thumb|upright|right|Gram-negative bacteria attempting to grow and divide in the presence of peptidoglycan synthesis-inhibiting antibiotics (e.g. penicillin) fail to do so, and instead end up forming spheroplasts. 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. Antibiotics that inhibit biochemical pathways directly upstream of peptidoglycan synthesis induce spheroplasts too (e.g. fosmidomycin, phosphoenolpyruvate). EDTA acts as a permeabilizer by binding to divalent ions such as Ca<sup>2+</sup> and removing them from the outer membrane.
The yeast Candida albicans can be converted to spheroplasts using the enzymes lyticase, chitinase and β-glucuronidase.
Uses and applications
Antibiotic discovery
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. In this screen devised by Eugene Dulaney, growing bacteria were exposed to test substances under hypertonic conditions. Inhibitors of cell wall synthesis caused growing bacteria to form spheroplasts. This screen enabled the discovery of fosfomycin, cephamycin C, thienamycin and several carbapenems. After a period of time, the cell walls of the filaments are digested, and the bacteria collapse into very large spheres surrounded by just their cytoplasmic and outer membranes. The membranes can then be analyzed on a patch clamp apparatus to determine the phenotype of the ion channels embedded in it. It is also common to overexpress a particular channel to amplify its effect and make it easier to characterize.
The technique of patch clamping giant E. coli spheroplasts has been used to study the native mechanosensitive channels (MscL, MscS, and MscM) of E. coli. It has been extended to study other heterologously expressed ion channels and it has been shown that the giant E. coli spheroplast can be used as an ion-channel expression system comparable to the Xenopus oocyte.
Cell lysis
Yeast cells are normally protected by a thick cell wall which makes extraction of cellular proteins difficult. Enzymatic digestion of the cell wall with zymolyase, creating spheroplasts, renders the cells vulnerable to easy lysis with detergents or rapid osmolar pressure changes. Upon conducting experiments following a modified Hanahan protocol using calcium chloride in E. coli, it was determined that spheroplasts may be able to transform at a frequency of 4.9 × 10<sup>−4</sup>.
See also
- Bacterial morphological plasticity
- Protoplast
- L-form bacteria