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Public Library of Science, PLoS Pathogens, 3(6), p. e1000794, 2010

DOI: 10.1371/journal.ppat.1000794

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Direct Visualization by Cryo-EM of the Mycobacterial Capsular Layer: A Labile Structure Containing ESX-1-Secreted Proteins

Journal article published in 2010 by Musa Sani, Edith N. G. Houben ORCID, Jeroen Geurtsen, Jason Pierson, Karin de Punder, Maaike van Zon, Brigitte Wever, Sander R. Piersma, Connie R. Jiménez, Mamadou Daffé, Ben J. Appelmelk, Wilbert Bitter ORCID, Nicole van der Wel ORCID, Peter J. Peters
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

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 ill-characterized 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, alpha-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.