Packed Like Sardines - How Surface Crowdedness Impacts Accessibility to Peptidoglycan of Staphylococcus aureus

Bacterial cell walls are essential barriers that protect bacteria against the onslaught of potentially lethal molecules from the outside. Small molecule therapeutics, proteins from bacterial foes, and host immune proteins must navigate past a dense layer of bacterial biomacromolecules (e.g., capsular proteins, teichoic acids, and anchored proteins) to reach the peptidoglycan (PG) layer of Gram-positive bacteria. A subclass of molecules (e.g., antibiotics with intracellular targets) must also permeate through the PG (in a molecular sieving manner) to reach the cytoplasmic membrane. In the case of Staphylococcus aureus (S. aureus), teichoic acids are the major biopolymers that decorate bacterial cell surfaces. Despite the biological and therapeutic importance of surface accessibility, systematic analyses in live bacterial cells have been lacking. We describe a novel live cell fluorescence assay that reports on the permeability of molecules to and within the PG scaffold. The assay has robust reproducibility, is readily adoptable to any Gram-positive organism, and is compatible with high-throughput sample processing. Analysis of the factors controlling permeability to S. aureus and the methicillin resistant MRSA revealed that molecular flexibility plays a central role in molecular permeability. Moreover, teichoic acids impeded permeability of molecules of a wide range of sizes and chemical composition.