Hydrostatic Filtration for the Isolation of Extracellular Vesicles from Lacticaseibacillus casei BL23

Technical Summary: Hydrostatic Filtration for the Isolation of Extracellular Vesicles from Lacticaseibacillus casei BL23

Problem Statement
Bacterial extracellular vesicles (EVs) are critical mediators of host-microbe interactions and probiotic functionality. However, their study is hindered by the technical limitations of the "gold-standard" isolation method, ultracentrifugation (UC). UC presents drawbacks including potential EV lysis, limited scalability, and high equipment costs. While membrane-based technologies like tangential flow filtration (TFF) offer industrial scalability, they require specialized instrumentation prohibitive for routine bench-scale research. Furthermore, there is a lack of comprehensive comparative data on low-shear, accessible alternatives like Hydrostatic Filtration (HF) for isolating EVs from lactic acid bacteria. Additionally, the concept of non-vesicular extracellular particles (NVEPs), well-characterized in eukaryotes (e.g., exomeres and supermeres), remains largely unexplored in prokaryotes, creating a gap in understanding the full spectrum of bacterial secretomes.

Methodology
The study isolated EVs from Lacticaseibacillus casei BL23 using two distinct methods:

1. Ultracentrifugation (UC): The conventional gold standard involving pre-concentration via ultrafiltration followed by high-speed centrifugation (110,000 ×g, 2 h).
2. Hydrostatic Filtration (HF): A gravity-driven, low-shear method utilizing a vertical column with a 1,000 kDa molecular weight cut-off (MWCO) dialysis membrane. This allowed passive permeation of solvent and low-molecular-weight analytes while retaining larger particles.

The isolated samples were subjected to a rigorous, multi-modal characterization panel to satisfy MISEV2023 recommendations, including:

• Quantification: Nanoparticle Tracking Analysis (NTA) and fluorescence microscopy (EVAnalyzer).
• Biochemical Analysis: SDS-PAGE for protein profiling (specifically p40 and p75), Bradford assay for total protein, and Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy for macromolecular composition.
• Morphological Analysis: Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM).
• Size Distribution: Dynamic Light Scattering (DLS).
• Fractionation: Differential ultracentrifugation was applied to the HF supernatant to isolate sub-20 nm populations (adapted from eukaryotic exomere/supermere protocols).
• Proteomics: Mass spectrometry (nanoHPLC-Orbitrap MS) was used to characterize the protein composition of the isolated fractions.

Key Results

• Yield and Purity: UC yielded approximately 4-fold more particles than HF (confirmed by NTA and fluorescence). Conversely, HF concentrates contained 8-fold more total protein than UC samples, indicating a higher co-isolation of non-vesicular soluble proteins and a lower particle-to-protein ratio.
• Structural Integrity: Despite lower yield, HF successfully isolated EVs with conserved bilayer membrane integrity, spherical morphology, and the presence of key probiotic proteins p40 and p75. ATR-FTIR spectroscopy confirmed that the overall macromolecular identity (lipids, proteins, carbohydrates) was statistically indistinguishable between UC and HF isolates.
• Discovery of a Sub-20 nm Population: DLS analysis revealed a bimodal size distribution in HF samples: a standard EV population and a distinct sub-20 nm population. This sub-20 nm fraction was absent in UC pellets.
• Characterization of Sub-20 nm Particles: Using differential ultracentrifugation (553,000 ×g, 16 h), the authors isolated a "supermere-like" population with a volume-mode diameter of 8 nm. Proteomic analysis of this fraction revealed a distinct profile enriched in cell wall hydrolases (p40, p75) and the moonlighting adhesin glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Unlike the total EV proteome (103 proteins), this fraction contained a core set of 9 proteins.
• Nomenclature: Due to the lack of confirmed bilayer enclosure for the sub-20 nm population, the authors classify them as Extracellular Particles (EPs) rather than EVs, adhering to MISEV2023 guidelines.

Significance and Claims
The paper claims three primary contributions:

1. Validation of Hydrostatic Filtration: HF is established as a robust, low-cost, and structurally conservative alternative to UC for isolating probiotic EVs, maintaining bilayer integrity and key functional markers despite lower particle recovery.
2. Methodological Bias: The study demonstrates that standard UC workflows may actively exclude distinct nanometric populations (specifically the sub-20 nm EPs) that are retained by HF. This suggests that isolation methodology fundamentally dictates which extracellular entities are experimentally visible.
3. Novel Prokaryotic Observation: The identification and partial characterization of a sub-20 nm, non-vesicular EP population in a prokaryote is presented as a novel observation, unprecedented in bacterial research to the authors' knowledge. These particles share functional proteins with EVs but possess a distinct proteomic signature enriched in cell wall hydrolases and metabolic enzymes.

The authors maintain a modest tone regarding the biological role of these sub-20 nm EPs, noting that their designation as "supermere-like" is descriptive and analogical to eukaryotes. They emphasize that further investigation (e.g., cryo-EM, RNA sequencing) is required to confirm their particulate nature and biological function before proposing specific mechanisms in probiotic-host interactions.