The Streptococcus pneumoniae Pilus-1 Displays a Biphasic Expression Pattern

The Streptococcus pneumoniae pilus-1 is encoded by pilus islet 1 (PI-1), which has three clonal variants (clade I, II and III) and is present in about 30% of clinical pneumococcal isolates. In vitro and in vivo assays have demonstrated that pilus-1 is involved in attachment to epithelial cells and virulence, as well as protection in mouse models of infection. Several reports suggest that pilus-1 expression is tightly regulated and involves the interplay of numerous genetic regulators, including the PI-1 positive regulator RlrA. In this report we provide evidence that pilus expression, when analyzed at the single-cell level in PI-1 positive strains, is biphasic. In fact, the strains present two phenotypically different sub-populations of bacteria, one that expresses the pilus, while the other does not. The proportions of these two phenotypes are variable among the strains tested and are not influenced by genotype, serotype, growth conditions, colony morphology or by the presence of antibodies directed toward the pilus components. Two sub-populations, enriched in pilus expressing or not expressing bacteria were obtained by means of colony selection and immuno-detection methods for five strains. PI-1 sequencing in the two sub-populations revealed the absence of mutations, thus indicating that the biphasic expression observed is not due to a genetic modification within PI-1. Microarray expression profile and western blot analyses on whole bacterial lysates performed comparing the two enriched sub-populations, revealed that pilus expression is regulated at the transcriptional level (on/off regulation), and that there are no other genes, in addition to those encoded by PI-1, concurrently regulated across the strains tested. Finally, we provide evidence that the over-expression of the RrlA positive regulator is sufficient to induce pilus expression in pilus-1 negative bacteria. Overall, the data presented here suggest that the observed biphasic pilus expression phenotype could be an example of bistability in pneumococcus

Reachable Set Bounding for Linear Discrete-Time Systems with Delays and Bounded Disturbances

This paper addresses the problem of reachable set bounding for linear discrete-time systems that are subject to state delay and bounded disturbances. Based on the Lyapunov method, a sufficient condition for the existence of ellipsoid-based bounds of reachable sets of a linear uncertain discrete system is derived in terms of matrix inequalities. Here, a new idea is to minimize the projection distances of the ellipsoids on each axis with different exponential convergence rates, instead of minimization of their radius with a single exponential rate. A smaller bound can thus be obtained from the intersection of these ellipsoids. A numerical example is given to illustrate the effectiveness of the proposed approach. © 2012 Springer Science+Business Media New York

On sliding dynamics bounding for discrete-time systems with state delay and disturbances

This paper addresses the problem of bounding the reachable set of the sliding dynamics in discrete-time systems subject to time-varying state delay and bounded external disturbances. The sliding motion is determined from the equivalent dynamics chosen from a desired eigen-structure via the pole placement technique. New delay-dependent conditions are derived to guarantee that the trajectories in the sliding mode are prescribed in an ellipsoid with a minimal bound on each coordinate. Finally, the proposed approach is illustrated by a numerical example of a quasi-sliding mode controller. © Institution of Engineers Australia 2012

Robust exponential stabilization of underactuated mechanical systems in the presence of bounded disturbances using sliding mode control

This article addresses the robust exponential stabilization problem of underactuated mechanical systems in the presence of bounded external disturbances using sliding mode control. Based on the Lyapunov method, a sufficient condition for the existence of the smallest possible ball which bounds the reduced-order sliding mode dynamics, is first derived in terms of linear matrix inequality (LMI). A sliding mode controller is then synthesized to guarantee that system state trajectories are exponentially convergent to another ball with a prespecified convergence rate. A case study of the Pendubot is provided to illustrate the effectiveness of the proposed approach. © 2013 IEEE

Partial state estimation for linear systems with output and input time delays

This paper deals with the problem of partial state observer design for linear systems that are subject to time delays in the measured output as well as the control input. By choosing a set of appropriate augmented Lyapunov-Krasovskii functionals with a triple-integral term and using the information of both the delayed output and input, a novel approach to design a minimal-order observer is proposed to guarantee that the observer error is ε-convergent with an exponential rate. Existence conditions of such an observer are derived in terms of matrix inequalities for the cases with time delays in both the output and input and with output delay only. Constructive design algorithms are introduced. Numerical examples are provided to illustrate the design procedure, practicality and effectiveness of the proposed observer. © 2013 ISA

Improved reachable set bounding for linear systems with discrete and distributed delays

This paper addresses the problem of reachable set bounding for linear systems in the presence of both discrete and distributed delays. The time delay is assumed to be differentiable and vary within an interval. By using the Lyapunov-Krasovskii approach and delay decomposition technique, improved delay-dependent conditions for the existence of an ellipsoid-based bound of reachable sets of the system trajectories are derived in terms of matrix inequalities. Here, the new idea is to minimize the ellipsoids' projection distances on each axis with different exponential convergence rates, instead of minimizing the ellipsoidal radius with a single exponential rate. A smaller bound can thus be obtained from the intersection of these ellipsoids. The effectiveness of the proposed approach is illustrated by a numerical example. © 2012 IEEE