Birth and Resuscitation of (p)ppGpp Induced Antibiotic Tolerant Persister Cells

Abstract Transient antibiotic treatment typically eradicates most sensitive bacteria except a few survivors called persisters. The second messenger (p)ppGpp plays a key role in persister formation in Escherichia coli populations but the underlying mechanisms have remained elusive. In this study we induced (p)ppGpp synthesis by modulating tRNA charging and then directly observed the stochastic appearance, antibiotic tolerance, and resuscitation of persister cells using live microscopy. Different physiological parameters of persister cells as well as their regularly growing ancestors and sisters were continuously monitored using fluorescent reporters. Our results confirmed previous findings that high (p)ppGpp levels are critical for persister formation, but the phenomenon remained strikingly stochastic without any correlation between (p)ppGpp levels and antibiotic tolerance on the single-cell level. We could not confirm previous notions that persisters exhibit markedly low concentrations of intracellular ATP or were linked to post-transcriptional effects of (p)ppGpp through the activation of small genetic elements known as toxin-antitoxin (TA) modules. Instead, we suggest that persister cell formation under regular conditions is driven by the transcriptional response to increased (p)ppGpp levels

Parametrically excited helicopter ground resonance dynamics with high blade asymmetries

The present work is aimed at verifying the influence of high asymmetries in the variation of in-plane lead-lag stiffness of one blade on the ground resonance phenomenon in helicopters. The periodical equations of motions are analyzed by using Floquet's Theory (FM) and the boundaries of instabilities predicted. The stability chart obtained as a function of asymmetry parameters and rotor speed reveals a complex evolution of critical zones and the existence of bifurcation points at low rotor speed values. Additionally, it is known that when treated as parametric excitations; periodic terms may cause parametric resonances in dynamic systems, some of which can become unstable. Therefore, the helicopter is later considered as a parametrically excited system and the equations are treated analytically by applying the Method of Multiple Scales (MMS). A stability analysis is used to verify the existence of unstable parametric resonances with first and second-order sets of equations. The results are compared and validated with those obtained by Floquet's Theory. Moreover, an explanation is given for the presence of unstable motion at low rotor speeds due to parametric instabilities of the second order

Determination of Modal Parameters in Complex Nonlinear Systems

This paper describes a methodology for evaluating the modal parameters of complex nonlinear systems. It combines four different tools: the Coleman post-processing, the partial Floquet analysis, the moving window analysis, and the signal synthesis algorithm. The approach provides a robust estimation of the linearized modal parameters and qualitative information about the nonlinear behavior of the system. It operates on one or multiple discrete time signals and is able to deal with both time-invariant and periodic systems. The method is computationally inexpensive and can be used with multiphysics computational tools, and in principle, with experimental data. The proposed approach is validated using a simple, four degree of freedom model of a wind turbine. The predictions for the linear system are validated against an exact solution of the problem. For the nonlinear system, it is demonstrated that qualitative information concerning the nonlinear behavior of the system is obtained using the proposed method. Finally, the nonlinear behavior of a realistic, three-bladed horizontal axis wind turbine model is investigated