR V. cholerae. First, MIC and MBC values were evaluated by broth microdilution techniques against 45 V. cholerae strains. The checkerboard assay was then used to determine the synergistic effect of EGCG and tetracycline. The pharmaceutical mode of action of EGCG was clarified by time-killing kinetics and membrane disruption assay. Our results revealed that all of the 45 clinical isolates were susceptible to EGCG, with MIC and MBC values in the range of 62.5–250 µg/mL and 125–500 µg/mL, respectively. Furthermore, the combination of EGCG and tetracycline was greater than either treatment alone, with a fractional inhibitory concentration index (FICI) of 0.009 and 0.018 in the O1 and O139 representative serotypes, respectively. Time-killing kinetics analysis suggested that EGCG had bactericidal activity for MDR V. cholerae after exposure to at least 62.5 µg/mL EGCG within 1 h. The mode of action of EGCG might be associated with membrane disrupting permeability, as confirmed by scanning electron microscopy. This is the first indication that EGCG is a viable anti-MDR V. cholerae treatment.

pH-Responsive Antimicrobial Peptide with Selective Killing Activity for Bacterial Abscess Therapy

The unusual acidic pH of the abscess milieu is an adverse factor that decreases the therapeutic efficacy of traditional antibiotics. Moreover, avoiding both the undesired killing of commensal bacteria and the development of drug resistance remains difficult during abscess therapy. Hence, we synthesized a series of pH-responsive antimicrobial peptides equipped with efficient bacterial killing activity at pH 6.5 and inactivity at pH 7.4. Among the peptides, F5 exhibited outstanding pH-responsive antimicrobial activity and low toxicity. Fluorescence spectroscopy and electron microscopy illustrated that F5 killed bacteria via a membrane-disruptive mechanism at acidic pH values. Mouse cutaneous abscesses revealed that F5 was equipped with excellent therapeutic ability to reduce the bacterial load and cytokines without causing skin toxicity. In summary, this study reveals a strategy for selectively killing bacteria under the pathologic conditions of abscess sites while avoiding the elimination of commensal bacteria under normal physiological pH levels.

Binding of cationic analogues of α-MSH to lipopolysaccharide and disruption of the cytoplasmic membranes caused bactericidal action against Escherichia coli

In earlier reports, we have shown the antimicrobial activity of a host neuropeptide, alpha-melanocyte stimulating hormone (α-MSH) and its cationic analogues against Staphylococcus aureus. These analogues of α-MSH showed enhanced staphylocidal activity without any significant mammalian cell toxicity. Therefore, here, we explored the antimicrobial activity of α-MSH and its cationic analogues against Escherichia coli. Though the presence of lipopolysaccharide (LPS) in Gram-negative bacteria enables them to resist most conventional antibiotics, encouragingly α-MSH and its four analogues showed killing of both logarithmic and stationary phase E. coli cells in a time, dose and cationicity-dependent manner. In fact, the most cationic analogue, KKK-MSH with a + 5 charge, demonstrated successful eradication of 10⁵ CFU/mL of E. coli cells within 15 min at a concentration as low as 1 µM. BC displacement experiment revealed that cationicity of the peptides was directly related to the killing efficacy of these α-MSH analogues against E. coli cells via initial LPS-binding, leading to rapid disruption of the LPS-outer membrane complex followed by inner bacterial membrane damage and eventual cell death. Here, we propose α-MSH based cationic peptides as promising future agents with broad-spectrum antibacterial efficacy against both Gram-negative and Gram-positive pathogens.

Antifungal activity and mode of action of synthetic peptides derived from the tick OsDef2 defensin

