Background Accurate determination of the efficacy of antimicrobial agents requires neutralization of residual antimicrobial activity in the samples before microbiological assessment of the number of surviving bacteria. Sodium polyanethol sulfonate (SPS) is a known neutralizer for the antimicrobial activity of aminoglycosides and polymyxins. In this study, we evaluated the ability of SPS to neutralize residual antimicrobial activity of antimicrobial peptides [SAAP-148 and pexiganan; 1% (wt/v) in PBS], antibiotics [mupirocin (Bactroban) and fusidic acid (Fucidin) in ointments; 2% (wt/wt))] and disinfectants [2% (wt/wt) silver sulfadiazine cream (SSD) and 0.5% (v/v) chlorhexidine in 70% alcohol].
Methods Homogenates of human skin models that were exposed to various antimicrobial agents for 1 h were pipetted on top of Methicillin-resistant Staphylococcus aureus (MRSA) on agar plates to demonstrate the presence of residual antimicrobial activity in tissue samples. To determine the efficacy of SPS in neutralization, various antimicrobial agents were mixed with PBS, 0.05%, 0.1%, 0.5% or 1% (wt/v) SPS in PBS. Subsequently, 10^5 colony forming units (CFU) MRSA were added to the mixtures and these were incubated for 30 min to determine the antimicrobial effect. Ex vivo excision wound models were inoculated with 10^5 CFU MRSA for 1 h and exposed to SAAP-148, pexiganan, chlorhexidine or PBS for 1 h to study the effect of SPS-neutralization on the bacterial killing of these antimicrobials. To quantify the number of surviving bacteria, 10-fold serial dilutions of the homogenates were cultured overnight on agar plates.
Results All tested antimicrobials displayed residual activity in tissue samples, resulting in a lower recovery of surviving bacteria on agar. SPS concentrations at ≥0.05% (wt/v) were able to neutralize the antimicrobial activity of SAAP-148, pexiganan and chlorhexidine, resulting in the complete recovery of bacteria. However, SPS did not neutralize the activity of SSD, Bactroban and Fucidin. Finally, SPS-neutralization in ex vivo efficacy tests of SAAP-148 (p<0.001), pexiganan (p<0.05) and chlorhexidine (p<0.01) resulted in at least 10-fold higher numbers of MRSA compared to control samples without SPS-neutralization.
Conclusion SPS can be used for the neutralization of residual activity of different antimicrobials, including SAAP-148, pexiganan and chlorhexidine and this prevents an overestimation of their efficacy.