Display of proteins on Bacillus subtilis endospores

Abstract

The targeting and anchoring of heterologous proteins and peptides to the outer surface of bacteriophages and cells is becoming increasingly important, and has been employed as a tool for fundamental and applied research in microbiology, molecular biology, vaccinology, and biotechnology. Less known are endospores or spores produced by some Gram-positive species. Spores of Bacillus subtilis are surrounded by a spore coat on their outside, and a few proteins have been identified being located on the outside layer and have been successfully used to immobilize antigens and some other proteins and enzymes. The major advantage of spores over the other published systems is their synthesis within the cytoplasm of the bacterial cell. Therefore, any heterologous protein to be anchored on the outside does not have to cross any membrane. Furthermore, spores are extremely resistant against high temperature, irradiation and many chemicals, and can be stored for many years at room temperature.

Fig. 1

Stages of sporulation. a Vegetative cell in the phase 0; b cell at stage II where the asymmetric septum has been formed; c cell at stage III where the cytoplasmic membrane of the mother cell has engulfed the forespore; d cell at stage IV where formation of the coat has already started; the spore is about to be released from the lysed mother cell. Figure adapted from [104]

Fig. 2

Structure of the spore showing the different compartments. The innermost compartment, the core, contains the DNA and is filled with small acid-soluble proteins that saturate the DNA and protect it and all the other components present in the cytoplasm in a largely dehydrated state. The spore core is surrounded by two membranes sandwiching a specialized peptidoglycan called cortex keeping the water activity relatively low. The spore coat consists of two layers and is composed of about 70 different proteins. Pathogenic and some saprophytic Bacilli possess an additional layer called exosporium consisting of a paracrystalline basal layer and a hair-like nap. A thin section electron micrograph showing a spore with its exosporium has been presented [108]