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Concept
Spheroplast
Summary
A spheroplast (or sphaeroplast in British usage) is a microbial cell from which the cell wall has been almost completely removed, as by the action of penicillin or lysozyme. According to some definitions, the term is used to describe Gram-negative bacteria. According to other definitions, the term also encompasses yeasts. The name spheroplast stems from the fact that after the microbe's cell wall is digested, membrane tension causes the cell to acquire a characteristic spherical shape. Spheroplasts are osmotically fragile, and will lyse if transferred to a hypotonic solution.
When used to describe Gram-negative bacteria, the term spheroplast refers to cells from which the peptidoglycan component but not the outer membrane component of the cell wall has been removed.
Various antibiotics convert Gram-negative bacteria into spheroplasts. These include peptidoglycan synthesis inhibitors such as fosfomycin, vancomycin, moenomycin, lactivicin and the β-lactam antibiotics. Antibiotics that inhibit biochemical pathways directly upstream of peptidoglycan synthesis induce spheroplasts too (e.g. fosmidomycin, phosphoenolpyruvate).
In addition to the above antibiotics, inhibitors of protein synthesis (e.g. chloramphenicol, oxytetracycline, several aminoglycosides) and inhibitors of folic acid synthesis (e.g. trimethoprim, sulfamethoxazole) also cause Gram-negative bacteria to form spheroplasts.
The enzyme lysozyme causes Gram-negative bacteria to form spheroplasts, but only if a membrane permeabilizer such as lactoferrin or ethylenediaminetetraacetate (EDTA) is used to ease the enzyme's passage through the outer membrane. EDTA acts as a permeabilizer by binding to divalent ions such as Ca2+ and removing them from the outer membrane.
The yeast Candida albicans can be converted to spheroplasts using the enzymes lyticase, chitinase and β-glucuronidase.
From the 1960s into the 1990s, Merck and Co. used a spheroplast screen as a primary method for discovery of antibiotics that inhibit cell wall biosynthesis.
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