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Biomedical subjects

S G Johnson

Publications and source records attributed to S G Johnson.

At least 19 recordsLinked to original sources
PubMed2004

Anomalous dispersion relations by symmetry breaking in axially uniform waveguides.

We show that modes of axially uniform waveguides of arbitrary cross section can be made to have anomalous dispersion relations resulting from strong repulsion between two modes. When the axial wave vector k is 0, the two modes have different TE/TM symmetry and thus can be brought arbitrarily close to an accidental frequency degeneracy. For nonzero k, the symmetry is broken causing the modes to repel. When the modes are sufficiently close together this repulsion leads to unusual features such as extremely flattened dispersion relations, backward waves, zero group velocity for nonzero k, atypical divergence of the density of states, and nonzero group velocity at k=0.

Journal Article↗
PubMed2003

Polarization-independent linear waveguides in 3D photonic crystals.

Using a symmetry-based approach, we have designed polarization-independent waveguides in a 3D photonic crystal. A comprehensive series of numerical experiments, involving the propagation of pulsed signals through long straight waveguide sections and sharp bends, quantitatively evaluates the bend-transmission coefficient over the entire bandwidth of the corresponding guided modes. High (approximately 95%) polarization-independent bend transmission is achieved within a certain frequency range.

Journal Article↗
PubMed2000

Three-dimensional control of light in a two-dimensional photonic crystal slab.

Optoelectronic devices are increasingly important in communication and information technology. To achieve the necessary manipulation of light (which carries information in optoelectronic devices), considerable efforts are directed at the development of photonic crystals--periodic dielectric materials that have so-called photonic bandgaps, which prohibit the propagation of photons having energies within the bandgap region. Straightforward application of the bandgap concept is generally thought to require three-dimensional (3D) photonic crystals; their two-dimensional (2D) counterparts confine light in the crystal plane, but not in the perpendicular z direction, which inevitably leads to diffraction losses. Nonetheless, 2D photonic crystals still attract interest because they are potentially more amenable to fabrication by existing techniques and diffraction losses need not seriously impair utility. Here we report the fabrication of a waveguide-coupled photonic crystal slab (essentially a free-standing 2D photonic crystal) with a strong 2D bandgap at wavelengths of about 1.5 microm, yet which is capable of fully controlling light in all three dimensions. These features confirm theoretical calculations on the possibility of achieving 3D light control using 2D bandgaps, with index guiding providing control in the third dimension, and raise the prospect of being able to realize unusual photonic-crystal devices, such as thresholdless lasers.

Journal Article↗
PubMed2000

Population structure, parasitism, and survivorship of sexual and autodiploid parthenogenetic Campeloma limum.

Two theories for the maintenance of sexual reproduction, the Red Queen hypothesis and mutation accumulation, suggest that the dispersal rates of sexuals and asexuals may determine the elimination or persistence of asexuals. Under higher dispersal rates of asexuals, asexuals may temporarily escape virulent parasites and reduce the effects of deleterious mutations. In the present study, I examine the population structure, parasite loads, and juvenile survivorship of Campeloma limum sexuals and autodiploid parthenogens from the southeastern U.S. Atlantic coastal plain. Using mtDNA sequence variation, it is shown that parthenogenetic haplotypes with limited sequence divergence are geographically widespread throughout this region and there is no significant population differentiation over a broad geographical scale. Sexual C. limum populations show significant mtDNA differentiation among and within river drainages and there is significant isolation by distance. These patterns are consistent with a recent origin and range expansion of parthenogens. Prevalence of infection by digenetic trematodes is significantly higher in autodiploid parthenogens, and the variance of prevalence is also higher in autodiploid parthenogens. I argue that the latter pattern indicates that unparasitized parthenogens have temporarily escaped these virulent parasites, but recolonization of these populations by trematodes results in high infection levels (> 40%), possibly due to reduced variation in resistance genes. I also examined whether the survivorship of juvenile sexuals and parthenogens varied under different stress levels. Sexual juveniles had twofold higher survivorship in all environments. Compared to polyploid parthenogens, autodiploid parthenogens may be less buffered against the effects of deleterious recessive alleles. I propose that the combined effects of higher parasitism and reduced juvenile survivorship of these autodiploid parthenogens accounts for the spatial distribution of sexual and parthenogenetic C. limum in the Atlantic coastal plain. Parthenogens may persist by higher dispersal rates into marginal habitats where there is a temporary escape from digenetic trematodes and competition with sexuals.

