The opportunistic human pathogen Staphylococcus aureus can evade
antibiotics by acquiring antibiotic resistance genes or by entering into a non-growing dormant state. Moreover, the particular circumstances of a specific
infection site, such as acidity or anaerobicity, often weaken
antibiotic potency. Decreased bacterial susceptibility combined with diminished
antibiotic potency is responsible for high failure rates when treating S. aureus
infections. Here, we report that the membrane-active
antimicrobial agent nTZDpa does not only exhibit enhanced
antibiotic activity against multidrug-resistant Gram-positive pathogens in acidic pH, but also retains antimicrobial potency under anaerobic conditions. This agent completely eradicated highly
antibiotic-tolerant cells and biofilms formed by methicillin-resistant S. aureus at pH 5.5 at concentrations at which it was not potent at pH 7.4. Furthermore, nTZDpa was more potent at synergistically potentiating
gentamicin killing against
antibiotic-tolerant MRSA cells at low pH than at high pH. All-atom molecular dynamics simulations combined with membrane-permeabilization assays revealed that the neutral form of nTZDpa, which contains
carboxylic acid, is more effective than the deprotonated form at penetrating the bacterial membrane and plays an essential role in membrane activity. An acidic pH increases the proportion of the neutrally charged nTZDpa, which results in antimicrobial enhancement. Our results provide key insights into rational design of pH-sensitive membrane-active antimicrobials and
antibiotic adjuvants that are effective in an
infection environment. These findings demonstrate that nTZDpa is a promising lead compound for developing new
therapeutics against hard-to-cure
infections caused by drug-resistant and -tolerant S. aureus.