Mimicking Nonribosomal Peptides from the Marine Actinomycete Streptomyces sp. H-KF8 Leads to Antimicrobial Peptides

Microorganisms within the marine environment have been shown to be very effective sources of naturally produced antimicrobial peptides (AMPs). Several nonribosomal peptides were identified based on genome mining predictions of Streptomyces sp. H-KF8, a marine Actinomycetota isolated from a remote Northern Chilean Patagonian fjord. Based on these predictions, a series of eight peptides, including cyclic peptides, were designed and chemically synthesized. Six of these peptides showed antimicrobial activity. Mode of action studies suggest that two of these peptides potentially act on the cell membrane via a novel mechanism allowing the passage of small ions, resulting in the dissipation of the membrane potential. This study shows that though structurally similar peptides, determined by NMR spectroscopy, the incorporation of small sequence mutations results in a dramatic influence on their bioactivity including mode of action. The qualified hit sequence can serve as a basis for more potent AMPs in future studies.

Substrate-derived Sortase A inhibitors: targeting an essential virulence factor of Gram-positive pathogenic bacteria

The bacterial transpeptidase Sortase A (SrtA) is a surface enzyme of Gram-positive pathogenic bacteria. It has been shown to be an essential virulence factor for the establishment of various bacterial infections, including septic arthritis. However, the development of potent Sortase A inhibitors remains an unmet challenge. Sortase A relies on a five amino acid sorting signal (LPXTG), by which it recognizes its natural target. We report the synthesis of a series of peptidomimetic inhibitors of Sortase A based on the sorting signal, supported by computational binding analysis. By employing a FRET-compatible substrate, our inhibitors were assayed in vitro. Among our panel, we identified several promising inhibitors with IC50 values below 200 μM, with our strongest inhibitor - LPRDSar - having an IC50 of 18.9 μM. Furthermore, it was discovered that three of our compounds show an effect on growth and biofilm inhibition of pathogenic Staphylococcus aureus, with the inclusion of a phenyl ring seemingly key to this effect. The most promising compound in our panel, BzLPRDSar, could inhibit biofilm formation at concentrations as low as 32 μg mL-1, manifesting it as a potential future drug lead. This could lead to treatments for MRSA infections in clinics and diseases such as septic arthritis, which has been directly linked with SrtA.

Role of sodium salicylate in Staphylococcus aureus quorum sensing, virulence, biofilm formation and antimicrobial susceptibility

The widespread threat of antibiotic resistance requires new treatment options. Disrupting bacterial communication, quorum sensing (QS), has the potential to reduce pathogenesis by decreasing bacterial virulence. The aim of this study was to investigate the influence of sodium salicylate (NaSa) on Staphylococcus aureus QS, virulence production and biofilm formation. In S. aureus ATCC 25923 (agr III), with or without serum, NaSa (10 mM) downregulated the agr QS system and decreased the secretion levels of alpha-hemolysin, staphopain A and delta-hemolysin. Inhibition of agr expression caused a downregulation of delta-hemolysin, decreasing biofilm dispersal and increasing biofilm formation on polystyrene and titanium under static conditions. In contrast, NaSa did not increase biofilm biomass under flow but caused one log₁₀ reduction in biofilm viability on polystyrene pegs, resulting in biofilms being twice as susceptible to rifampicin. A concentration-dependent effect of NaSa was further observed, where high concentrations (10 mM) decreased agr expression, while low concentrations (≤0.1 mM) increased agr expression. In S. aureus 8325-4 (agr I), a high concentration of NaSa (10 mM) decreased hla expression, and a low concentration of NaSa (≤1 mM) increased rnaIII and hla expression. The activity of NaSa on biofilm formation was dependent on agr type and material surface. Eight clinical strains isolated from prosthetic joint infection (PJI) or wound infection belonging to each of the four agr types were evaluated. The four PJI S. aureus strains did not change their biofilm phenotype with NaSa on the clinically relevant titanium surface. Half of the wound strains (agr III and IV) did not change the biofilm phenotype in the 3D collagen wound model. In addition, compared to the control, ATCC 25923 biofilms formed with 10 mM NaSa in the collagen model were more susceptible to silver. It is concluded that NaSa can inhibit QS in S. aureus, decreasing the levels of toxin production with certain modulation of biofilm formation. The effect on biofilm formation was dependent on the strain and material surface. It is suggested that the observed NaSa inhibition of bacterial communication is a potential alternative or adjuvant to traditional antibiotics.