Candida albicans is the principal opportunistic fungal pathogen in nosocomial settings and resistance to antifungal drugs is on the rise. Antimicrobial peptides from natural sources are promising novel therapeutics against C. albicans. OsDef2 defensin was previously found to be active against only Gram-positive bacteria, whereas derived fragments Os and its cysteine-free analogue, Os-C, are active against Gram-positive and Gram-negative bacteria at low micromolar concentrations. In this study, OsDef2-derived analogues and fragments were screened for anticandidal activity with the aim to identify peptides with antifungal activity and in so doing obtain a better understanding of the structural requirements for activity and modes of action. Os, Os-C and Os(11-22)NH₂ , a Os-truncated carboxy-terminal-amidated fragment, had the most significant antifungal activities, with minimum fungicidal concentrations (MFCs) in the micromolar range (6-28 μM). C. albicans killing was rapid and occurred within 30-60 min. Further investigations showed all three peptides interacted with cell wall derived polysaccharides while both Os and Os(11-22)NH₂ permeabilized fungal liposomes. Confocal laser scanning microscopy confirmed that Os-C and Os(11-22)NH₂ could enter the cytosol of live cells and subsequent findings suggest that the uptake of Os and Os-C, in contrast to Os(11-22)NH₂ , is energy dependent. Although Os, Os-C and Os(11-22)NH₂ induced the production of reactive oxygen species (ROS), co-incubation with ascorbic acid revealed that only ROS generated by Os-C and to a lesser extent Os(11-22)NH₂ resulted in cell death. Overall, Os, Os-C and Os(11-22)NH₂ are promising candidacidal agents.

Bioevaluation and Targeted Modification of Temporin-FL From the Skin Secretion of Dark-Spotted Frog (Pelophylax nigromaculatus)

Bioactive proteins secreted by the granular glands of amphibian skin play a self-defensive role, and exhibit various bioactivities in vitro and in vivo. In light of the severity of the problem of antibiotic resistance for treating infections, many antimicrobial peptides (AMPs) have been developed and applied in clinical microbial treatments. We identified a naturally derived and potent antimicrobial peptide, temporin-FL, obtained from the skin secretion of Pelophylax nigromaculatus via “shotgun” cloning. Two truncated analogues of this peptide were chemically synthesized to explore their structural-functional relationships. The results of a functional evaluation showed that all of the tested AMPs were active against Gram-positive bacteria and fungi and demonstrated antibiofilm activity against methicillin-resistant Staphylococcus aureus (MRSA) but did not have an effect on Gram-negative bacteria. Moreover, temporin-FLa demonstrated a higher level of hydrophobicity and enhanced antimicrobial efficiency, as well as hemolytic activity and cell cytotoxicity than the parent peptide. Temporin-FLb, which evidenced significantly less α-helicity, was less potent against various microbes but exhibited lower cytotoxicity relating to mammalian cells. Both of the synthesized analogues possessed a higher therapeutic index than the original peptide. Moreover, the membrane permeability assay and the measuring membrane depolarization assay declared that temporin-FL and its analogues induced membrane fracture and depolarization; the quantitative biofilm formation assay and the observations of MRSA biofilms using scanning electron microscopy revealed that the AMPs caused biofilm disruption and blocked biofilm formation, the former experiments all confirming their antimicrobial and antibiofilm properties. Hence, the optimization of temporin-FL offers insights for the discovery of new drugs for treating MRSA infections.

Improving the Activity of Antimicrobial Peptides Against Aquatic Pathogen Bacteria by Amino Acid Substitutions and Changing the Ratio of Hydrophobic Residues

With the increasing number of drug-resistant bacteria, there is an urgent need for new antimicrobial agents, and antimicrobial peptides (AMPs), which exist in the human non-specific immune system, are one of the most promising candidates. It is an effective optimization strategy to modify antimicrobial peptides (AMPs) according to the distribution of amino acids and hydrophobic characteristics. The addition of bacterial pheromones to the N short peptide can increase the ability to recognize bacteria. In this study, we designed and synthesized AMP1–6 by amino acid substitution of mBjAMP1. Additionally, P-6, S-6, and L-6 were designed and synthesized by adding bacterial pheromones based on 1–6. Functional tests showed that the four AMPs had the ability to kill Gram-negative Vibrio anguillarum, Pseudomonas mendocina, and Vibrio parahaemolyticus, and Gram-positive Micrococcus luteus and Listeria monocytogenes. Additionally, all four AMPs induced permeabilization and depolarization of bacterial cell membranes and increased intracellular reactive oxygen species (ROS) levels. Importantly, they had little or no mammalian cytotoxicity. At the same time, 1–6 and L-6 protected the stability of intestinal flora in Sebastes schlegelii and increased the relative abundance of Lactobacillaceae. In summary, our results indicate that the designed AMPs have broad application prospects as a new type of polypeptide antimicrobial agent.

