Organization of Peptidoglycan Synthesis in Nodes and Separate Rings at Different Stages of Cell Division of Streptococcus pneumoniae

ABSTRACTBacterial peptidoglycan (PG) synthesis requires strict spatial and temporal organization to reproduce specific cell shapes. In the ovoid-shaped, pathogenic bacterium Streptococcus pneumoniae (Spn), septal and peripheral (sidewall-like) PG synthesis occur simultaneously at midcell. To uncover the organization of proteins and activities that carry out these two modes of PG synthesis, we examined Spn cells vertically oriented onto their poles to image the division plane at the high lateral resolution of 3D-SIM (structured-illumination microscopy). Using fluorescent D-amino acid (FDAA) probes, we show that areas of new transpeptidase (TP) activity catalyzed by penicillin-binding proteins (PBPs) separate into a pair of concentric rings early in division, representing peripheral PG (pPG) synthesis (outer ring) and the leading-edge (inner ring) of septal PG (sPG) synthesis. Fluorescently tagged PBP2x or FtsZ locate primarily to the inner FDAA-marked ring, whereas PBP2b and FtsX remain in the outer ring, suggesting roles in sPG or pPG synthesis, respectively. Short pulses of FDAA labeling revealed an arrangement of separate regularly spaced “nodes” of TP activity around the division site of predivisional cells. Control experiments in wild-type and mutant strains support the interpretation of nodal spacing of TP activity, and statistical analysis confirmed that the number of nodes correlates with different ring diameters. This nodal pattern of FDAA labeling is conserved in other ovoid-shaped species. Tagged PBP2x, PBP2b, and FtsX proteins also exhibited nodal patterns with spacing comparable to that of FDAA labeling. Together, these results reveal a highly ordered PG synthesis apparatus in ovococcal bacteria at different stages of division.SIGNIFICANCEThe spatial organization of PBPs and their TP activity at division septa is not well understood. In some bacteria, TP activity and PBP localization seem to be nodal (also called punctate), whereas in other bacteria, discrete foci of PBP activity are infrequently or not observed. Here we report two basic properties of the organization of PBPs and TP activity in the ovoid-shaped bacterium Spn. First, there is distinct spatial separation of the sPG machine, including FtsZ, from the pPG synthesis machine at the midcell of dividing Spn cells. Second, in predivisional cells, PBPs and TP activity are organized heterogeneously into regularly spaced nodes, whose number and dynamic distribution are likely driven by the PG synthesis of PBP:SEDS complexes.

Low-Frequency Seiche in a Large Bay

Short-term observations of sea surface elevations η along the 10-m isobath and long-term observations inside and outside of a large bay (Monterey Bay, CA) were obtained to describe the nodal structure of the modes 0–3 seiches within the bay and the low-frequency (<346 cpd) seiche forcing mechanism. The measured nodal pattern validates previous numerical estimates associated with a northern amplitude bias, though variability exists across the modal frequency band, particularly for modes 0 and 1. Low-frequency oceanic η white noise within seiche frequency bands (24–69 cpd) provides a continuous resonant forcing of the bay seiche with a η2 (variance) amplification of 16–40 for the different modes. The temporal variation of the oceanic η white noise is significantly correlated (R2 = 0.86) at the 95% confidence interval with the bay seiche η that varies seasonally. The oceanic η white noise is hypothesized as being from low-frequency, free, infragravity waves that are forced by short waves.

Fatigue Failures of Centrifugal Impeller Disks

An unusual centrifugal impeller disk failure has been encountered in one of the stages of a multistage air compressor. The failure occurred when a triangular piece of metal separated from the disk at the rim of the impeller disk. Stress analyses show that the steady hoop stresses at the rim were very low. Metallurgical observations indicate that the failure was caused by high cycle fatigue in cast 17-4 PH steel. A modal analysis shows that the failure can be explained by a resonance condition. The excitation of the backward rotating tangential nodal pattern by the stationary vaned diffusers just downstream of the impeller rim causes this resonance condition. Avoiding the resonance condition by changing the number of diffuser vanes has eliminated the failures.

Theory of Receptance Applied to Modal Analysis of a Spinning Disk With Interior Multi-Point Supports

The theory of receptance applied to the modal analysis of a spinning disk with interior multi-point supports is developed. Traveling modes are first introduced for the vibration analysis of a spinning disk. The receptance method is then applied to join the spinning disk and the point supports. The frequency equation and mode shapes are then formulated. An important property, conjugate pairing of the cross receptances is derived. This feature has never been described before in the literature. Numerical results for various numbers of point supports are illustrated. The results show that the nodal pattern of a spinning disk with point supports is varying periodically. The change of rotation speed further causes the mode exchange phenomenon. The change of first critical speed due to point supports is also discussed.

A nodal determination of the Hall conductance

We investigate the nodal structure of the electronic states which arise in the case of a uniform magnetic field and a weak periodic potential. By making use of the continuous motion of these zeros as the K -value labelling the state is varied, we introduce a simple method of obtaining the quantized value of the Hall conductance of a filled band from the nodal pattern of the band wave function. This method is demonstrated to give the correct results in the cases where the Hall conductances are known, given by the solutions of a Diophantine equation.

The Flexural Vibrations of Thin Cylinders

The flexural vibrations of the walls of thin cylinders are considered. In this type of vibration many forms of nodal pattern may exist owing to the combination of circumferential and axial nodes. Theoretical expressions are developed for the natural frequencies of cylinders with freely-supported and fixed ends and a comparison is made with the frequencies obtained experimentally. In practice, the ends of cylinders are subjected to a certain degree of fixing by end-plates, flanges, etc., and the natural frequencies thus lie between the corresponding values for freely-supported and fixed ends. To make possible the estimation of such frequencies, a method is devised in which an equivalent wavelength factor is used. This factor represents the wavelength of the freely-supported cylinder that would have the same frequency as the cylinder under consideration when vibrating in the same mode. The results of experimental investigations with various end thicknesses and flange dimensions are recorded, and from these the equivalent factors are derived. Sets of curves calculated for cylinders with freely-supported ends and covering a range of cylinder thicknesses are given. From these it is possible to obtain close approximation to the frequencies of cylinders under other end conditions by the use of an appropriate factor. An example is given of frequency calculations for a large air-receiver for which two frequencies were identified by experiment.