New anti-infective compounds against Staphylococcus aureus

Abstract

This publication is an edited version of my submitted dissertation “New anti-infective compounds against Staphylococcus aureus” that was accepted by the faculty on 14.12.2022 and published on TOBIAS-lib on 17.01.2023. Owing to the development of antibiotic resistance in microorganisms, new anti-microbial and anti-infective agents are of dire need to keep these pathogens at bay. These agents are mainly derived from the pathogen (bacteria or fungi) itself or from different plant extracts. In our previous work, it was observed that Lipoprotein like lipoprotein (Lpl) is an essential virulence factor in Staphylococcus aureus. Lpl is membrane bound and encoded in a genomic island called νSaα. Lpl causes the invasion of S. aureus into the HaCaT keratinocyte cells through its interaction with Hsp90α receptor. Lpl1 administration also surges the murine kidney abscess - bacterial burden. Our aim was to identify peptides that can block Hsp90 and reduce the S.aureus pathogenicity caused by Lpl. Here, we show 2 small peptides, named L15 and its derivative, L13, both part of Lpl1 have anti-infective action towards Staphylococcus aureus. Pretreatment with L15 and L13 reduced the invasion of S. aureus USA300 into HaCaT keratinocytes, N/TERT-1 primary keratinocytes and S. aureus phagocytosis in human monocytes. The peptides were water soluble, non-cytotoxic and non-hemolytic at the tested concentrations. Dot blot studies showed that there is direct interaction of the peptides with Hsp90α. L15 and L13 significantly decreased lethality of S. aureus bacteremia in Galleria mellonella insect larvae model, but did not affect growth or hemolytic activity of S. aureus in vitro. In a mouse bacteremia model L15 was found to significantly decrease weight loss and lethality. In effect the small peptides showed the opposite effect from their mother protein, in terms of the invasion of S. aureus into host cell and mice bacteremia model. Although the molecular bases behind the protective effect of L15 remain unclear, in vitro data indicate that treatment with L15 or L13 and simultaneous infection with S. aureus significantly increase IL-6 production in host immune cells. Our findings highlight Hsp90-interacting peptides as potential anti-infective agents In a separate work, non-peptide, anti-microbial phytochemicals- Rhodomyrtone (Rom) and Polycyclic polyprenylated acylphloroglucinols (PPAP), capable of inhibiting S.aureus in vitro are described. A single point mutation at farR makes S.aureus resistant to Rom. The antibacterial activity of Rom was targeted towards the cell membrane by interacting with its phospatidylglycerol (PG). RomR mutants were seen to excrete far more PGs, thereby neutralizing the Rom before it reaches the cell membrane. PPAP, though very much active in vitro, failed to rescue G. mellonella larvae from S.aureus infection. The reason was found to be the neutralization of PPAPs by the serum albumin proteins found in the larval coelomic fluid. In silico docking studies showed that PPAP is binding to the Heme binding pocket of bovine serum albumin. On the other hand, PPAP was observed to have a beneficial but not fully protective effect in S. aureus septic arthritis mouse model.