A Seed-Endophytic Bacillus safensis Strain With Antimicrobial Activity Has Genes for Novel Bacteriocin-Like Antimicrobial Peptides

Bacteriocins are a highly diverse group of antimicrobial peptides that have been identified in a wide range of commensal and probiotic organisms, especially those resident in host microbiomes. Rising antibiotic resistance have fueled renewed research into new drug scaffolds such as antimicrobial peptides for use in therapeutics. In this investigation, we examined mung bean seeds for endophytes possessing activity against human and plant pathogens. We isolated a novel strain of Bacillus safensis, from the contents of surface-sterilized mung bean seed, which we termed B. safensis C3. Genome sequencing of C3 identified three distinct biosynthetic systems that produce bacteriocin-based peptides. C3 exhibited antibacterial activity against Escherichia coli, Xanthomonas axonopodis, and Pseudomonas syringae. Robust antimicrobial activity of B. safensis C3 was observed when C3 was co-cultured with Bacillus subtilis. Using the cell-free supernatant of C3 and cation exchange chromatography, we enriched a product that retained antimicrobial activity against B. subtilis. The peptide was found to be approximately 3.3 kDa in size by mass spectrometry, and resistant to proteolysis by Carboxypeptidase Y and Endoproteinase GluC, suggesting that it is a modified variant of an AS-48 like bacteriocin. Our findings open new avenues into further development of novel bacteriocin-based scaffolds for therapeutic development, as well as further investigations into how our discoveries of bacteriocin-producing plant commensal microorganisms may have the potential for an immediate impact on the safety of food supplies.

Surfactin inhibits the growth of Propionibacterium acnes by destroying the cell wall and membrane

Propionibacterium acnes plays a major role in acne vulgaris. In the pre-experiment, the growth of P. acnes was inhibited effectively using surfactin; however, the antibacterial mechanism has not been described. Therefore, the aim of this study was to evaluate antibacterial activity and analyse the mechanism of surfactin against P. acnes. Minimum inhibitory concentration, time-killing kinetics and scanning electron microscopy were used to evaluate the activity of surfactin against P. acnes, which showed that 128 μg ml^-1 effectively inhibited growth. Cell wall permeability was evaluated by detecting the extracellular alkaline phosphatase activity, which increased to 1·83- and 2·32-fold after incubating with 128 and 256 μg ml^-1 of surfactin for 10 h, respectively. Propidium iodide fluorescence, leakage of nucleic acid, protein, K⁺ , and Ca²⁺ , membrane potential and the leakage of calcein from small unilamellar vesicles all increased after incubation with surfactin, indicating that its strong biological activities act mainly by altering membrane integrity. In a mouse model of acne, surfactin significantly reduced P. acnes-induced epidermal swelling and erythema. These results indicate that surfactin effectively inhibited the growth of P. acnes by destroying the cell wall and membrane, and is a potential candidate for acne treatment.

Outer Membrane Vesicles Protect Gram-Negative Bacteria against Host Defense Peptides

Host defense peptides (HDPs) are part of the innate immune system and constitute a first line of defense against invading pathogens. They possess antimicrobial activity against a broad spectrum of pathogens. However, pathogens have been known to adapt to hostile environments. Therefore, the bacterial response to treatment with HDPs was investigated. Previous observations suggested that sublethal concentrations of HDPs increase the release of outer membrane vesicles (OMVs) in Escherichia coli. First, the effects of sublethal treatment with HDPs CATH-2, PMAP-36, and LL-37 on OMV release of several Gram-negative bacteria were analyzed. Treatment with PMAP-36 and CATH-2 induced release of OMVs, but treatment with LL-37 did not. The OMVs were further characterized with respect to morphological properties. The HDP-induced OMVs often had disc-like shapes. The beneficial effect of bacterial OMV release was studied by determining the susceptibility of E. coli toward HDPs in the presence of OMVs. The minimal bactericidal concentration was increased in the presence of OMVs. It is concluded that OMV release is a means of bacteria to dispose of HDP-affected membrane. Furthermore, OMVs act as a decoy for HDPs and thereby protect the bacterium. IMPORTANCE Antibiotic resistance is a pressing problem and estimated to be a leading cause of mortality by 2050. Antimicrobial peptides, also known as host defense peptides (HDPs), and HDP-derived antimicrobials have potent antimicrobial activity and high potential as alternatives to antibiotics due to low resistance development. Some resistance mechanisms have developed in bacteria, and complete understanding of bacterial defense against HDPs will aid their use in the clinic. This study provides insight into outer membrane vesicles (OMVs) as potential defense mechanisms against HDPs, which will allow anticipation of unforeseen resistance to HDPs in clinical use and possibly prevention of bacterial resistance by the means of OMVs.

