Structure of the cell envelope of corynebacteria: importance of the non-covalently bound lipids in the formation of the cell wall permeability barrier and fracture plane

With the recent success of the heterologous expression of mycobacterial ant igens in corynebacteria, in addition to the importance of these bacteria in biotechnology and medicine, a better understanding of the structure of the ir cell envelopes was needed. A combination of molecular compositional anal ysis, ultrastructural appearance and freeze-etch electron microscopy study was used to arrive at a chemical model, unique to corynebacteria but consis tent with their phylogenetic relatedness to mycobacteria and other members of the distinctive suprageneric actinomycete taxon. Transmission electron m icroscopy and chemical analyses showed that the cell envelopes of the repre sentative strains of corynebacteria examined consisted of (i) an outer laye r composed of polysaccharides (primarily a high-molecular-mass glucan and a rabinomannans), proteins, which include the mycoloyltransferase PS1, and li pids; (ii) a cell wall glycan core of peptidoglycan-arabinogalactan which m ay contain other sugar residues and was usually esterified by corynomycolic acids; and (iii) a typical plasma membrane bilayer. Freeze-etch electron m icroscopy showed that most corynomycolate-containing strains exhibited a ma in fracture plane in their cell wall and contained low-molecular-mass porin s, while the fracture occurred within the plasma membrane of strains devoid of both corynomycolate and pore-forming proteins. Importantly, in most str ains, the amount of cell wall-linked corynomycolates was not sufficient to cover the bacterial surface; interestingly, the occurrence of a cell wall f racture plane correlated with the amount of non-covalently bound lipids of the strains. Furthermore, these lipids were shown to spontaneously form lip osomes, indicating that they may participate in a bilayer structure. Altoge ther, the data suggested that the cell wall permeability barrier in coryneb acteria involved both covalently linked corynomycolates and non-covalently bound lipids of their cell envelopes.