A spheroplast is a modified microbial cell, typically bacterial or fungal, that has undergone a significant alteration to its outer protective layer. This form arises when most, but not all, of the rigid cell wall is removed. The resulting structure is characterized by its altered shape and increased fragility. This modification makes spheroplasts a valuable tool in various scientific investigations.
Understanding Spheroplasts
A spheroplast is a cell that has lost the majority of its cell wall, but not every component of it. This partial removal of the cell wall causes the cell to adopt a characteristic spherical shape, which gives it its name. The absence of a rigid cell wall makes spheroplasts highly susceptible to osmotic shock, meaning they can easily burst or shrivel if the surrounding environment’s water concentration changes too drastically.
To survive, spheroplasts must be maintained in an isotonic solution, an environment where the concentration of solutes outside the cell is similar to that inside the cell. This balanced environment prevents water from excessively moving into or out of the cell, thus preserving its structural integrity. Without the protective cell wall, the delicate cell membrane is exposed, making the cell vulnerable to environmental stresses like mechanical forces or changes in osmotic pressure.
How Spheroplasts are Created
Spheroplasts are generated through controlled enzymatic or chemical treatments that selectively degrade the cell wall. A common method for bacteria involves using enzymes such as lysozyme, which specifically targets and breaks down peptidoglycan, a major component of bacterial cell walls. For fungal cells, enzymes like glucanases are employed to digest their cell wall components.
Chemical methods can also induce spheroplast formation, such as exposing bacteria to penicillin, an antibiotic that inhibits the synthesis of new peptidoglycan. As the cells attempt to grow and divide in the presence of penicillin, they fail to properly construct their cell walls, leading to the formation of spheroplasts. Throughout this process, it is necessary to maintain an osmotically balanced environment, often by adding agents like sucrose or magnesium sulfate to the solution. This prevents the cells from lysing due to osmotic pressure changes as their protective wall is compromised.
Spheroplasts Compared to Protoplasts
While both spheroplasts and protoplasts are forms of cells that lack their full cell walls, a clear distinction exists. A spheroplast retains some residual cell wall material, particularly the outer membrane in the case of Gram-negative bacteria, even after enzymatic or chemical treatment.
In contrast, a protoplast has its entire cell wall completely removed, leaving only the cell membrane as the outermost boundary. Gram-negative bacteria are associated with spheroplast formation because their cell wall structure includes an outer membrane that remains intact during the process. Conversely, Gram-positive bacteria and plant cells, which lack this outer membrane, form true protoplasts when their cell walls are fully digested.
Practical Uses of Spheroplasts
Spheroplasts are valuable tools across various scientific fields, particularly in molecular biology and biotechnology. Their compromised cell wall makes them more permeable, facilitating the introduction of foreign DNA into cells, a process known as genetic transformation. This enhanced permeability is also beneficial for cell fusion, a technique used to combine cells and create hybrid organisms, notably in yeast research.
Spheroplasts are also used in studying cell wall synthesis, providing a model to understand how these structures are built and how certain antimicrobial agents might interfere with their formation. Their exposed cell membrane allows for easier isolation and study of membrane-bound proteins, which are difficult to access in intact cells. Spheroplasts are also used in drug screening assays, particularly for identifying new antimicrobial compounds that target bacterial cell wall biosynthesis.