Bacterial-Specific Aggregation and Killing of Immunomodulatory Host Defense Peptides

This study involves the design and development of disulfide bridge-linked antimicrobial peptides using the host defense protein Angiogenin 4 (chAng4) as a template. The mini peptides derived from chAng4 (mCA4s) were evaluated for their antibacterial efficacies in various pathogenic bacterial strains, and the role of the oxidation state of thiols in the peptide sequence and its implication on antibacterial properties were explored. A remarkable property of these synthetic mCA4 peptides is their capability to flocculate bacteria and mediate bacterial-specific killing, in the absence of any other external stimulus. mCA4s were further evaluated for their cellular uptake, hemolytic activities, toxicities, and immunomodulatory activities in different eukaryotic cell lines. The results indicate that disulfide bridge-containing cationic amphipathic peptides show superior antibacterial efficacies, are nontoxic and nonhemolytic, and mediate bacterial flocculation and killing, in the absence of external stimuli.

Recombinant active Peptides and their Therapeutic functions

Recombinant active peptides are utilized as diagnostic and biotherapeutics in various maladies and as bacterial growth inhibitors in the food industry. This consequently stimulated the need for recombinant peptides' production, which resulted in about 19 approved biotech peptides of 1-100 amino acids commercially available. While most peptides have been produced by chemical synthesis, the production of lengthy and complicated peptides comprising natural amino acids has been problematic with low quantity. Recombinant peptide production has become very vital, cost-effective, simple, environmentally friendly with satisfactory yields. Several reviews have focused on discussing expression systems, advantages, disadvantages, and alternatives strategies. Additionally, the information on the antimicrobial activities and other functions of multiple recombinant peptides is challenging to access and is scattered in literature apart from the food and drug administration (FDA) approved ones. From the reports that come to our knowledge, there is no existing review that offers substantial information on recombinant active peptides developed by researchers and their functions. This review provides an overview of some successfully produced recombinant active peptides of ≤100 amino acids by focusing on their antibacterial, antifungal, antiviral, anticancer, antioxidant, antimalarial, and immune-modulatory functions. It also elucidates their modes of expression that could be adopted and applied in future investigations. We expect that the knowledge available in this review would help researchers involved in recombinant active peptide development for therapeutic uses and other applications.

Porcine Myeloid Antimicrobial Peptides: A Review of the Activity and Latest Advances

Traditional antibiotics have made great contributions to human health and animal husbandry since the discovery of penicillin in 1928, but bacterial resistance and drug residues are growing threats to global public health due to the long-term uncontrolled application of antibiotics. There is a critical need to develop new antimicrobial drugs to replace antibiotics. Antimicrobial peptides (AMPs) are distributed in all kingdoms of life, presenting activity against pathogens as well as anticancer, anti-inflammatory, and immunomodulatory activities; consequently, they have prospects as new potential alternatives to antibiotics. Porcine myeloid antimicrobial peptides (PMAPs), the porcine cathelicidin family of AMPs, have been reported in the literature in recent years. PMAPs have become an important research topic due to their strong antibacterial activity. This review focuses on the universal trends in the biochemical parameters, structural characteristics and biological activities of PMAPs.

Bioinformatic Analysis of Genome-Predicted Bat Cathelicidins

Bats are unique in their potential to serve as reservoir hosts for intracellular pathogens. Recently, the impact of COVID-19 has relegated bats from biomedical darkness to the frontline of public health as bats are the natural reservoir of many viruses, including SARS-Cov-2. Many bat genomes have been sequenced recently, and sequences coding for antimicrobial peptides are available in the public databases. Here we provide a structural analysis of genome-predicted bat cathelicidins as components of their innate immunity. A total of 32 unique protein sequences were retrieved from the NCBI database. Interestingly, some bat species contained more than one cathelicidin. We examined the conserved cysteines within the cathelin-like domain and the peptide portion of each sequence and revealed phylogenetic relationships and structural dissimilarities. The antibacterial, antifungal, and antiviral activity of peptides was examined using bioinformatic tools. The peptides were modeled and subjected to docking analysis with the region binding domain (RBD) region of the SARS-CoV-2 Spike protein. The appearance of multiple forms of cathelicidins verifies the complex microbial challenges encountered by these species. Learning more about antiviral defenses of bats and how they drive virus evolution will help scientists to investigate the function of antimicrobial peptides in these species.

