Antimicrobial Activity and Action Mechanisms of Arg-Rich Short Analog Peptides Designed from the C-Terminal Loop Region of American Oyster Defensin (AOD)

Semi-purified Antimicrobial Proteins from Oyster Hemolymph Inhibit Pneumococcal Infection

Pneumococcal infections caused by Streptococcus pneumoniae are a leading cause of morbidity and mortality globally, particularly among children. The ability of S. pneumoniae to form enduring biofilms makes treatment inherently difficult, and options are further limited by emerging antibiotic resistance. The discovery of new antibiotics, particularly those with antibiofilm activity, is therefore increasingly important. Antimicrobial proteins and peptides (AMPs) from marine invertebrates are recognised as promising pharmacological leads. This study determined the in vitro antibacterial activity of hemolymph and unique protein fractions from an Australian oyster (Saccostrea glomerata) against multi-drug-resistant S. pneumoniae. We developed a successful method for hemolymph extraction and separation into 16 fractions by preparative HPLC. The strongest activity was observed in fraction 7: at 42 µg/mL protein, this fraction was bactericidal to S. pneumoniae and inhibited biofilm formation. Proteomic analysis showed that fraction 7 contained relatively high abundance of carbonic anhydrase, cofilin, cystatin B-like, and gelsolin-like proteins, while surrounding fractions, which showed lower or no antibacterial activity, contained these proteins in lower abundance or not at all. This work supports traditional medicinal uses of oysters and contributes to further research and development of novel hemolymph/AMP-based treatments for pneumococcal infections.

High-Yield Preparation of American Oyster Defensin (AOD) via a Small and Acidic Fusion Tag and Its Functional Characterization

The marine peptide, American oyster defensin (AOD), is derived from Crassostrea virginica and exhibits a potent bactericidal effect. However, recombinant preparation has not been achieved due to the high charge and hydrophobicity. Although the traditional fusion tags such as Trx and SUMO shield the effects of target peptides on the host, their large molecular weight (12-20 kDa) leads to the yields lower than 20% of the fusion protein. In this study, a short and acidic fusion tag was employed with a compact structure of only 1 kDa. Following 72 h of induction in a 5 L fermenter, the supernatant exhibited a total protein concentration of 587 mg/L. The recombinant AOD was subsequently purified through affinity chromatography and enterokinase cleavage, resulting in the final yield of 216 mg/L and a purity exceeding 93%. The minimum inhibitory concentrations (MICs) of AOD against Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus galactis ranged from 4 to 8 μg/mL. Moreover, time-killing curves indicated that AOD achieved a bactericidal rate of 99.9% against the clinical strain S. epidermidis G-81 within 0.5 h at concentrations of 2× and 4× MIC. Additionally, the activity of AOD was unchanged after treatment with artificial gastric fluid and intestinal fluid for 4 h. Biocompatibility testing demonstrated that AOD, at a concentration of 128 μg/mL, exhibited a hemolysis rate of less than 0.5% and a cell survival rate of over 83%. Furthermore, AOD's in vivo therapeutic efficacy against mouse subcutaneous abscess revealed its capability to restrain bacterial proliferation and reduce bacterial load, surpassing that of antibiotic lincomycin. These findings indicate AOD's potential for clinical usage.

3-Hydroxy coumarin demonstrates anti-biofilm and anti-hyphal efficacy against Candida albicans via inhibition of cell-adhesion, morphogenesis, and virulent genes regulation