Animals↗
PubMed1998

Substituted salicylanilides as inhibitors of two-component regulatory systems in bacteria.

A new class of inhibitors of the two-component regulatory systems (TCS) of bacteria was discovered based on the salicylanilide screening hits, closantel (1) and tetrachlorosalicylanilide (9). A systematic SAR study versus a model TCS, KinA/Spo0F, demonstrated the importance of electron-attracting substituents in the salicyloyl ring and hydrophobic groups in the anilide moiety for optimal activity. In addition, derivatives 8 and 16, containing the 2, 3-dihydroxybenzanilide structural motif, were potent inhibitors of the autophosphorylation of the KinA kinase, with IC50s of 2.8 and 6. 3 &microM, respectively. Compound 8 also inhibited the TCS mediating vancomycin resistance (VanS/VanR) in a genetically engineered Enterococcus faecalis cell line at concentrations subinhibitory for growth. Closantel (1), tetrachlorosalicylanilide (9), and several related derivatives (2, 7, 10, 11, 20) had antibacterial activity against the drug-resistant organisms, methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecium (VREF).

Anti-Bacterial Agents↗
PubMed1994

Brooding and the evolution of parthenogenesis: strategy models and evidence from aquatic invertebrates.

Developmental defects are expected to be common and severe in the early evolution of parthenogenesis, and they could help to explain the predominance of sexual forms of reproduction. It is difficult, however, to see how such defects might explain the ecological and phylogenetic correlates of sex. Here we suggest that internally fertilized animals that brood their young may be more susceptible to invasion by parthenogenetic mutants. The reason is that brooders could establish 'selective arenas' in which developmentally defective embryos are competitively displaced. Brooders could also selectively abort defective embryos, and replace them with minimal cost. Consistent with these ideas, we found a striking association between brooding and parthenogenesis in aquatic invertebrates. For example, in the Cnidaria and Mollusca, parthenogenesis is significantly more common in lineages that retain their young through the early stages of development. Hence brooding and ecological factors (such as escape from parasites) might combine to explain the initial spread, long-term persistence, and phylogenetic distribution of parthenogenetic reproduction.

Animals↗
PubMed1992

An in vitro study on the effect of UVA radiation on human gingival fibroblasts.

The recent availability of a 99.9% UVA source has made possible studies that show that low energy wavelengths, previously considered innocuous, significantly affected wound healing in hairless guinea pigs. Decreased wound tensile strength and a slower rate of wound contraction in irradiated animals were among the changes noted. Because of their advocated role in the wound healing process, fibroblasts were chosen to study the effects of pure UVA exposure at a cellular level. 3H-thymidine uptake levels were measured in 8 groups of fibroblast cultures (12 samples/group). The cultures were exposed to varying concentrations of pure UVA. Previously incorporated 14C-thymidine levels were used to compensate for differences in cell numbers between samples. At a fluence of 3.65 x 10(-3) watts/cm2, a significant decrease in 3H-thymidine incorporation (compared to controls) was seen for all exposure periods and there was a dose-dependent decrease only in 3H-thymidine uptake for cells exposed to 1-4 min of UVA. Using post-exposure incubations of 2-16 h, a time-dependent recovery of 3H-thymidine uptake was also demonstrated, from 40% of control at 4 h, to 75% at 8 h, and 99% at 16 h. The near-complete recovery at 16 h was seen in exposures up to 2.73 joules/cm2 (12 min), whereas higher concentrations showed only partial recovery. These studies demonstrated the deleterious, though reversible, effects of UVA on fibroblasts and suggest a possible pathophysiologic process for UVA's effect on wound healing in this animal model.

Cells, Cultured↗
PubMed1992

The effects of ultraviolet irradiation on wound contraction in the hairless guinea pig.