Inhibition of Dermatophyte Fungi by Australian Jarrah Honey

Superficial dermatophyte infections, commonly known as tineas, are the most prevalent fungal ailment and are increasing in incidence, leading to an interest in alternative treatments. Many floral honeys possess antimicrobial activity due to high sugar, low pH, and the production of hydrogen peroxide (H2O2) from the activity of the bee-derived enzyme glucose oxidase. Australian jarrah (Eucalyptus marginata) honey produces particularly high levels of H2O2 and has been found to be potently antifungal. This study characterized the activity of jarrah honey on fungal dermatophyte species. Jarrah honey inhibited dermatophytes with minimum inhibitory concentrations (MICs) of 1.5-3.5% (w/v), which increased to ≥25% (w/v) when catalase was added. Microscopic analysis found jarrah honey inhibited the germination of Trichophyton rubrum conidia and scanning electron microscopy of mature T. rubrum hyphae after honey treatment revealed bulging and collapsed regions. When treated hyphae were stained using REDOX fluorophores these did not detect any internal oxidative stress, suggesting jarrah honey acts largely on the hyphal surface. Although H2O2 appears critical for the antifungal activity of jarrah honey and its action on fungal cells, these effects persisted when H2O2 was eliminated and could not be replicated using synthetic honey spiked with H2O2, indicating jarrah honey contains agents that augment antifungal activity.

A Thermostable, Modified Cathelicidin-Derived Peptide With Enhanced Membrane-Active Activity Against Salmonella enterica serovar Typhimurium

Foodborne illness caused by consumption of food contaminated with Salmonella is one of the most common causes of diarrheal disease and affects millions of people worldwide. The rising emergence and spread of antimicrobial resistance, especially in some serotypes of Salmonella, has raised a great awareness of public health issues worldwide. To ensure safety of the food processing chain, the development of new food preservatives must be expedited. Recently, thermal- and pH-stable antimicrobial peptides have received much attention for use in food production, and represent safe alternatives to chemical preservatives. A 12-mer cathelicidin-derived, α-helical cationic peptide, P7, displayed rapid killing activity, against strains of drug-resistant foodborne Salmonella enterica serovar Typhimurium and its monophasic variant (S. enterica serovar 4,5,12:i:-) and had minimal toxicity against mouse fibroblast cells. P7 tended to form helical structure in the membrane-mimic environments as evaluated by circular dichroism (CD) spectroscopy. The action mode of P7 at the membrane-level was affirmed by the results of flow cytometry, and confocal, scanning and transmission electron microscopy. P7 killed bacteria through binding to bacterial membranes, penetration and the subsequent accumulation in S. enterica serovar Typhimurium cytoplasm. This induced membrane depolarization, permeabilization, and sequential leakage of intracellular substances and cell death. Except for sensitivity to proteolytic digestive enzymes, P7 maintained its inhibitory activity against S. enterica serovar Typhimurium in the presence of different conditions [various salts, extreme pHs and heat (even at 100°C)]. Moreover, the peptide is unlikely to induce bacterial resistance in vitro. Taken together, this study demonstrated that the membrane-permeabilizing P7 peptide has much potential as a new antimicrobial agent for use in food processing and preservation.

