Direct visualization by Cryo-EM of the mycobacterial capsular layer: A labile structure containing ESX-1-secreted proteins

The cell envelope of mycobacteria,a group of gram positive bacteria,is composed of a plasma membrane and a gram-negative-like outer membrane containing mycolic acids. in addition,the surface of the mycobacteria is coated with an illcharacterized layer of extractable,non-covalently linked glycans,lipids and proteins,collectively known as the capsule,whose occurrence is a matter of debate. by using plunge freezing cryo-electron microscopy technique,we were able to show that pathogenic mycobacteria produce a thick capsule,only present when the cells were grown under unperturbed conditions and easily removed by mild detergents. this detergent-labile capsule layer contains arabinomannan,α-glucan and oligomannosyl-capped glycolipids. further immunogenic and proteomic analyses revealed that mycobacterium marinum capsule contains high amounts of proteins that are secreted via the esx-1 pathway. finally,cell infection experiments demonstrated the importance of the capsule for binding to cells and dampening of pro-inflammatory cytokine response. together,these results show a direct visualization of the mycobacterial capsular layer as a labile structure that contains esx-1-secreted proteins. © 2010 sani et al.