Targeting Wall Teichoic Acid in Situ with Branched Polyethylenimine Potentiates ß-Lactam Efficacy against MRSA.

Methicillin-resistant Staphylococcus aureus (MRSA) is a medical concern. Here, we show that branched polyethylenimine (BPEI), a nontoxic, cationic polymer, restores MRSA's susceptibility to ß-lactam antibiotics. Checkerboard assays with MRSA demonstrated synergy between BPEI and ß-lactam antibiotics. A time-killing curve showed BPEI to be bactericidal in combination with oxacillin. BPEI did not potentiate efficacy with vancomycin, chloramphenicol, or linezolid. When exposed to BPEI, MRSA increased in size and had difficulty forming septa. BPEI electrostatically binds to wall teichoic acid (WTA), a cell wall anionic polymer of Gram-positive bacteria that is important for localization of certain cell wall proteins. Lack of potentiation in a WTA knockout mutant supports the WTA-based mechanism. These data suggest that BPEI may prevent proper localization of cell wall machinery by binding to WTA; leading to cell death when administered in combination with ß-lactam antibiotics. Negligible in vitro toxicity suggests the combination could be a viable treatment option.

Efficacy of ampicillin against methicillin-resistant Staphylococcus aureus restored through synergy with branched poly(ethylenimine).

ß-Lactam antibiotics kill Staphylococcus aureus bacteria by inhibiting the function of cell wall penicillin-binding proteins (PBPs) 1 and 3. However, ß-lactams are ineffective against PBP2a, used by methicillin-resistant S. aureus (MRSA) to perform essential cell wall crosslinking functions. PBP2a requires teichoic acid to properly locate and orient the enzyme, and thus MRSA is susceptible to antibiotics that prevent teichoic acid synthesis in the bacterial cytoplasm. As an alternative, we have used branched poly(ethylenimine), BPEI, to target teichoic acid in the bacterial cell wall. The result is restoration of MRSA susceptibility to the ß-lactam antibiotic ampicillin with a MIC of 1 µg ml-1, superior to that of vancomycin (MIC=3.7 µg ml-1). A checkerboard assay shows synergy of BPEI and ampicillin. NMR data show that BPEI alters the teichoic acid chemical environment. Laser scanning confocal microscopy images show BPEI residing on the bacterial cell wall, where teichoic acids and PBPs are located.