A Gold Nanocluster Constructed Mixed-Metal Metal-Organic Network Film for Combating Implant-Associated Infections

The development of modular strategies for programming self-assembled supramolecular architectures with distinct structural and functional features is of immense scientific interest. We reported on the intrinsic antibacterial capability of anionic amphiphilic gold nanoclusters (GNCs) capped by para-mercaptobenzoic acid, which was closely related to the protonation level of terminal carboxylate groups. By using of the metal-ligand coordination-driven and solvent evaporation-induced self-assembly, we constructed GNCs-based mixed-metal metal-organic network (MM-MON) films on titanium disks as antibacterial nanocoatings. Taking the reasonable utilization of tetravalent metal ions M⁴⁺ (Ti, Zr, Hf; hard Lewis acid) and bactericidal divalent metal ions M²⁺ (Cu, Zn; borderline acid) co-incorporated metal-carboxylate coordination bonds, the MM-MON films exhibited superior stability due to the robust M⁴⁺-O bonds and M²⁺ releasing behavior resulting from the labile M²⁺-O coordinating. Together, the MM-MON films integrated the bacteria-responsive character of GNCs, exceptional chemical stability, and greatly enhanced antibacterial activity, ultimately killing adherent bacteria and initiating a self-defensive function. In a rat model for subcutaneous implant-associated infection, the MM-MON nanocoating showed an approximately 2 and 1 log lower multidrug-resistant Staphylococcus aureus implant and tissue colonization, respectively. The generalizable modular strategy of the GNC-metal networks is amenable to facilitate the functionalization of metal surfaces for combating implant-associated infections.

Synthetic Peptide Libraries Designed From a Minimal Alpha-Helical Domain of AS-48-Bacteriocin Homologs Exhibit Potent Antibacterial Activity

The circularized bacteriocin enterocin AS-48 produced by Enterococcus sp. exhibits antibacterial activity through membrane disruption. The membrane-penetrating activity of enterocin AS-48 has been attributed to a specific alpha-helical region on the circular peptide. Truncated, linearized forms containing these domains have been shown to preserve limited bactericidal activity. We utilized the amino acid sequence of the active helical domain of enterocin AS-48 to perform a homology-based search of similar sequences in other bacterial genomes. We identified similar domains in three previously uncharacterized AS-48-like bacteriocin genes in Clostridium sordellii, Paenibacillus larvae, and Bacillus xiamenensis. Enterocin AS-48 and homologs from these bacterial species were used as scaffolds for the design of a minimal peptide library based on the active helical domain of each bacteriocin sequence. 95 synthetic peptide variants of each scaffold peptide, designated Syn-enterocin, Syn-sordellicin, Syn-larvacin, and Syn-xiamensin, were designed and synthesized from each scaffold sequence based on defined biophysical parameters. A total of 384 total peptides were assessed for antibacterial activity against Gram-negative and Gram-positive bacteria. Minimal Inhibitory Concentrations (MICs) as low as 15.6 nM could be observed for the most potent peptide candidate tested, with no significant cytotoxicity to eukaryotic cells. Our work demonstrates for the first time a general workflow of using minimal domains of natural bacteriocin sequences as scaffolds to design and rapidly synthesize a library of bacteriocin-based antimicrobial peptide variants for evaluation.

Hybrid Antimicrobial Peptide Targeting Staphylococcus aureus and Displaying Anti-infective Activity in a Murine Model

Broad-spectrum antimicrobial peptides (AMPs) kill bacteria indiscriminately, increasing the possibility of an ecological imbalance in the microbiota. To solve this problem, new types of AMPs, which kill pathogenic bacteria without breaking the micro-ecological balance of the body, were proposed. Here, we successfully designed a targeting AMP, S2, which is a fusion peptide composed of a species-specific targeting domain and broad-spectrum AMP domain. In the current study, S2 showed specific killing activity against Staphylococcus aureus, and almost no resistance induced compared to penicillin. Mechanism studies indicated that S2 killed S. aureus by destroying the bacterial membrane. Meanwhile, S2 possessed excellent salt-tolerance properties and biocompatibility. Importantly, S2 exhibited perfect treatment efficacy against an S. aureus subcutaneous infection model and remained nontoxic. In conclusion, this study provides a promising strategy for designing specific AMPs against growing bacterial infections.

