The MBC was also determined using the CLSI procedure. Briefly, 100 μL from the MIC, two times MIC (MIC × 2), four times MIC (MIC × 4), and eight times MIC PRMT inhibitor (MIC × 8) wells were plated on Luria Bertani (LB) agar and incubated at 37°C overnight. MIC of Vancomycin was determined for a panel of S. aureus isolates that represented the MIC range of P128 (1-64 μg/mL) using the CLSI broth microdilution method. Vancomycin was tested at concentrations of 0.125-256 μg/mL, and MICs were read manually

after 24 h of incubation. MBC was also determined using the CLSI procedure. The reference strain, S. aureus ATCC 25923 was used for quality control of the assay, in case of both P128 and Vancomycin MIC and MBC determinations. Time-kill curve studies The kinetics of P128 bactericidal activity were assessed in vitro using six S. aureus strains: SBI-0206965 BK#13237, BK#9894, BK#14780, BK#8374, BK#9918, and BK#19069. The cryopreserved test strains were plated on LB agar plate and incubated overnight at 37°C. Several well-isolated colonies were picked up and suspended in MHB broth;

the turbidity was then adjusted to 0.5 McFarland standard (about 108 CFU/mL). The initial inoculum was prepared by inoculating 10 μL of each test bacterial suspension into 20 mL MHB supplemented with 0.1% BSA. After 1 h in a shaker incubator (37°C, 200 rpm), 2.7 mL aliquots of the culture were dispensed into four tubes, and 0.3 mL P128 was added. A 0.3 mL aliquot was immediately removed to determine

the initial CFU (0 h). Incubation was continued, and 0.3 mL aliquots were taken at 1, 2, 4, 8, and 24 h. The cultures were serially diluted in sterile saline immediately after sampling and plated on MHB agar. After overnight incubation of the plates, CFU were determined. The time-kill curve was plotted based on bacterial survival at the sampling intervals [25]. Efficacy of P128 hydrogel applied to S. aureus on agar surface P128 before hydrogel was formulated with hydroxyethyl cellulose (0.42%), propylene glycol (0.75%), and glycerin (2.25%) as the main excipients along with P128 protein. A formulation that contained physiological saline in place of P128 (referred to as buffer gel) served as a negative control. LB agar was poured into 24-well tissue culture plates (Tarson). S. aureus (BK#13237) cells at 103 CFU/well (Figure 1) and 102 CFU/well (Figure 1) were seeded on LB agar in the microwells. P128 gel was diluted two-fold in buffer gel to contain P128 protein at a concentration range of 100 to 1.56 μg/mL. P128 gel preparations were applied to wells and the plates were incubated at 37°C for 18 h. At the end of incubation, 20 μL iodonitrotetrazolium chloride (INT dye; Loba Chemie) prepared in 50 mM sodium phosphate buffer, pH 7.0 (30 mg/mL) was added to the wells to visualize viable cells. Figure 1 Efficacy of P128 gel formulation applied to S. aureus on agar surface. A hydrogel formulation containing P128 protein (100 to 1.