Mycobacteria Modify Their Cell Size Control under Sub-Optimal Carbon Sources

Abstract

The decision to divide is the most important one that any cell must make. Recent single cell studies suggest that most bacteria follow an “adder” model of cell size control, incorporating a fixed amount of cell wall material before dividing. Mycobacteria, including the causative agent of tuberculosis Mycobacterium tuberculosis, are known to divide asymmetrically resulting in heterogeneity in growth rate, doubling time, and other growth characteristics in daughter cells. The interplay between asymmetric cell division and adder size control has not been extensively investigated. Moreover, the impact of changes in the environment on growth rate and cell size control have not been addressed for mycobacteria. Here, we utilize time-lapse microscopy coupled with microfluidics to track live Mycobacterium smegmatis cells as they grow and divide over multiple generations, under a variety of growth conditions. We demonstrate that, under optimal conditions, M. smegmatis cells robustly follow the adder principle, with constant added length per generation independent of birth size, growth rate, and inherited pole age. However, the nature of the carbon source induces deviations from the adder model in a manner that is dependent on pole age. Understanding how mycobacteria maintain cell size homoeostasis may provide crucial targets for the development of drugs for the treatment of tuberculosis, which remains a leading cause of global mortality.

Analysis of Cell Size Homeostasis at the Single-Cell and Population Level

“…as has been reported for many microbes such as Escherichia coli [2], Caulobacter crescentus [3], B. subtilis [7], M. smegmatis [9] and S. cerevisiae [8]. Using this dependence we can expand the total derivative to obtain…”

“…A phenomenological linear relation between birth and division size has been observed in experiments [9,15,28] …”

“…The weights (1a) follow from the fact that the probability of choosing such a lineage from N T binary trees is 2 −D j /N T and there are D j measurements in the jth lineage (see [9,29] for details). A similar lineage would be observed in the mother machine [2] assuming mother and daughter cells are statistically indistinguishable.…”

“…Since exponential growth is ubiquitously observed in microbes [2,3,[7][8][9], cells must control their division sizes to achieve homeostasis [10][11][12]. A possible strategy for such a control, called a sizer, is to set a stochastic threshold.…”

“…Many microbes, however, rather grow by a constant size from birth to division, called the adder control [3,[13][14][15]. Other mixed strategies may be described by sizer-or timer-like controls [9,15,16].…”