## 1 Introduction  
During the last decade, interest in the use of pDNA vaccines has strongly increased [1]. The pDNA vaccines are believed to be potentially useful in a wide range of applications including preventive and therapeutic vaccines for viral, bacterial, and parasitic diseases as well as cancer [2,3]. The production of pDNA follow the steps of fermentation, primary and intermediate recovery, and purification. After the fermentation step, a sequence of operations is followed to recover pDNA molecules from the host cells and to remove impurities and contaminants from the lysate, until the desired level of purity and other specifications are obtained [4]. Cell disruption is the first and the most critical step in the bioprocess since this step influences the yield and quality of the product [5,6]. The cell disruption method for plasmid isolation must be chosen such that minimal damage is caused on the pDNA product, and in most cases, it is also desired to avoid shearing of the host cell genomic DNA (gDNA) into smaller fragments that are more difficult to separate from pDNAA [7]. Typically, the disintegration of bacterial cells for pDNA production is performed by alkaline treatment described by [PERSON] and [PERSON] [8]. The method successfully removes most of the cell walls, organelles, proteins, and gDNA leaving RNA as the main contaminant of the target plasmid together with some proteins [9]. During this procedure, cells are subjected to an alkaline solution such as NaOH containing a detergent as SDS. Under this condition, cell wall structures are solubilized, thereby releasing the plasmid and other cell-related compounds, resulting in a highly viscous solution. While gDNA and proteins are denatured at the resulting pH-value, pDNA stays intact if homogeneous mixing is provided to avoid local pH-extremes that would lead to an irreversible denaturalization of the pDNA and the formation of undesired isoforms and pDNA loss trapped in flocs [7]. Several works aimed to improve the alkaline lysis method have been reported. [PERSON] et al. [10], developing a one-step approach for isolate pDNA, found that non-ionic detergent (NID) plasmid DNA performs better than alkaline treatment in many downstream applications. [PERSON] et al. [11] developed an improved method for the extraction of community DNA from different environmental and human origin samples by a combination of physical, chemical, and mechanical lysis methods for proper lysis of microbial DNA. A novel approach to lyse cells on-chip through the application of electric discharges can be used in plasmid recovery [12]. The combined action of enzymes, detergents, and high temperatures constitutes an interesting alternative for the large-scale disruption of _E. coli_ cells and plasmid release [13]. Although the method works well at the laboratory scale, the implementation of the high-temperature batch incubation at a process scale is clearly problematic [5]. The heat lysis is possibly the most commonly used method after alkaline lysis [14]. Recently, [PERSON] et al. [15] described various methods and devices aimed at performing alkaline lysis at a large scale. Unfortunately, the commonly used cell lysis methods (non-mechanical: chemical, physical, or enzymatic) prove efficient only at the laboratory scale [16; 17] but fail to address the needs of large-scale cell disruption processes [18]. Mechanical methods, on the other hand, are promising as these can be scaled up with minimum or no pre-treatment/chemical additives and can be used in batch or continuous modes. Bead milling is one of the most preferable mechanical cell lysis methods at the industrial scale due to its ease of operation, controllability, and ability to load concentrated cell slurry [17; 19]. This method has been a subject of continuing research in batch and continuous modes [19; 20].