Inhibitory effect of silk sericin on biofilm formation by methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates

In chronic wounds such as bedsores, bacterial infection prolongs wound healing, and biofilms are a particularly important inhibitory factor in wound healing. Therefore, wound management requires inhibition of biofilm formation, and research is being conducted to develop products with anti-biofilm properties. Silk is composed of two types of proteins, fibroin and sericin, and it has been reported that fibroin promotes wound healing and sericin has antibacterial activity. Therefore, we wondered whether sericin could inhibit biofilm formation and investigated the inhibitory effect of sericin on thebiofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from wounds in vitro. Two MRSA strains with high biofilm forming ability and a standard strain were used. Sericin was extracted from silk by high-pressure steam sterilization. Sericin was added to the biofilm forming process of MRSA strains and its inhibitory effect on biofilm formation was evaluated. Morphological observation showed that the pigment in the stained biofilm was attenuated. Biofilm assay using a microtiter plate showed that the mean biofilm index was significantly inhibited by 66.1% in the sericin 250 μg/mL group compared to the control group (0.417 vs. 1.231, p < 0.01). These results show that silk sericin has an inhibitory effect on the biofilm formation by MRSA strains. Sericin contains many positively charged amino acids with hydrophilic amino acid residues, which are easy to combine with the negatively charged bacterial surface. The feature of molecular structure was thought to be related to its anti-biofilm properties.