## 1 Introduction  
A biofilm is tightly grouped mass of microorganisms encased in extracellular polymeric substances to provide cellular protection against host cellular and chemical responses [1]. Biofilms are formed on the surface of food products, making food spoilage and leading to serious health problems and economic losses [2]. The non-thermal sterilization techniques commonly applied in the food industry, such as sodium hypochloride and peroxyacetic acid, are very efficient at controlling foodborne pathogens existing in the planktonic state [3, 4]. However, most pathogens are found in the biofilm matrix [5], which is 10 to 1000 times more resistant to sanitizer than in the planktonic state [6]. Due to the limitations of existing non-thermal methods for the control of bacterial biofilms in food,such as high cost and negative impact on food, human, and the environment, it is of interest in exploring new non-polluting and effective anti-biofilm methods that can be used in the food industry.  
The formation of bacterial biofilms is a dynamic process of repeated cycles and can be divided into four stages: initial attachment, early formation, maturation, and shedding [7]. In the initial attachment, planktonic bacteria attach to the substrate surface through fimbria and/or flagella. This process can be considered as the reversible adhesion [8; 9], involving the proliferation of bacteria cells, the increase in extracellular polymers, and the interaction between bacteria and matrix turned from weak to permanent. In the early stage of biofilm formation, bacterial cells grow closely and begin to secrete a polymer matrix to the surroundings, further transforming into discrete groups of cells [10]. For the maturation, small colonies aggregate into large colonies, forming a specific biofilm structure with the extracellular polymer matrix as a protective barrier. Finally, mature biofilm cells shed and disperse into planktonic cells or form new biofilms [11].  
Cold plasma is a relatively new non-thermal sterilization technology and has been drawing more interest in food sterilization due to its low cost and 'green' feature [12]. Cold plasma can directly treat contaminated surfaces or generate a solution with antimicrobial activity to sanitize surfaces of food and/or processing equipment. When the liquid is exposed to plasma, the reactive species generated in the gas phase are transferred to the liquid phase, such as reactive oxygen species (ROS) and nitrogen species [13]. The treated liquid is named a 'plasma-activated solution' (PAS). PAS has been proven to have good antibacterial activity against planktonic bacteria. Gram-positive bacteria are more resistant to PAS than Gram-negative bacteria [14]. Therefore, _Staphylococcus aureus_ biofilms were used as the model to investigate the inactivation effect of the plasma-activated hydrogen peroxide solution (PAH) in the present study. The bactericidal mechanism of PAH was illustrated with the changes in biofilm structures, ROS content in biofilms, the metabolic capacity of biofilms, and intracellular DNA integrity. This study provided a theoretical basis for the effective treatment of _S. aureus_ biofilms with PAH.