The attachment of bacteria to sediment particles has a significant impact on the level of bacterial contamination in water. In this work, the attachment of Escherichia coli to particles was investigated with emphasis on the effect of particle size. To exclude the impact of other facts, e.g., the irregularity of particles, we used regular spherical glass particles in addition to natural sediment particles. Both types of particles are mainly composed of SiO₂ and are similar in density. Through a bacterial tracking method together with microfluidic techniques, the attachment of single Escherichia coli cells on the particles was observed. The results showed that only a small portion of the cells that approach the particles remain attached and that the attachment probability per approach increases with surface area for both sediment and glass particles within the size range (8–62 μm) examined in this study. Therefore, finer sediments with more surface area have a higher E. coli attachment capacity. The attachment probability is higher on sediment particles than on glass microspheres of equivalent size, indicating preferential attachment of E. coli to sediment particles. The partition coefficient of the commonly used linear partition model was calculated based on microscopic measurements and the obtained relation of the partition coefficient with attachment probability and particle size was validated with data from the published literature.

Virus-like particles (VLPs), nanoscale self-assembled structures lacking viral genetic material, have emerged as a versatile platform for vaccines, targeted delivery systems, and gene-editing applications owing to their strong immunogenicity, favorable biosafety profile, and high engineerability. However, the complex architecture of VLPs, their significant size heterogeneity, and the diversity of process- and product-related impurities generated in different expression systems make downstream purification a major bottleneck limiting product quality, yield, and manufacturability. This review systematically discusses advanced downstream purification strategies for VLPs from the perspective of three major objectives: preservation of structure and biological activity, control of product heterogeneity, and assurance of viral safety. First, strategies for maintaining VLP integrity and function are examined, including optimization of solution conditions, adoption of gentle yet efficient separation operations, and integration of process analytical technology (PAT) to reduce process-induced damage. Second, the review summarizes multi-step purification approaches—spanning clarification, ultrafiltration/diafiltration (UF/DF), chromatography, and disassembly/reassembly—to remove host cell proteins, host cell DNA, and product-related impurities while improving particle homogeneity and stability. Third, viral safety is discussed primarily from the perspective of downstream virus clearance under host-dependent risk, with particular attention to orthogonal clearance steps tailored to VLP properties and expression systems such as CHO cells and insect cell–baculovirus platforms. Overall, this review provides a CQA-oriented framework and practical guidance for the development of robust, scalable, and GMP-compliant downstream purification processes for VLP-based products.