Candida albicans, a common fungus of human flora, can become an opportunistic pathogen and causes invasive candidiasis in immunocompromised individuals. Biofilm formation is the prime cause of antibiotic resistance during C. albicans infections and treating biofilm-forming cells is challenging due to their intractable and persistent nature. The study intends to explore the therapeutic potential of naturally produced compounds by competitive marine bacteria residing in marine biofilms against C. albicans biofilm. To this end, 3-hydroxy coumarin (3HC), a compound identified from the cell-free culture supernatant of the marine bacterium Brevundimonas abyssalis, was found to exhibit anti-biofilm and anti-hyphal activity against both reference and clinical isolates of C. albicans. The compound demonstrated significant inhibitory effects on biofilms and impaired the yeast-to-hyphal transition, wrinkle, and filament morphology at the minimal biofilm inhibitory concentration (MBIC) of 250 µg mL^-1. Intriguingly, quantitative PCR analysis of 3HC-treated C. albicans biofilm revealed significant downregulation of virulence genes (hst7, ume6, efg1, cph1, ras1, als1) associated with adhesion and morphogenesis. Moreover, 3HC displayed non-fungicidal and non-toxic characteristics against human erythrocytes and buccal cells. In conclusion, this study showed that marine biofilms are a hidden source of diverse therapeutic drugs, and 3HC could be a potent drug to treat C. albicans infections.

Structure-Activity Relationship Studies of Substitutions of Cationic Amino Acid Residues on Antimicrobial Peptides

Antimicrobial peptides (AMPs) have received considerable attention as next-generation drugs for infectious diseases. Amphipathicity and the formation of a stabilized secondary structure are required to exert their antimicrobial activity by insertion into the microbial membrane, resulting in lysis of the bacteria. We previously reported the development of a novel antimicrobial peptide, 17KKV, based on the Magainin 2 sequence. The peptide was obtained by increasing the amphipathicity due to the replacement of amino acid residues. Moreover, we studied the structural development of 17KKV and revealed that the secondary structural control of 17KKV by the introduction of non-proteinogenic amino acids such as α,α-disubstituted amino acids or side-chain stapling enhanced its antimicrobial activity. Among them, peptide 1, which contains 2-aminobutyric acid residues in the 17KKV sequence, showed potent antimicrobial activity against multidrug-resistant Pseudomonus aeruginosa (MDRP) without significant hemolytic activity against human red blood cells. However, the effects of cationic amino acid substitutions on secondary structures and antimicrobial activity remain unclear. In this study, we designed and synthesized a series of peptide 1 by the replacement of Lys residues with several types of cationic amino acids and evaluated their secondary structures, antimicrobial activity, hemolytic activity, and resistance against digestive enzymes.

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.

Myticofensin, a novel antimicrobial peptide family identified from Mytilus coruscus

In this study, seven transcripts representing a novel antimicrobial peptide (AMP) family with structural features similar to those of arthropod defensins were identified from Mytilus coruscus. These novel defensins from the Mytilus AMP family were named myticofensins. To explore the possible immune-related functions of these myticofensins, we examined their expression profiles in different tissues and larval stages, as well as in three immune-related tissues under the threat of different microbes. Our data revealed that the seven myticofensins had relatively high expression levels in immune-related tissues. Most myticofensins were undetectable, or had low expression levels, in different larval mussel stages. Additionally, in vivo microbial challenges significantly increased the expression levels of myticofensins in M. coruscus hemocytes, gills, and digestive glands, showing different immune response patterns under challenges from different microbes. Our data indicates that different myticofensins may have different immune functions in different tissues. Furthermore, peptide sequences corresponding to the beta-hairpin, alpha-helix, and N-terminal loop of myticofensin were synthesized and the antimicrobial activities of these peptide fragments were tested. Our data confirms the diversity of defensins in Mytilus and reports the complex regulation of these defensins in the mussel immune response to different microbes in immune-related tissues. The immune system of Mytilus has been studied for years as they are a species with strong environmental adaptations. Our data can be regarded as a step forward in the study of the adaptation of Mytilus spp. to an evolving microbial world.

