Analysis of cell wall architecture and metabolism of a vancomycin resistant Staphylococcus aureus strain
Final Report Abstract
Infections with methicillin resistant Staphylococcus aureus strains are commonly treated with the glycopeptide antibiotic vancomycin. This antibiotic inhibits cell wall biosynthesis by binding to the cell wall building block lipid II at the surface of the membrane. We set out to characterize the mechanisms leading to decreased vancomycin susceptibility in a laboratory generated strain, S. aureus VC40 by investigating the biochemistry of its cell wall polymers. S. aureus VC40 has acquired 79 mutations among them two exchanges in the sensor kinase VraS, that up-regulates the so-called “cell wall stress regulon” and will accelerate cell wall biosynthesis in the presence of antibiotics. This strain possesses an altered cell wall architecture with a thick cell murein layer with very low cross-linking. The free D-Ala-D-Ala ends, that are not part of cross-links, provide decoy binding sites for vancomycin in the cell wall. Subsequently, the strong adsorption of vancomycin in the outer cell wall layers will decrease the speed with which vancomycin transverses the peptidoglycan layer, the “clogging effect”. On the other hand, the low cross-linking, necessary for this resistance mechanism, decreases the stability of the cell wall against lytic enzymes, which normally separate the daughter cells. In S. aureus VC40, another cell wall polymer, the wall teichoic acid, provides protection against these enzymes. Here the teichoic acids contain an unusually high amount of D-alanine and β-N-acetyl-D-glucosamine instead of α-N-acetyl-D-glucosamine that is present in the parent strain. We could show that the peptidoglycan of this strain does not bind the major autolysin AtlA any longer. A susceptible revertant of S. aureus VC40, called S. aureus VC40R, has acquired an additional mutation in VraR, which seems to dampen the activity of VraS. This strain shows a significantly lower resistance and has teichoic acids, which are - again - decorated with α-N-acetyl-D- glucosamine. When we increased the proportion of β-N-acetyl-D-glucosamine through the induction of salt stress in S. aureus VC40R, it showed again the full resistance of its parent strain. These results indicate that the β-conformation of the sugars plays a vital role in the resistance of S. aureus VC40 against vancomycin.
Publications
- (2021) The role of β-glycosylated wall teichoic acids in the reduction of vancomycin susceptibility in vancomycin-intermediate Staphylococcus aureus, Microbiology Spectrum Oct 31;9(2):e0052821
Hort M, Bertsche U, Nozinovic S, Dietrich A, Schrötter AS, Mildenberger L, Axtmann K, Berscheid A and Bierbaum G
(See online at https://doi.org/10.1128/spectrum.00528-21)
