Session F58: Implications of Single-cell Variability: From Cells to Populations

The effects of stochasticity at the single-cell level and cell size control on the population growth

Establishing a quantitative connection between the population growth rate and the generation time distribution of single cells is a prerequisite for understanding evolutionary dynamics of microbes. However, existing theories fail to account for the experimentally observed correlations between mother-daughter generation times that are unavoidable when cell size is controlled for - which is typically the case. Here, we study population-level growth in the presence of cell size control. We derive a closed formula for the population growth rate and demonstrate that it only depends on the single-cell growth rate variability, not other sources of stochasticity or the details of the size control mechanism. We corroborate our theory using experimental data on E. coli. Our work provides an evolutionary rationale for the narrow growth rate distributions often observed in nature: when single-cell growth rates are less variable but have a fixed mean, the population will exhibit an enhanced population growth rate.   

Adding it up: mycobacteria growth heterogeneity and antibiotic susceptibility

Mycobacterium tuberculosis infects billions of people worldwide and kills more than 1.5 million per year. TB remains extremely difficult to treat with antibiotics, requiring months to years of therapy for cure. The variable course of disease and treatment response suggests that functionally heterogeneous populations of mycobacteria respond differently to stress. Using a quantitative single-cell approach, we show that mycobacteria deterministically generate diversity in their growth characteristics through asymmetric growth and maintain a controlled but asymmetric chromosomal organization pattern. Coupled with a unique mechanism of cell size regulation utilizing parallel adders from initiation, this asymmetry creates subpopulations of cells with distinct growth rates and cell sizes that are differentially susceptible to clinically relevant classes of antibiotics. We find that combinations of inherent and temporal properties of individual cells describe subpopulations susceptible to different antibiotics. Thus, the polar growth pattern intrinsic to mycobacteria deterministically creates a diverse population structure that may underlie phenotypes previously thought to be controlled by external stressors. Mathematical models based on measurements from new reporters of cell state enable us to better understand how mycobacteria manage growth variation and generate subpopulations with distinct behaviors arising from asymmetric growth and division.   

Population Dynamics of Antimicrobial Peptides are Driven by Single-cell Heterogeneities and Retention of Peptides in Dead Cells

Antimicrobial peptides (AMPs) are natural antibiotics of multicellular systems that utilize electrostatics to target bacteria selectively. Like most antibiotics, AMPs need a minimum concentration to inhibit the growth of a bacterial culture. Unlike other antibiotics, AMPs’ distribution and kinetics in the culture is dictated by electrostatics. In this talk, I present our recent data mapping a quantitative picture of these dynamics. Surprisingly, we evidence that the minimum inhibitory concentration (MIC) of AMPs is strongly dependent on the cell density, even in dilute cultures where direct cell-to-cell interactions are minimal. We hypothesize that this dependence is due to absorption of a significant number of AMPs in individual cells, which can reduce the effective concentration of AMPs in the culture. To investigate this hypothesis, we utilized a single-cell imaging platform to track dye-tagged AMPs and the time evolution of their translocation into the bacteria. Our single-cell analysis confirms that bacteria not only absorb a significant fraction of AMPs, but also retain them after cell death, which sequesters AMPs’ availability for attacking more cells. Further, we developed a theoretical model of which recapitulates experimental behavior using AMP retention hypothesis.   

How diversity modulates collective migration and vice versa

Phenotypic diversity and collective behavior are important properties of living communities that provide selective advantages. While collective behavior requires coordination between individuals, phenotypic diversity tends to reduce coordination. I will report on our recent experimental and theoretical results that used bacterial chemotaxis as model system to examine how phenotypic heterogeneity modulates group performance and how collective behavior affects the amount of diversity in the group.   

Lineages, Growth, and Selection in Heterogeneous Populations

This talk will highlight theoretical and experimental results on the relation between single cell lineages and the selective forces acting on phenotypic states within growing bacterial populations.  The main focus will be on the empirical problem of measuring the strength of selection acting on quantitative traits, such as protein expression levels and production rates, using single cell lineage data.  A general method will be presented and applied to infer the strength of selection acting on the expression dynamics of an antibiotic resistance gene.