Mechanisms of daptomycin resistance in Staphylococcus aureus: role of the cell membrane and cell wall

Abstract

The bactericidal, cell membrane-targeting lipopeptide antibiotic daptomycin (DAP) is an important agent in treating invasive Staphylococcus aureus infections. However, there have been numerous recent reports of development of daptomycin resistance (DAP-R) during therapy with this agent. The mechanisms of DAP-R in S. aureus appear to be quite diverse. DAP-R strains often exhibit progressive accumulation of single nucleotide polymorphisms in the multipeptide resistance factor gene (mprF) and the yycFG components of the yycFGHI operon. Both loci are involved in key cell membrane (CM) events, with mprF being responsible for the synthesis and outer CM translocation of the positively charged phospholipid, lysyl-phosphotidylglycerol (L-PG), while the yyc operon is involved in the generalized response to stressors such as antimicrobials. In addition, other perturbations of the CM have been identified in DAP-R strains, including extremes in CM order, resistance to CM depolarization and permeabilization, and reduced surface binding of DAP. Moreover, modifications of the cell wall (CW) appear to also contribute to DAP-R, including enhanced expression of the dlt operon (involved in d-alanylation of CW teichoic acids) and progressive CW thickening.

Figure 1

Chemical structure of calcium-DAP based on NMR analyses. (A) Basic chemical structure of the entire lipo-peptide DAP molecule; (B) and (C) Model of the apostructure and calcium-conjugated structure, respectively. Negatively charged side chains are colored red, while positively charged side chains are colored blue. (E) and (F) Surface representation of the apostructure and calcium-conjugated structure of DAP, respectively, with red representing negative charges, blue representing positive charges, and white representing uncharged regions. Modified from data in Refs. 27–30a.

Figure 2

Proposed tri-domain structure-function topology of the MprF molecule by TOPCONS algorithm construction (i.e., C-terminal synthase domain; N-terminal flippase domain; and central bifunctional domains). The sites and amino acid modifications of the five SNPs most commonly observed in association with DAP-R are represented by the star-burst symbols. Modified from Ernst et al. (Reproduced with permission of C. Ernst and A. Peschel).

Figure 3

(A) Cartoon of enzymes and bactroprenol-bound substrates comprising the partially overlapping machineries for biosynthesis of CW teichoic acids (left), peptidoglycan (middle) and for cell division (right). CM areas in which these pathways take place are rich negatively-charged bactoprenol-phosphate/pyrophosphate and PG (indicated in yellow), and thus attract DAP and other CAPs. (B) Biosynthetic pathways generating negatively-charges lipids and CW components, and reactions involved in modulation of the surface charge by D-alanylation and lysinylation (positive charges indicated in red) resulting in reduced DAP activity and DAP-R development. (C) Sensor systems involved in controlling CW integrity and proposed regulons; mutations in yycG (vicK) and vraS frequently observed in DAP-R mutants may reduce precision in CW structure and function, and allow for growth of impaired, but viable cells.

Figure 3

(A) Cartoon of enzymes and bactroprenol-bound substrates comprising the partially overlapping machineries for biosynthesis of CW teichoic acids (left), peptidoglycan (middle) and for cell division (right). CM areas in which these pathways take place are rich negatively-charged bactoprenol-phosphate/pyrophosphate and PG (indicated in yellow), and thus attract DAP and other CAPs. (B) Biosynthetic pathways generating negatively-charges lipids and CW components, and reactions involved in modulation of the surface charge by D-alanylation and lysinylation (positive charges indicated in red) resulting in reduced DAP activity and DAP-R development. (C) Sensor systems involved in controlling CW integrity and proposed regulons; mutations in yycG (vicK) and vraS frequently observed in DAP-R mutants may reduce precision in CW structure and function, and allow for growth of impaired, but viable cells.

Figure 3

(A) Cartoon of enzymes and bactroprenol-bound substrates comprising the partially overlapping machineries for biosynthesis of CW teichoic acids (left), peptidoglycan (middle) and for cell division (right). CM areas in which these pathways take place are rich negatively-charged bactoprenol-phosphate/pyrophosphate and PG (indicated in yellow), and thus attract DAP and other CAPs. (B) Biosynthetic pathways generating negatively-charges lipids and CW components, and reactions involved in modulation of the surface charge by D-alanylation and lysinylation (positive charges indicated in red) resulting in reduced DAP activity and DAP-R development. (C) Sensor systems involved in controlling CW integrity and proposed regulons; mutations in yycG (vicK) and vraS frequently observed in DAP-R mutants may reduce precision in CW structure and function, and allow for growth of impaired, but viable cells.