Daptomycin

Daptomycin requires calcium (Ca2+) for activity. In the presence of Ca2+, daptomycin undergoes conformational change allowing its decanoyl tail to insert into bacterial cell membranes. Multiple daptomycin-Ca2+ molecules oligomerize in the membrane, forming an amphipathic channel. Ion flux through the channel rapidly depolarizes the membrane potential, halting ATP synthesis, DNA, RNA, and protein synthesis simultaneously. The bactericidal activity is rapid (within 1 hour) and does not require cell lysis, distinguishing it from beta-lactams.

Daptomycin resistance in MRSA involves mutations in mprF (multiple peptide resistance factor), which adds lysylphosphatidylglycerol (L-PG) to the membrane, increasing positive charge and repelling cationic daptomycin. YycFG two-component system mutations also alter cell wall stress responses. Resistance frequency is low (approximately 1 in 10^10 per generation) but can emerge on therapy in serious infections, typically requiring dose escalation or combination with beta-lactams (beta-lactam exposure paradoxically enhances daptomycin activity against some MRSA strains through PBP2a inhibition).

Daptomycin is FDA-approved for S. aureus bacteremia including right-sided endocarditis at 6 mg/kg once daily, non-inferior to standard-of-care vancomycin plus gentamicin. For left-sided endocarditis and osteomyelitis, higher doses (8-10 mg/kg) are used off-label. The Clinical and Laboratory Standards Institute (CLSI) breakpoint for susceptibility is MIC <=1 ug/mL. Dose escalation protocols for daptomycin-non-susceptible MRSA have been developed.

Daptomycin is one of very few antibiotics active against vancomycin-resistant S. aureus (VRSA) and remains active against most VRE strains. For VRE bacteremia and urinary infections, daptomycin at 6 mg/kg provides comparable outcomes to linezolid. The novel mechanism entirely bypasses the vanA/vanB resistance systems that confer glycopeptide resistance, making cross-resistance absent.