To test for synergy between protamine and vancomycin by analyzing their bactericidal activities against slime-producing Staphylococcus epidermidis ATCC 35983 under planktonic and biofilm conditions.

We evaluated the activity of vancomycin and protamine separately against planktonic S epidermidis in broth by measuring the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) for each agent according to the standard macrobroth dilution method. To assess the possibility of synergy between these two agents, planktonic S epidermidis was exposed to vancomycin and protamine together in varying concentrations. Biofilms containing S epidermidis were then prepared and subjected to incubation with vancomycin and protamine separately as well as combined in varying concentrations. The bacterial viability of S epidermidis in the planktonic and biofilm phases after exposure to these two agents was assessed by qualitative culture and determination of viable colony blots.

Standard antibacterial susceptibility tests revealed that the MICs of protamine and vancomycin were 1 and 2 μg/mL, respectively, and their MBCs were 4 μg/mL. The MICs were unchanged when protamine and vancomycin were combined in varying concentrations. Neither agent exhibited significant bactericidal activity against S epidermidis in the biofilm phase at concentrations ≤32 μg/mL However, a combination of both agents, each at 32 μg/mL, resulted in a 7-log decrease in viable bacterial counts.

Protamine alone exhibited significant antibacterial activity against planktonic S epidermidis. No synergy was noted between protamine and vancomycin against S epidermidis in the planktonic phase. However, synergy was demonstrated when a combination of protamine and vancomycin was used on S epidermidis in the biofilm phase. Thus, protamine shows promise as an adjunctive agent to vancomycin in the treatment of S epidermidis in biofilms.