Gausemycin Antibiotic Family Acts via Ca2+-Dependent Membrane Targeting

We report the molecular mechanism of action of gausemycins and the isolation of new members of the family, gausemycins C (1c), D (1d), E (1e), and F (1f), the minor components of the mixture. To elucidate the mechanism of action of gausemycins, we investigated the antimicrobial activity of the most active compounds, gausemycins A and B, in the presence of Ca2+, other metal ions, and phosphate. Gausemycins require a significantly higher Ca2+ concentration for maximum activity than daptomycin but lower than that required for malacidine and cadasides. Species-specific antimicrobial activity was found upon testing against a wide panel of Gram-positive bacteria. Membranoactivity of gausemycins was demonstrated upon their interactions with model lipid bilayers and micelles. The pore-forming ability was found to be dramatically dependent on the Ca2+ concentration and the membrane lipid composition. An NMR study of gausemycin B in zwitterionic and anionic micelles suggested the putative structure of the gausemycin/membrane complex and revealed the binding of Ca2+ by the macrocyclic domain of the antibiotic.

Staphylococcus aureus is able to generate resistance to novel lipoglycopeptide antibiotic gausemycin A

Gausemycin A is the first member of the novel lipoglycopeptides family produced by Streptomyces roseoflavus INA-Ac-5812. Gausemycin A has a pronounced bactericidal activity against methicillin-resistant Staphylococcus aureus. However, the ability of S. aureus to be resistant to gausemycin A has not been investigated yet. Using serial passaging, we have obtained the resistant variant S. aureus 5812R, which is 80 times more resistant compared to the parent strain. Susceptibility testing of S. aureus 5812R revealed the acquisition of cross-resistance to daptomycin, cefazolin, tetracycline, and gentamicin, while the resistance to vancomycin, nisin, and ramoplanin was absent. Whole genome sequencing revealed single nucleotide polymorphism (SNP) and deletions in S. aureus 5812R, among which are genes encoding efflux pump (sepA), the two-component Kdp system (kdpE), and the component of isoprenoid biosynthesis pathway (hepT). Phenotypically, S. aureus 5812R resembles a small-colony variant, as it is slow-growing, forms small colonies, and is deficient in pigments. Profiling of fatty acids (FA) composition constituting the cytoplasmic membrane of S. aureus 5812R revealed the prevalence of anteiso-branched FA, while straight FA was slightly less present. The evidence also showed that the gausemycin A-resistant strain has increased expression of the cls2 gene of the cardiolipin synthase. The performed checkerboard assay pointed out that the combination of gausemycin A and ciprofloxacin showed a synergistic effect against S. aureus 5812R.

Gausemycins A,B: Cyclic Lipoglycopeptides from Streptomyces sp.*

We report a novel family of natural lipoglycopeptides produced by Streptomyces sp. INA-Ac-5812. Two major components of the mixture, named gausemycins A and B, were isolated, and their structures were elucidated. The compounds are cyclic peptides with a unique peptide core and several remarkable structural features, including unusual positions of d-amino acids, lack of the Ca²⁺ -binding Asp-X-Asp-Gly (DXDG) motif, tyrosine glycosylation with arabinose, presence of 2-amino-4-hydroxy-4-phenylbutyric acid (Ahpb) and chlorinated kynurenine (ClKyn), and N-acylation of the ornithine side chain. Gausemycins have pronounced activity against Gram-positive bacteria. Mechanistic studies highlight significant differences compared to known glyco- and lipopeptides. Gausemycins exhibit only slight Ca²⁺ -dependence of activity and induce no pore formation at low concentrations. Moreover, there is no detectable accumulation of cell wall biosynthesis precursors under treatment with gausemycins.

A Novel Peptide Antibiotic Produced by Streptomyces roseoflavus Strain INA-Ac-5812 With Directed Activity Against Gram-Positive Bacteria

In this work, we report the isolation and detailed functional characterization for the new non-ribosomally synthesized antibiotic 5812-A/C, which was derived from metabolites of Streptomyces roseoflavus INA-Ac-5812. According to its chemical structure, the studied 5812-A/C preliminary is composed of a cyclic peptide part covalently bounded with an arabinose residue. N-terminal amino acid sequencing of the native peptide has identified its partial structure of Leu-Asp-Gly-Ser-Gly and consisting of a Tyr residue that is supposed to have a two-component peptide nature for the molecule studied. However, the structural analysis of the antibiotic complex derived from S. roseoflavus INA-Ac-5812 is still ongoing. The mechanism of action of 5812-A/C was assessed in comparison with its most related analog, the lipopeptide antibiotic daptomycin, given the presence in both antimicrobials of an L-kynurenine amino acid residue. The inhibitory activity of 5812-A/C against Gram-positive bacteria including methicillin-resistant strain of Staphylococcus aureus was similar to daptomycin. The mechanism of action of 5812-A/C was associated with the disruption of membrane integrity, which differs in comparison with daptomycin and is most similar to the antimicrobial membrane-disturbing peptides. However, 5812-A/C demonstrated a calcium-dependent mode of action. In addition, unlike daptomycin, 5812-A/C was able to penetrate mature biofilms and inhibit the metabolic activity of embedded S. aureus cells. At the same time, 5812-A/C has no hemolytic activity toward erythrocyte, but possessed weak cytotoxic activity represented by heterochromatin condensation in human buccal epithelium cells. The biological properties of the peptide 5812-A/C suggest its classification as a calcium-dependent antibiotic effective against a wide spectrum of Gram-positive pathogenic bacteria.

[Irumamicin Produced by Streptomyces roseoflavus INA-1278]

The strain Streptomyces roseoflavus INA-1278 is described as a new irumamicin producer. Irumamicin 1278 is different by the antifungal activity from irumamicin produced by the world-known strain Streptomyces subflavus subsp. Irumaensis subps. nov. AM-3603.