RTX proteins: a highly diverse family secreted by a common mechanism

Repeats-in-toxin (RTX) exoproteins of Gram-negative bacteria form a steadily growing family of proteins with diverse biological functions. Their common feature is the unique mode of export across the bacterial envelope via the type I secretion system and the characteristic, typically nonapeptide, glycine- and aspartate-rich repeats binding Ca2+ ions. In this review, we summarize the current state of knowledge on the organization of rtx loci and on the biological and biochemical activities of therein encoded proteins. Applying several types of bioinformatic screens on the steadily growing set of sequenced bacterial genomes, over 1000 RTX family members were detected, with the biological functions of most of them remaining to be characterized. Activities of the so far characterized RTX family members are then discussed and classified according to functional categories, ranging from the historically first characterized pore-forming RTX leukotoxins, through the large multifunctional enzymatic toxins, bacteriocins, nodulation proteins, surface layer proteins, up to secreted hydrolytic enzymes exhibiting metalloprotease or lipase activities of industrial interest

Phagocytic activity of peripheral blood and crevicular phagocytes in health and periodontal disease

<b>Background:</b> Neutrophils constitute the main phagocytic cell system in mammalian host defense against an infecting agent. Abnormalities in leukocyte number and function are associated with increased susceptibility to periodontal diseases. The purpose of this study is to evaluate the <i>in vitro</i> phagocytic properties of crevicular and peripheral blood neutrophils in healthy and periodontitis subjects. <b> Patients and Methods:</b> A total of 30 subjects, that is, 10 patients in each of the following three groups: healthy controls, chronic periodontitis (CP), and localized aggressive periodontitis (LAP), were included in the study. The neutrophils were isolated from the peripheral blood and gingival crevice and tested for phagocytosis of <i>Candida albicans</i>. The percentage of leukocytes with ingested <i>C. albicans</i> was determined by light microscopy. <b> Results:</b> A significant reduction in the phagocytic activity of crevicular fluid polymorphonuclear neutrophils (CF-PMN) of LAP subjects (mean: 54.3&#177;7)(<i>P</i>&#60; 0.001) was observed, compared to healthy controls (mean: 74.2&#177;9) and chronic periodontitis subjects (mean: 69&#177;9)(<i>P</i>=0.352). The mean percentage of peripheral blood polymorphonuclear neutrophils (PMNs) with phagocytosis of opsonized <i>C. albicans</i> in LAP patients was significantly reduced (mean: 74.9&#177;5)(<i>P</i>&#60; 0.0068) compared to the phagocytic activity of neutrophils from controls (mean:82.1&#177;3) and chronic periodontitis subjects (mean: 82.0&#177;5)(<i>P</i>=0.970). There was no significant reduction in the phagocytic activity of CF PMNs (mean: 69&#177;9) (<i>P</i>=0.35) and peripheral blood PMNs (mean: 82.5)(<i>P</i>=0.97) in the chronic periodontitis group when compared to the control group. <b> Conclusion:</b> The phagocytic activity of both crevicular and peripheral neutrophils in subjects with periodontitis is altered, increasing the susceptibility to periodontitis. Thus individual susceptibility may be an additional and important modifying factor in the pathogenesis of periodontal disease

Estimation of temporal variations in the magnetic field arising from the motional induction that accompanies seismic waves at a large distance from the epicentre

Temporal variations in the electromagnetic field that accompany earthquakes are generated by various mechanisms, of which this study focuses on variations in the magnetic field arising from motionally induced electric currents that accompany seismic waves at large distances (several hundred kilometres) from the epicentre. A simple situation is considered in which seismic waves are approximated by plane waves and the electrical conductivity of the Earth's crust has a stratified structure. Solutions of Maxwell's equations corresponding to this situation have analytical expressions. Analysis of the solutions verifies that SH waves do not generate variations in the EM field above the ground surface, thereby implying that Rayleigh waves are dominant at a significant distance from earthquake epicentres. Numerical examples demonstrate that the amplitudes of the variations in the magnetic field monotonically increase with increasing conductivity, although attenuation because of the skin effect cannot be ignored. The amplitudes of the generated magnetic field can be sensitive to the conductivity of both the shallow and deep crust. Nevertheless, calculations assuming a simplified conductivity structure provide an upper limit to the possible amplitudes of variations in the magnetic field because of seismic waves. For example, the amplitudes of variations in the magnetic field arising from a Rayleigh wave with displacement amplitude of 10 cm and a period of 30 s are as large as 0.1 nT, close to the limit of detection under typical observation conditions. It is also suggested that phase differences between seismic ground motions and variations in the magnetic field are insignificantly influenced by details of conductivity structures, and they occur within a rather narrow range of values determined by the direction orientation of the ambient geomagnetic field. In the future, if a detection limit of 0.01 nT becomes available, phase difference may be used to distinguish variations arising from the motional induction, from variations arising from other mechanisms

Intra- and Interspecies Regulation of Gene Expression by Actinobacillus actinomycetemcomitans LuxS

The cell density-dependent control of gene expression is employed by many bacteria for regulating a variety of physiological functions, including the generation of bioluminescence, sporulation, formation of biofilms, and the expression of virulence factors. Although periodontal organisms do not appear to secrete acyl-homoserine lactone signals, several species, e.g., Porphyromonas gingivalis, Prevotella intermedia, and Fusobacterium nucleatum, have recently been shown to secrete a signal related to the autoinducer II (AI-2) of the signal system 2 pathway in Vibrio harveyi. Here, we report that the periodontal pathogen Actinobacillus actinomycetemcomitans expresses a homolog of V. harveyi luxS and secretes an AI-2-like signal. Cell-free conditioned medium from A. actinomycetemcomitans or from a recombinant Escherichia coli strain (E. coli AIS) expressing A. actinomycetemcomitans luxS induced luminescence in V. harveyi BB170 >200-fold over controls. AI-2 levels peaked in mid-exponential-phase cultures of A. actinomycetemcomitans and were significantly reduced in late-log- and stationary-phase cultures. Incubation of early-log-phase A. actinomycetemcomitans cells with conditioned medium from A. actinomycetemcomitans or from E. coli AIS resulted in a threefold induction of leukotoxic activity and a concomitant increase in leukotoxin polypeptide. In contrast, no increase in leukotoxin expression occurred when cells were exposed to sterile medium or to conditioned broth from E. coli AIS(−), a recombinant strain in which luxS was insertionally inactivated. A. actinomycetemcomitans AI-2 also induced expression of afuA, encoding a periplasmic iron transport protein, approximately eightfold, suggesting that LuxS-dependent signaling may play a role in the regulation of iron acquisition by A. actinomycetemcomitans. Finally, A. actinomycetemcomitans AI-2 added in trans complemented a luxS knockout mutation in P. gingivalis by modulating the expression of the luxS-regulated genes uvrB and hasF in this organism. Together, these results suggest that LuxS-dependent signaling may modulate aspects of virulence and the uptake of iron by A. actinomycetemcomitans and induce responses in other periodontal organisms in mixed-species oral biofilm