Arginine Homopeptide of 11 Residues as a Model of Cell-Penetrating Peptides in the Interaction with Bacterial Membranes

Cell-penetrating peptides rich in arginine are good candidates to be considered as antibacterial compounds, since peptides have a lower chance of generating resistance than commonly used antibiotics. Model homopeptides are a useful tool in the study of activity and its correlation with a secondary structure, constituting an initial step in the construction of functional heteropeptides. In this report, the 11-residue arginine homopeptide (R11) was used to determine its antimicrobial activity against Staphylococcus aureus and Escherichia coli and the effect on the secondary structure, caused by the substitution of the arginine residue by the amino acids Ala, Pro, Leu and Trp, using the scanning technique. As a result, most of the substitutions improved the antibacterial activity, and nine peptides were significantly more active than R11 against the two tested bacteria. The cell-penetrating characteristic of the peptides was verified by SYTOX green assay, with no disruption to the bacterial membranes. Regarding the secondary structure in four different media-PBS, TFE, E. coli membrane extracts and DMPG vesicles-the polyproline II structure, the one of the parent R11, was not altered by unique substitutions, although the secondary structure of the peptides was best defined in E. coli membrane extract. This work aimed to shed light on the behavior of the interaction model of penetrating peptides and bacterial membranes to enhance the development of functional heteropeptides.

Antimicrobial potential of a ponericin-like peptide isolated from Bombyx mori L. hemolymph in response to Pseudomonas aeruginosa infection

The main effectors in the innate immune system of Bombyx mori L. are antimicrobial peptides (AMPs). Here, we infected B. mori with varied inoculum sizes of Pseudomonas aeruginosa ATCC 25668 cells to investigate changes in morpho-anatomical responses, physiological processes and AMP production. Ultraviolet-visible spectra revealed a sharp change in λ_max from 278 to 285 nm (bathochromic shift) in the hemolymph of infected B. mori incubated for 24 h. Further, Fourier Transform InfraRed studies on the hemolymph extracted from the infected B. mori showed a peak at 1550 cm^-1, indicating the presence of α-helical peptides. The peptide fraction was obtained through methanol, acetic acid and water mixture (90:1:9) extraction, followed by peptide purification using Reverse Phase High Performance Liquid Chromatography. The fraction exhibiting antibacterial properties was collected and characterized by Matrix-Assisted Laser Desorption/Ionization-Time of Flight. A linear α-helical peptide with flexible termini (LLKELWTKMKGAGKAVLGKIKGLL) was found, corresponding to a previously described peptide from ant venom and here denominated as Bm-ponericin-L1. The antibacterial activity of Bm-ponericin-L1 was determined against ESKAPE pathogens. Scanning electron microscopy confirmed the membrane disruption potential of Bm-ponericin-L1. Moreover, this peptide also showed promising antibiofilm activity. Finally, cell viability and hemolytic assays revealed that Bm-ponericin-L1 is non-toxic toward primary fibroblasts cell lines and red blood cells, respectively. This study opens up new perspectives toward an alternative approach to overcoming multiple-antibiotic-resistance by means of AMPs through invertebrates' infection with human pathogenic bacteria.

Marine Compounds with Anti-Candida sp. Activity: A Promised “Land” for New Antifungals

Candida albicans is still the major yeast causing human fungal infections. Nevertheless, in the last decades, non-Candida albicans Candida species (NCACs) (e.g., Candida glabrata, Candida tropicalis, and Candida parapsilosis) have been increasingly linked to Candida sp. infections, mainly in immunocompromised and hospitalized patients. The escalade of antifungal resistance among Candida sp. demands broadly effective and cost-efficient therapeutic strategies to treat candidiasis. Marine environments have shown to be a rich source of a plethora of natural compounds with substantial antimicrobial bioactivities, even against resistant pathogens, such as Candida sp. This short review intends to briefly summarize the most recent marine compounds that have evidenced anti-Candida sp. activity. Here, we show that the number of compounds discovered in the last years with antifungal activity is growing. These drugs have a good potential to be used for the treatment of candidiasis, but disappointedly the reports have devoted a high focus on C. albicans, neglecting the NCACs, highlighting the need to perform outspreading studies in the near future.