Deploying Soft Drugs in the Fight against MRSA with Detailed Biological Evaluation of 4-(<i>n</i>-Alkoxy)-phenoxy Betaine Amphiphiles Active against MDR <i>Staphylococcus aureus</i> and <i>Enterococcus</i> sp.

Antimicrobial resistance is one of the major challenges faced worldwide by the healthcare systems, negatively impinging upon global health and the economy. Methicillin-resistant <i>Staphylococcus aureus</i> (MRSA), which causes skin and mucosal infections, contributes to 64% more death as compared to drug-susceptible infections, which forces researchers to continuously search for new antimicrobials. In this context, we report the synthesis and antimicrobial activity of novel amphiphilic cationic 1,4-alkoxy-phenol derivatives. We synthesized a library of 12 small amphiphilic compounds containing an l-carnitine or betaine as a cationic headgroup and investigated the effect of the hydrocarbon chain length on the antimicrobial activity against a critical and high-priority human pathogen panel. The amphiphilic molecules with a C-10 alkyl chain length (<b>2f</b> and <b>3f</b>) showed substantial antibacterial activity at very low concentrations (1-2 μg/mL). Based on low cytotoxicity (Selectivity Index 20) and MIC of amphiphilic series, we further explored <b>3f</b>, which shows a concentration-dependent bactericidal activity at 10× MIC; it completely wiped out <i>S. aureus</i> ATCC 29213 at 1 h, and when combined with daptomycin, it showed a reduction of ∼6 log₁₀ CFU/mL as compared with untreated at 24 h against <i>S. aureus</i> ATCC 29213 even at 1× MICs. In order to discriminate between nonspecificity in detergent-like action, we further determined the CMC of 3f and found that it is 36-fold higher than MIC, thus further emphasizing its specificity. <b>3f</b> exhibits biofilm eradication properties that are comparable to levofloxacin and better than vancomycin while intracellular killing efficacy of <b>3f</b> was found to be better than vancomycin and similar to levofloxacin. When tested <i>in vivo</i> in the murine skin infection model, it shows activity comparable to fusidic acid as it reduces ∼2 log₁₀ CFU/g compared with that untreated. Considering the potent antimicrobial properties of <b>3f</b>, we hypothesize that it opens up a new avenue for discovery and development of novel antimicrobials active against MDR <i>S. aureus</i> and <i>Enterococcus</i> sp.