The objective of this experiment was to investigate the inhibitory effect and mechanism of mammary-derived extracellular vesicles(MmEVs)from mastitis dairy cows on the biofilm for-mation of Staphylococcus aureus SA1.The biofilm-forming ability of Staphylococcus aureus SA1 was confirmed using Congo red staining,and the biofilm growth curve of S.aureus SA1 was plot-ted using the crystal violet staining method.The minimum inhibitory concentration(MIC)and minimum biofilm inhibitory concentration(MBIC)of MmEVs against S.aureus SA1 were deter-mined.After treating S.aureus SA1 with different concentrations of MmEVs,the cell morphology of S.aureus SA1 was observed using transmission electron microscopy.The effects of MmEVs on S.aureus SA1 under low pH(pH value=5)or heat stress(58℃)were investigated.The hydro-phobicity index was explored using the microbial adhesion to hydrocarbons(MATH)assay.Bacte-rial conductivity was measured.The expression levels of biofilm-related genes(SarA,icaB,FnbA,ClfB,CidA,and gyrB)were detected using quantitative real-time PCR(qPCR).The results showed that MIC of MmEVs against the biofilm of S.aureus SA1 was 1 000 mg/L,and the MBIC was 500 mg/L.Under the influence of MmEVs,the internal substances of S.aureus SA1 leaked,the biofilm boundary became blurred,and the cell wall separated.At the MBIC concentration,MmEVs significantly reduced the tolerance of S.aureus SA1 to low pH(P＜0.001)and high tem-perature(P＜0.001),decreased hydrophobicity(P＜0.001),and increased bacterial conductivity(P＜0.001).At the MBIC concentration,MmEVs significantly downregulated the gene expression of Sa rA(P＜0.001),icaB(P＜0.001),FnbA(P＜0.001),ClfB(P＜0.001),and CidA(P＜0.001)in S.aureus SA1,while no significant effect was observed on the expression of the gyrB gene.In summary,MmEVs inhibit the formation of Staphylococcus aureus SA1 biofilms by sup-pressing the gene expression of SarA,icaB,FnbA,ClfB,and CidA within the biofilm.This dis-ruption damages the biofilm's morphological structure,reduces its tolerance to low pH and high temperature,decreases hydrophobicity,and increases bacterial conductivity,thereby ultimately in-hibiting the formation of S.aureus SA1 biofilms.

The objective of this experiment was to investigate the inhibitory effect and mechanism of mammary-derived extracellular vesicles(MmEVs)from mastitis dairy cows on the biofilm for-mation of Staphylococcus aureus SA1.The biofilm-forming ability of Staphylococcus aureus SA1 was confirmed using Congo red staining,and the biofilm growth curve of S.aureus SA1 was plot-ted using the crystal violet staining method.The minimum inhibitory concentration(MIC)and minimum biofilm inhibitory concentration(MBIC)of MmEVs against S.aureus SA1 were deter-mined.After treating S.aureus SA1 with different concentrations of MmEVs,the cell morphology of S.aureus SA1 was observed using transmission electron microscopy.The effects of MmEVs on S.aureus SA1 under low pH(pH value=5)or heat stress(58℃)were investigated.The hydro-phobicity index was explored using the microbial adhesion to hydrocarbons(MATH)assay.Bacte-rial conductivity was measured.The expression levels of biofilm-related genes(SarA,icaB,FnbA,ClfB,CidA,and gyrB)were detected using quantitative real-time PCR(qPCR).The results showed that MIC of MmEVs against the biofilm of S.aureus SA1 was 1 000 mg/L,and the MBIC was 500 mg/L.Under the influence of MmEVs,the internal substances of S.aureus SA1 leaked,the biofilm boundary became blurred,and the cell wall separated.At the MBIC concentration,MmEVs significantly reduced the tolerance of S.aureus SA1 to low pH(P＜0.001)and high tem-perature(P＜0.001),decreased hydrophobicity(P＜0.001),and increased bacterial conductivity(P＜0.001).At the MBIC concentration,MmEVs significantly downregulated the gene expression of Sa rA(P＜0.001),icaB(P＜0.001),FnbA(P＜0.001),ClfB(P＜0.001),and CidA(P＜0.001)in S.aureus SA1,while no significant effect was observed on the expression of the gyrB gene.In summary,MmEVs inhibit the formation of Staphylococcus aureus SA1 biofilms by sup-pressing the gene expression of SarA,icaB,FnbA,ClfB,and CidA within the biofilm.This dis-ruption damages the biofilm's morphological structure,reduces its tolerance to low pH and high temperature,decreases hydrophobicity,and increases bacterial conductivity,thereby ultimately in-hibiting the formation of S.aureus SA1 biofilms.