In previous studies we have shown spherical aggregates of 3 to firstly adhere to the surface of both Gram-negative and Gram-positive bacteria before forming a coating over the surface of the microbe and subsequently permeating through the cell membrane(s) into the cell interior. It is this interaction with the cell wall, specifically the interactions with the phospholipid bilayer that we hypothesise to contribute to the antimicrobial activity of SSAs. To gauge the activity and selectivity of 1 -5 and a -j against microbial membranes, a series of synthetic model vesicle systems were prepared, containing a variety of common bacterial membrane phospholipid headgroups: (i) phosphatidylglycerol (PG) only; (ii) phosphatidylethanolamine (PE):PG 3:1 and; (iii) PE:PG 1:1. Following our previous work in this area, we first quantified the ability of 1 -5 and co-formulations a -j to lyse the aforementioned synthetic model phospholipid vesicle systems. A summary of these results are shown in Table , Figures and Tables present in the ESI (Section S13 -S14). Here the fluorescent dye calcein was loaded into synthetic vesicles at a concentration where the dye self-quenches (70 mM). When the vesicle is lysed, the calcein is released to the extravesicular solution, effectively reducing calcein concentration to a point where the dye no longer undergoes self-quenching. Through comparison to a positive control experiment, in which 100 % vesicle lysis was achieved through the addition of 2 % X-100 Triton, the proportion of vesicles to undergo lysis in the presence or absence of SSA can be calculated. The data was then preferentially fitted to the Hill equation to determine the effective concentration of SSA needed to lyse 50 % of the vesicles present in solution (EC50) (Section S14). All SSAs and SSA co-formulations exhibited vesicle lysis EC50 values > 0.1 mM at 20 mins against the phospholipid vesicles of varying headgroup composition, apart from 1 and SSA coformulations a -d, which all contain 1. The results for 1 and a -d, summarised in Table , show the vesicle lysis process to be time dependent, with the amount of SSA required to lyse 50 % of the vesicles present decreasing over the 20-minute time frame of this experiment. SSA 1 was found to exhibit the greatest vesicle lysis properties across all synthetic microbial membranes tested, with a preference demonstrated towards the lysis of PE containing vesicle systems. However, co-formulation a (addition of a carboxylate, thiourea incorporated SSA) demonstrates the greatest degree of selectivity towards the lysis of PE:PG 1:1 membranes, while co-formulation c (addition of a sulfonate, squaramide incorporated SSA) demonstrates selectivity towards the lysis of PE:PG 3:1 membranes after 20 mins incubation. Therefore, we provide evidenc e that SSA co-formulation enables us to tailor the phospholipid vesicle lysis properties of the individual SSA. Next, we sought to understand the effects of 1, 2, 4 and 5 and a, c, d, f, g, j on synthetic vesicle phospholipid membrane fluidity using a fluorescence polarization (FP) assay. Here, the fluorescent reporter dye 1,6-diphenyl-1,3,5-hexatriene (DPH) is incorporated into the vesicles phospholipid bilayer. As membrane fluidity is increased, the DPH reporter dye moves more freely within the phospholipid bilayer environment, resulting in a decreased FP value. However, should the membrane become more rigid, the movement of the dye is decreased and the FP value increases. As 3 is intrinsically fluorescent and overlaps with the fluorescence excitation/emission properties of the DPH, this SSA and co-formulations incorporating this compound were not included.