A Novel Dual-Targeted α-Helical Peptide With Potent Antifungal Activity Against Fluconazole-Resistant Candida albicans Clinical Isolates

Due to compromised immune system, fungal infection incidences have markedly increased in the last few decades. Pathogenic fungi have developed resistance to the clinically available antifungal agents. Antifungal resistance poses a great challenge to clinical treatment and has stimulated the demand for novel antifungal agents. A promising alternative to the treatment of fungal diseases is the use of antimicrobial peptides (AMPs). However, the antifungal activities of AMPs have not been fully determined. Therefore, this study aimed at designing and screening α-helical peptides with potential antifungal activities. The effects of key physicochemical parameters on antifungal activities were also investigated. A series of lengthened and residue-substituted derivatives of the template peptide KV, a hexapeptide truncated from the α-helical region of porcine myeloid antimicrobial peptide-36, were designed and synthesized. Enhancement of hydrophobicity by introducing aromatic hydrophobic amino acids (tryptophan and phenylalanine) significantly increased the efficacies of the peptides against Candida albicans strains, including fluconazole-resistant isolates. Increased hydrophobicity also elevated the toxic properties of these peptides. RF3 with moderate hydrophobicity exhibited potent anticandidal activities (GM = 6.96 μM) and modest hemolytic activities (HC10 > 64 μM). Additionally, repeated exposure to a subinhibitory concentration of RF3 did not induce resistance development. The antifungal mechanisms of RF3 were due to membrane disruptions and induction of reactive oxygen species production. Such a dual-targeted mechanism was active against drug-resistant fungi. These results show the important role of hydrophobicity and provide new insights into designing and developing antifungal peptides. Meanwhile, the successful design of RF3 highlights the potential utility of AMPs in preventing the spread of drug-resistant fungal infections.

N-terminal Myristoylation Enhanced the Antimicrobial Activity of Antimicrobial Peptide PMAP-36PW

Drug-resistant bacteria infections and drug residues have been increasing and causing antibiotic resistance and public health threats worldwide. Antimicrobial peptides (AMPs) are novel antimicrobial drugs with the potential to solve these problems. Here, a peptide based on our previously studied peptide PMAP-36PW was designed via N-terminal myristoylation and referred to as Myr-36PW. The fatty acid modification provided the as-prepared peptide with good stability and higher antimicrobial activity compared with PMAP-36PW in vitro. Moreover, Myr-36PW exhibited effective anti-biofilm activity against Gram-negative bacteria and may kill bacteria by improving the permeability of their membranes. In addition, the designed peptide Myr-36PW could inhibit the bacterial growth of Staphylococcus aureus ATCC 25923 and Pseudomonas aeruginosa GIM 1.551 to target organs, decrease the inflammatory damage, show an impressive therapeutic effect on mouse pneumonia and peritonitis experiments, and promote abscess reduction and wound healing in infected mice. These results reveal that Myr-36PW is a promising antimicrobial agent against bacterial infections.

Innovaciones en la terapia antimicrobiana

La resistencia microbiana ha llevado a la búsqueda de innovadoras alternativas para su contención y dentro de las más promisorias están el uso de péptidos sintéticos, no sólo por sus características intrínsecas antimicrobianas, sino por  las interacciones sinérgicas y antagónicas que presenta con otros mediadores inmunológicos. Estas propiedades han permitido crear péptidos sintéticos reguladores de defensa innata que representan un nuevo enfoque inmunomodulador para el tratamiento de infecciones; sin embargo, sólo los diseñados con alto score antimicrobiano, han demostrado eficacia en estudios clínicos de Fase 3. Debido a su amplio espectro de actividad, un único péptido puede actuar contra bacterias Gram negativas, Gram positivas, hongos, e incluso virus y parásitos, aumentando el interés por investigar estas dinámicas moléculas.  Por otra parte, se encuentra el sistema CRISPR, para la edición de genomas bacterianos, permitirá reducir su actividad virulenta y diseñar antimicrobianos basados en nucleasas CRISPR-Cas 9 programables contra dianas específicas, las que representan un promisorio camino en el estudio de nuevas alternativas con alto potencial para eliminar la resistencia a antibióticos de bacterias altamente patógenas. Asimismo, se aborda la terapia con fagos, referida a la accion de virus que infectan bacterias, usados solos o en cocteles para aumentar el espectro de acción de estos, aprovechando su abundacia en la naturaleza, ya que se ha considerado que cada bacteria tiene un virus específico que podría emplearse como potente agente antibacteriano. Finalmente,  mientras se usen como principal medio de contención solo tratamientos convencionales antimicrobianos, incluso de manera oportuna y acertada, la microevolución en las bacterias se asegurará de seguir su curs

