Traditional Methods of Cell Lysis

Although physical methods have traditionally been used to disrupt cells, there are some inherent disadvantages to their use. Localized heating within a sample can occur with many of the techniques described, leading to protein denaturation and aggregation. To avoid this problem, it is essential to pre-chill equipment and keep samples on ice at all times. Reproducibility with homogenization and grinding methods can be challenging due to inexact terminology used to define sample handling. Furthermore, cells disrupt at different times, so the viscosity of the medium constantly changes and released subcellular components are subjected to disruptive forces. In addition to sample handling problems, some physical disruption methods require fairly expensive equipment, such as the French press and sonicator.

The characteristics of reagent-based methods for lysing cells include:

• It is a rapid, gentle, efficient, and reproducible method, and leads to a high protein yield.
• It can be used to extract total protein or subcellular fractions or organelles from various sample types.
• It works by disrupting the lipid membrane and/or cell wall.
• Some components may need to be removed for downstream analysis.
• High concentrations of salts and detergents are not compatible with protein assays and mass spectrometry.

Physical disruption methods characteristics

The characteristics of physical disruption methods for lysing cells include:

• It may require expensive equipment.
• It may be cumbersome to use.
• Reproducibility may vary.
• Mechanical methods are generally not compatible with high-throughput and small volumes.
• Protein denaturation and aggregation can occur.
• Cells disrupt at different times, so subcellular components may be subjected to ongoing disruptive forces.

Mechanical methods rely on the use of rotating blades to grind and disperse large amounts of complex tissue, such as liver or muscle. The Waring blender and the Polytron are commonly used for this purpose. Unlike the Waring blender, which is similar to a standard household blender, the Polytron draws tissue into a long shaft containing rotating blades. The shafts vary in size to accommodate a wide range of volumes, and can be used with samples as small as 1 mL.

Liquid-based homogenization is the most widely used cell disruption technique for small volumes and cultured cells. Cells are lysed by forcing the cell or tissue suspension through a narrow space, thereby shearing the cell membranes. Three different types of homogenizers are in common use. A Dounce homogenizer consists of a round glass pestle that is manually driven into a glass tube. A Potter-Elvehjem homogenizer consists of a manually or mechanically driven PTFE pestle shaped to fit a rounded or conical vessel. The number of strokes and the speed at which the strokes are administered influences the effectiveness of Dounce and Potter-Elvehjem homogenization methods. Both homogenizers can be obtained in a variety of sizes to accommodate a range of volumes. A French press consists of a piston that is used to apply high pressure to a sample volume of 40 to 250 mL, forcing it through a tiny hole in the press. Only two passes are required for efficient lysis due to the high pressures used with this process. The equipment is expensive, but the French press is often the method of choice for breaking bacterial cells mechanically.

Liquid homogenization lysis method.

Sonication is the third class of physical disruption commonly used to break open cells. The method uses pulsed, high frequency sound waves to agitate and lyse cells, bacteria, spores and finely diced tissue. The sound waves are delivered using an apparatus with a vibrating probe that is immersed in the liquid cell suspension. Mechanical energy from the probe initiates the formation of microscopic vapor bubbles that form momentarily and implode, causing shock waves to radiate through a sample. To prevent excessive heating, ultrasonic treatment is applied in multiple short bursts to a sample immersed in an ice bath. Sonication is best suited for volumes <100mL.

Sonication lysis method.

Manual grinding is the most common method used to disrupt plant cells. Tissue is frozen in liquid nitrogen and then crushed using a mortar and pestle. Because of the tensile strength of the cellulose and other polysaccharides comprising the cell wall, this method is the fastest and most efficient way to access plant proteins and DNA.

Manual grinding lysis method.

Cells can be treated with various agents to aid the disruption process. Lysis can be promoted by suspending cells in a hypotonic buffer, which cause them to swell and burst more readily under physical shearing. Lysozyme (200 µg/mL) can be used to digest the polysaccharide component of yeast and bacterial cell walls. Alternatively, processing can be expedited by treating cells with glass beads in order to facilitate the crushing of cell walls. This treatment is commonly used with yeast cells. Viscosity of a sample typically increases during lysis due to the release of nucleic acid material. DNase can be added to samples (25–50 µg/mL) along with RNase (50 µg/mL) to reduce this problem. Nuclease treatment is not required for sonicated material since sonication shears chromosomes. Finally, proteolysis can be a problem whenever cells are manipulated; therefore, protease inhibitors should be added to all samples undergoing lysis.

Hypotonic cell disruption. This is a relatively gentle method that can be used to lyse erythrocytes and extract periplasmic proteins from E. coli. Lysozyme can be used to digest the polysaccharide component of yeast and bacterial cell walls.

Lysozyme improves protein extraction efficiency. To demonstrate the benefit of using lysozyme and DNase I to process bacterial extracts, the extractions of two different-sized, over-expressed proteins were compared. Cell pellets from 50 mL culture of E. coli BL-21 over-expressing green fluorescent protein (GFP) or GST-Ral binding protein (GST-RalBP) were lysed using Thermo Scientific B-PER Reagent with and without lysozyme and DNase I. The soluble fractions were separated from the pellets, and the two fractions were analyzed by SDS-PAGE. Although GFP (32 kDa) was extracted equally well in the absence or presence of enzyme, GST-RalBP (75 kDa) required lysozyme and DNase I to be efficiently solubilized.

1. Walker JM (2009) The Protein Protocols Handbook. Third Edition. New York (NY): Springer-Verlag New York, LLC.