Bacterial Evolution in High-Osmolarity Environments

Abstract

Bacteria must maintain a cytosolic osmolarity higher than that of their environment in order to take up water. High-osmolarity environments therefore present formidable stress to bacteria. To explore the evolutionary mechanisms by which bacteria adapt to high-osmolarity environments, we selected Escherichia coli in media with a variety of osmolytes and concentrations for 250 generations. Adaptation was osmolyte dependent, with sorbitol stress generally resulting in increased fitness under conditions with higher osmolarity, while selection in high concentrations of proline resulted in increased fitness specifically on proline. Consistent with these phenotypes, sequencing of the evolved populations showed that passaging in proline resulted in specific mutations in an associated metabolic pathway that increased the ability to utilize proline for growth, while evolution in sorbitol resulted in mutations in many different genes that generally resulted in improved growth under high-osmolarity conditions at the expense of growth at low osmolarity. High osmolarity decreased the growth rate but increased the mean cell volume compared with growth on proline as the sole carbon source, demonstrating that osmolarity-induced changes in growth rate and cell size follow an orthogonal relationship from the classical Growth Law relating cell size and nutrient quality. Isolates from a sorbitol-evolved population that captured the likely temporal sequence of mutations revealed by metagenomic sequencing demonstrated a trade-off between growth at high osmolarity and growth at low osmolarity. Our report highlights the utility of experimental evolution for dissecting complex cellular networks and environmental interactions, particularly in the case of behaviors that can involve both specific and general metabolic stressors. IMPORTANCE For bacteria, maintaining higher internal solute concentrations than those present in the environment allows cells to take up water. As a result, survival is challenging in high-osmolarity environments. To investigate how bacteria adapt to high-osmolarity environments, we maintained Escherichia coli in a variety of high-osmolarity solutions for hundreds of generations. We found that the evolved populations adopted different strategies to improve their growth rates depending on the osmotic passaging condition, either generally adapting to high-osmolarity conditions or better metabolizing the osmolyte as a carbon source. Single-cell imaging demonstrated that enhanced fitness was coupled to faster growth, and metagenomic sequencing revealed mutations that reflected growth trade-offs across osmolarities. Our study demonstrated the utility of long-term evolution experiments for probing adaptation occurring during environmental stress.

GABA Production by Human Intestinal Bacteroides spp.: Prevalence, Regulation, and Role in Acid Stress Tolerance

“…to cope with the dynamic physicochemical conditions of the human large intestine. On one hand as a potential contributor to osmoregulation to survive osmotic shifts resulting from water absorption and transport of ions by colonic epithelial cells ( Costongs et al, 1985 ; Cesar et al, 2020 ); on the other hand to cope with the dynamic pH across the large intestine. On average, the pH of the human adult colon ranges from 5.7 (proximal) to 6.7 (distal; Fallingborg, 1999 ), but varies among individuals with values as low as 5.0 reported in healthy subjects ( Koziolek et al, 2015 ).…”

Competition for fluctuating resources reproduces statistics of species abundance over time across wide-ranging microbiotas

“…Our CR model and fitting procedure can also be used to aid the parametrization of other models such as Lotka-Volterra models ( Figure 4—figure supplements 1 – 5 ), comparisons among which can reveal the model details that are required to recapitulate experimental data. In the future, more detailed hypotheses can be generated by investigating how time series statistics are affected by modifications to baseline CR dynamics, such as the incorporation of metabolic cross-feeding ( Goldford et al, 2018 ; Li et al, 2020 ) or physical interactions such as type VI killing ( Verster et al, 2017 ), functional differentiation from genomic analysis ( Arkin et al, 2018 ; Machado et al, 2021 ; Pollak et al, 2021 ), and physical variables such as pH ( Aranda-Díaz et al, 2020 ; Ratzke and Gore, 2018 ), temperature ( Lax et al, 2020 ), and osmolality ( Cesar et al, 2020 ). In addition, recent studies have shown that evolution can substantially affect the dynamics of human gut microbiotas ( Garud et al, 2019 ; Yaffe and Relman, 2020 ; Zhao et al, 2019 ).…”

Application of data integration for rice bacterial strain selection by combining their osmotic stress response and plant growth-promoting traits

“…It has been shown earlier that under non-stress conditions, bacteria favor phytohormone synthesis rather than osmolyte accumulation, whereas the opposite occurs under stress conditions. Such a shift in metabolic activity is common in bacterial cells, allowing them to maintain homeostasis when there is an increase in osmotic pressure in the extracellular medium ( Varela et al, 2004 ; Lahtvee et al, 2014 ; Cesar et al, 2020 ).…”