CxxC Zinc Finger Protein Derived Peptide, MF18 Functions Against Biofilm Formation

The major threat in modern medicine was biofilm forming bacterial related infections and they were highly tolerant to conventional antibiotics and a boundless demand for new drugs. In this regard, antimicrobial peptide (AMP) have been considered as potential alternative agents to conventional antibiotics. In this study, we have reported a CxxC zinc finger protein derived peptide, MF18 and its various biological role including activity against biofilm forming bacteria. Zinc finger protein are important in regulation of several cellular processes and wide range of molecular functions. The CxxC zinc finger protein identified from the cDNA library of a teleost fish; further it was characterised using various online bioinformatics programs. During the in-silico analysis, an AMP named MF18 was identified from the CxxC zinc finger protein, then it was synthesised for further biological activity studies. The antimicrobial activity of MF18 was confirmed against the biofilm clinical isolates such as Staphylococcus aureus and Escherichia coli. The MIC of the antimicrobial peptide at the concentration of 320 µM was observed against these two biofilm bacteria. The mechanism of the peptides was determined using bacteria on its membrane permeabilization ability by scanning electron microscopy. It is exhibited that the MF18 potentially influenced in damaging the morphology of the bacteria. The toxicity of MF18 against the continuous cell line (RAW 264.7) was demonstrated by MTT assay and also using peripheral red blood cells by haemolytic assay; both assays showed that the peptide have no toxicity on the cells at lower concentration. Overall, the study showed the potential therapeutic application of the peptide in pharma industry.

Antimicrobial Peptide JH-3 Effectively Kills Salmonella enterica Serovar Typhimurium Strain CVCC541 and Reduces Its Pathogenicity in Mice

Salmonella is an important zoonotic pathogen and is a major cause of gastrointestinal diseases worldwide. The current serious problem of antibiotic abuse has prompted the search for new substitutes for antibiotics. JH-3 is a small antimicrobial peptide with broad-spectrum bactericidal activity. In this study, we showed that JH-3 has good bactericidal activity towards the clinical isolate Salmonella enterica serovar Typhimurium strain CVCC541. The minimum inhibitory concentration (MIC) of JH-3 against this bacterium was determined to be 100 μg/mL, which could decrease the number of CVCC541 cells by 1000-fold in vitro within 5 h. The transmission electron microscopy (TEM) results showed that JH-3 can damage the cell wall and membrane of CVCC541, leading to the leakage of cell contents and subsequent cell death. To measure the bactericidal activity of CVCC541-infected mice were treated intraperitoneally 40 or 10 mg/kg JH-3 at 2 h or 3 days postinfection. Our results showed that treatment with 40 mg/kg JH-3 at 2 h postinfection had the best therapeutic effect and could significantly protect mice from a lethal dose of CVCC541. Furthermore, the clinical symptoms, bacterial burden in blood and organs, and intestinal pathological changes were all decreased and were close to normal. This study examined the therapeutic effect of the antimicrobial peptide JH-3 against S. enterica CVCC541 infection for the first time and determined the therapeutic effect of different JH-3 doses and treatment times, laying the foundation for studies of new antimicrobial agents.

Protective efficacy of the recombinant lysozyme-PMAP36 fusion protein-inactivated Salmonella Typhimurium vaccine candidate via oral immunization in a murine model

The aim of this study was to evaluate protective efficacy of S. Typhimurium ghost vaccine candidate lysed by the recombinant lysozyme-PMAP36 fusion protein via oral immunization in a murine model. Sixty BALB/c mice were equally divided into 4 groups. Group A mice were inoculated with 20 μL of sterile phosphate-buffered saline (PBS). Groups B-D mice were immunized with approximately 1 × 10⁷, 1 × 10⁸, and 1 × 10⁹ cells of the vaccine candidate, respectively, in 20 μL of PBS. Salmonella-outermembrane-proteins-specific serum IgG was considerably higher in groups B-D than in group A. The interleukin-10 and interferon-γ levels in groups B-D were significantly higher than in group A. Following challenge with wild-type S. Typhimurium, all immunized groups showed a significant level of protection compared with group A. The highest protection was shown in group D. Overall, these results show that oral immunization with the candidate vaccine can effectively protect mice from S. Typhimurium infection.