Bacterial cell envelopes: Composition, architecture, and origin

Abstract

The bacterial envelope is a complex layered structure, whose primary function is to protect the cell from the environment. The Gram-staining procedure has been a fundamental method to classify the bacteria for more than 100 years. It was based on the effect of the structure and composition of the bacterial envelope on the staining procedure. It has made it possible to classify the bacteria in two main categories: the Gram-positive, which bind the Gram stain, and Gram-negative bacteria, which do not. Currently, the prokaryotes are classified in 30 phyla of Bacteria and in 5 phyla of Archaea. This classification does not consider the Gram-staining properties which was an excessive simplification. The growing amount of data concerning the genomic sequences of bacteria has significantly amended the view of the bacterial phylogeny. In addition, the access to the genetics of the biogenesis of the cell envelope has also allowed envisioning the bacterial Tree of Life in a different way. The bacterial envelopes are now defined with the number of membranes they contain: the cells are either diderms or monoderms. In addition, the presence or the absence of lipopolysaccharides (LPS) in the diderm envelopes is also a fundamental criterion. This chapter is an overview of current knowledge about the composition and architecture of bacterial envelopes in light of recent data showing that the diversity of structures allows reaching the same main objective, the survival and the protection of the bacterium from its environment.