Unveiling Unusual Features of Formation of Septal Partition and Constriction in Mycobacteria—an Ultrastructural Study

Abstract

The ultrastructural functions of the electron-dense glycopeptidolipid-containing outermost layer (OL), the arabinogalactanmycolic acid-containing electron-transparent layer (ETL), and the electron-dense peptidoglycan layer (PGL) of the mycobacterial cell wall in septal growth and constriction are not clear. Therefore, using transmission electron microscopy, we studied the participation of the three layers in septal growth and constriction in the fast-growing saprophytic species Mycobacterium smegmatis and the slow-growing pathogenic species Mycobacterium xenopi and Mycobacterium tuberculosis in order to document the processes in a comprehensive and comparative manner and to find out whether the processes are conserved across different mycobacterial species. A complete septal partition is formed first by the fresh synthesis of the septal PGL (S-PGL) and septal ETL (S-ETL) from the envelope PGL (E-PGL) in M. smegmatis and M. xenopi. The S-ETL is not continuous with the envelope ETL (E-ETL) due to the presence of the E-PGL between them. The E-PGL disappears, and the S-ETL becomes continuous with the E-ETL, when the OL begins to grow and invaginate into the S-ETL for constriction. However, in M. tuberculosis, the S-PGL and S-ETL grow from the E-PGL and E-ETL, respectively, without a separation between the E-ETL and S-ETL by the E-PGL, in contrast to the process in M. smegmatis and M. xenopi. Subsequent growth and invagination of the OL into the S-ETL of the septal partition initiates and completes septal constriction in M. tuberculosis. A model for the conserved sequential process of mycobacterial septation, in which the formation of a complete septal partition is followed by constriction, is presented. The probable physiological significance of the process is discussed. The ultrastructural features of septation and constriction in mycobacteria are unusually different from those in the well-studied organisms Escherichia coli and Bacillus subtilis.T he presence of a bilayered plasma membrane and a triplelayered cell wall, constituted by an electron-dense outermost layer (OL) containing mostly glycopeptidolipids (11, 37), an electron-transparent layer (ETL) below, consisting of an arabinogalactan-mycolic acid complex (22,36), and an electrondense peptidoglycan layer (PGL), has been observed and characterized through ultrastructural and biochemical studies in Mycobacterium smegmatis (7,12,16,27), Mycobacterium xenopi and other mycobacterial species (reviewed in references 2, 17, 24, and 27), Mycobacterium tuberculosis (3, 9, 33; reviewed in reference 5), Mycobacterium leprae (33), and Mycobacterium kansasii (25,27). Numerous studies had been carried out on the ultrastructures of the OL, ETL, and PGL, their molecular compositions, and the probable roles of these layers in the physiology, pathology, and virulence of different mycobacterial species (6-8; reviewed in references 3 and 5). However, the functional roles of these layers in cell division remained unclear. Studies on mycobacterial cell division (reviewed in refe...

Reduced Permeability to Rifampicin by Capsular Thickening as a Mechanism of Antibiotic Persistence inMycobacterium tuberculosis

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