Abstract: Staphylococcus aureus is the main cause of bacterial mastitis in dairy cows, and the formation of biofilm is the key factor of S. aureus persistence under adverse environmental conditions. Exploring the correlation between drug resistance and growth state of the same strain in biofilm and planktonic growth state can lay a foundation for further exploring the drug resistance mechanism of S. aureus. In this study, the biofilm of S. aureus was cultured and its formation was observed by optical microscope and scanning electron microscope. The minimum inhibitory concentrations of 9 kinds of antibiotics against 32 strains of S. aureus in biofilm and planktonic states were measured and compared. Transcriptome sequencing was performed on S. aureus under the two states to screen out cell signaling pathways and expressed genes with significant differences. At the same time, the main differentially expressed genes were verified by RT-qPCR. The results showed that in the early stage of biofilm formation, with the extension of culture time, the aggregation area of biofilm state bacteria observed under the microscope became larger and larger, and the structure was more and more compact. After 72 hours of culture, the biofilm gradually began to disperse. MIC results showed that the inhibitory concentration of planktonic bacteria was lower than that of biofilm bacteria. Transcriptome results showed that there were 1512 differentially expressed genes in the two states, among which 760 genes were up-regulated and 752 genes were down-regulated in the biofilm state. GO and KEGG enrichment analysis showed that, compared with plankplankton, metabolization-related pathways were significantly enriched in biofilm bacteria, followed by amino acid biosynthesis and ABC transporter pathways.Genes associated with biofilm formation, such as those encoding ABC transportersare up-regulated, while genes associated with metabolic pathways are down-regulated.Ten major differentially expressed genes were verified by RT-qPCR, and the differentially expressed trends were consistent with transcriptome sequencing results. These differences may be helpful to study the high drug resistance and virulence of S.aureus in biofilm state.

Key words: transcriptome sequencing, differentially expressed genes, biofilm, Staphylococcus aureus