Optimal proteome allocation and the temperature dependence of microbial growth laws

Although the effect of temperature on microbial growth has been widely studied, the role of proteome allocation in bringing about temperature-induced changes remains elusive. To tackle this problem, we propose a coarse-grained model of microbial growth, including the processes of temperature-sensitive protein unfolding and chaperone-assisted (re)folding. We determine the proteome sector allocation that maximizes balanced growth rate as a function of nutrient limitation and temperature. Calibrated with quantitative proteomic data for Escherichia coli, the model allows us to clarify general principles of temperature-dependent proteome allocation and formulate generalized growth laws. The same activation energy for metabolic enzymes and ribosomes leads to an Arrhenius increase in growth rate at constant proteome composition over a large range of temperatures, whereas at extreme temperatures resources are diverted away from growth to chaperone-mediated stress responses. Our approach points at risks and possible remedies for the use of ribosome content to characterize complex ecosystems with temperature variation.

Relationship between Escherichia coli growth rate and bacterial susceptibility to ciprofloxacin

The effect of Escherichia coli growth rate on its susceptibility to ciprofloxacin was investigated using bacteria grown on different carbon sources and harboring mutations in genes encoding tricarboxylic acid cycle enzymes. A 1-h treatment of the wild type (wt) grown on glucose, succinate, malate, α-ketoglutarate or acetate with 0.3 μg ml-1 ciprofloxacin decreased the number of surviving cells (CFU ml-1), 560, 110, 74, 62 and 5 times, respectively. Among the mutants tested, sucB strain, which grew 1.75 times slower than wt, was 7.4-fold more tolerant to 0.3 μg ml-1 of ciprofloxacin than wt. Strong inverse correlations between log(CFU ml-1) after 1-h exposure to 0.3 and 3.0 μg ml-1 ciprofloxacin and the specific growth rate prior to antibiotic treatment (r = - 0.93 and -0.96, respectively) were observed. Data from the current and previous studies on the inhibitory effect of ciprofloxacin on cultures exhibiting a wide range of growth rates (0.01-1.3 h-1) were collated. Statistical analysis revealed a significant inverse correlation between log(CFU ml-1) after exposure to 3.0 μg ml-1 of ciprofloxacin and the specific bacterial growth rate prior to antibiotic exposure (r = -0.92). These data may be used in a design of antibiotic treatment protocols.

Ciprofloxacin provokes SOS-dependent changes in respiration and membrane potential and causes alterations in the redox status of Escherichia coli

An in-depth understanding of the physiological response of bacteria to antibiotic-induced stress is needed for development of new approaches to combatting microbial infections. Fluoroquinolone ciprofloxacin causes phase alterations in Escherichia coli respiration and membrane potential that strongly depend on its concentration. Concentrations lower than the optimal bactericidal concentration (OBC) do not inhibit respiration during the first phase. A dose higher than the OBC provokes immediate SOS-independent inhibition of respiration and growth that can contribute to a decreased SOS response and lowered susceptibility to high concentrations of ciprofloxacin. Cells retain their metabolic activity, membrane potential and accelerated K⁺ uptake and produce low levels of superoxide and H₂O₂ during the first phase. The time before initiation of the second phase is inversely correlated with the ciprofloxacin concentration. The second phase is SOS-dependent and characterized by respiratory inhibition, membrane depolarization, K⁺ and glutathione leakage and cessation of glucose consumption and may be considered as cell death. atpA, gshA and kefBkefC knockouts, which perturb fluxes of protons and K⁺, can modify the degree and duration of respiratory inhibition and potassium retention. Loss of K⁺ efflux channels KefB and KefC enhances the susceptibility of E. coli to ciprofloxacin.