Asymmetric growth and division in Mycobacterium spp.: compensatory mechanisms for non‐medial septa

Abstract

SummaryMycobacterium spp., rod-shaped cells belonging to the phylum Actinomycetes, lack the Min-and Noc/Slm systems responsible for preventing the placement of division sites at the poles or over the nucleoids to ensure septal assembly at mid-cell. We show that the position for establishment of the FtsZ-ring in exponentially growing Mycobacterium marinum and Mycobacterium smegmatis cells is nearly random, and that the cells often divide non-medially, producing two unequal but viable daughters. Septal sites and cellular growth disclosed by staining with the membranespecific dye FM4-64 and fluorescent antibiotic vancomycin (FL-Vanco), respectively, showed that many division sites were off-centre, often over the nucleoids, and that apical cell growth was frequently unequal at the two poles. DNA transfer through the division septum was detected, and translocation activity was supported by the presence of a putative mycobacterial DNA translocase (MSMEG2690) at the majority of the division sites. Time-lapse imaging of single live cells through several generations confirmed both acentric division site placement and unequal polar growth in mycobacteria. Our evidence suggests that post-septal DNA transport and unequal polar growth may compensate for the non-medial division site placement in Mycobacterium spp.

Single-cell dynamics of the chromosome replication and cell division cycles in mycobacteria

“…1a and Supplementary Movie 2). Analysis of time-lapse image series confirms that the septum in mycobacteria is not always placed exactly at midcell 12,15,16 . In the majority of cells, septum position is skewed towards the new pole ( Fig.…”

“…Moreover, during the early stages of septum formation, visualized as an FM4-64-stained membrane invagination, we find that the septum is already placed asymmetrically. Similarly, it has been shown that initial positioning of the FtsZ ring, one of the earliest recognizable events in division-site selection, is also asymmetric 12 . These observations tend to support a second scenario in which asymmetric division is because of initially skewed positioning of the division septum towards the new cell pole rather than at midcell.…”

“…Consequently, the surface area that is available for insertion of new cell-wall material increases continuously from birth to next division. In contrast, rod-shaped Actinobacteria, including organisms in the genus Mycobacterium, grow by inserting new cell-wall material exclusively at the cell poles [9][10][11][12][13] . As these cells grow by extending their length while maintaining an approximately constant width, the surface area available for insertion of new cell-wall material at the cell poles does not change appreciably between birth and division.…”

“…Recently, it was reported that Mycobacterium smegmatis grows in an unusual unipolar manner by preferential extension of the old cell pole, whereas the new cell pole remains quiescent until the next cell division has been completed 12,15,16 . Unipolar growth was hypothesized to give rise to asymmetric cell division, resulting in sibling cells of unequal sizes and growth velocities, with larger old-pole cells growing faster than their smaller newpole siblings 15 .…”

Staying in Shape: the Impact of Cell Shape on Bacterial Survival in Diverse Environments

“…In contrast to the case in most rod-shaped bacteria, such as E. coli and B. subtilis, where longitudinal growth results from insertion of new cell wall material into the sidewall, polar growth results from new cell wall material inserted only at the cell poles. Interestingly, in mycobacteria this polar growth results in unequally sized daughter cells that elongate at different rates, resulting in population heterogeneity (71)(72)(73)(74). Several studies have reported that daughter cells who inherit the old pole are larger and grow faster than cells which inherit the new pole (71,74).…”