Ultraviolet radiation has been shown to alter wound tensile strength and evoke a number of intracellular changes in fibroblasts. We examined the effects of relatively high doses of ultraviolet radiation on subsequent wound contraction of circular wounds in the hairless guinea pig model. Female hairless guinea pigs were divided into two experimental groups receiving 80 J/cm2 or 480 J/cm2 every other day for 16 weeks. Age-matched unirradiated animals were used as controls. After exposure, all animals had either a 4 mm punch biopsy (80 J/cm2) or a 2.4 cm diameter ((480 J/cm2) Groups 3 and 4) circular area excised from the dorsum. The extent of wound enlargement immediately following wounding of the irradiated animals was decreased as compared to the controls. The rate of wound contraction was significantly lower during early stages of wound contraction in each group of irradiated animals, and wound contraction was significantly slower overall in both groups of irradiated animals compared to controls.

Animals↗
PubMed1992

Macrophage interleukin 1 response to injected silicone in a rat model.

Observations of silicone granuloma formation and migration of silicone to regional lymph nodes have indicated a need for more research into the possible immunological responses to silicone. The present study was undertaken to assess the effect of injected silicone particles on the ability of splenic macrophages to produce interleukin 1 (IL-1) and to determine the relative quantities produced. Lewis rats were divided into 4 groups: Group 1 animals (n = 3) were injected subcutaneously with sterile saline (2.5 ml) and served as control animals; Group 2 animals (n = 3) also served as control subjects, but macrophages isolated from these animals were exposed to lipopolysaccharide (LPS); Group 3 animals (n = 3) were injected subcutaneously with Freund's complete adjuvant (FCA) (2.5 ml) to serve as FCA control animals; and Group 4 animals (n = 3) received a subcutaneous injection of a sonicated slurry of equal parts FCA and silicone (2.5 ml each). IL-1 production was not significantly increased in splenic macrophages from animals exposed to the silicone slurry (p greater than 0.20) 8 months after injection as compared with control animals or animals given FCA alone. Macrophages exposed to LPS, a known mitogen, had significantly elevated IL-1 production. Subcutaneously injected silicone particles did not elicit an increase in IL-1 production in rat macrophages.

Animals↗
PubMed1990

Stimulation of the growth of Streptococcus sanguis (NCTC 7864) by zinc in vitro.

Zinc, which is present in many materials used in dentistry, including restorative and endodontic materials and mouthwashes, is reputedly bactericidal. However, the addition of this element at concentrations in the range 10-50 ppm to cultures of Streptococcus sanguis (NCTC 7864) stimulated the growth of the micro-organism relative to that of controls. A second antibacterial element, namely fluoride, reduced growth. Similar results were obtained whether the organism was grown aerobically or anaerobically. In view of the susceptibility of some organisms to zinc, the results of this study may help to explain conflicting reports in the literature; the results will vary depending on which organism is predominant.

Streptococcus mutans↗
PubMed1989

The modified model cavity method for assessing antibacterial properties of dental restorative materials.

A new in vitro method for assessing the antibacterial properties of dental restorative materials is described with ratios of test material/culture medium volume aiming to simulate conditions around a restoration in vivo. Antibacterial activity is determined by the reduction in optical density of the test culture relative to controls. The method was used for assessment of the antibacterial activity of five dental materials of different composition against five oral bacteria. Release of zinc and fluoride from these materials was also measured and correlated with antibacterial activity. There was a general trend toward greater antibacterial activity with increased zinc release, while fluoride release had a significant effect on only one organism. While all the materials, when freshly mixed, were strongly toxic to three out of the five bacteria studied, much of this activity was lost after the materials had set.

Anti-Infective Agents, Local↗
PubMed1987

Direct assay for testosterone in saliva: relationship with a direct serum free testosterone assay.

A direct non-extraction radioimmunoassay for salivary testosterone is described using a modified commercial kit procedure that is in use for total serum testosterone (T). Serum free testosterone was also measured by direct radioimmunoassay. A significant correlation (r = 0.83, p less than 0.01, n = 194) was obtained between salivary and serum free testosterone in matched serum and saliva samples over a wide range of concentrations. Within- and between-batch precision for the salivary testosterone method was 11% and 18%, respectively at a concentration of 170 pmol/l. Recovery of added T was 89% +/- 15% (mean +/- 2 SD) dilution of high samples showed parallelism. Salivary testosterone measured by direct radioimmunoassay offers a simple cheaper alternative to serum free testosterone measurement with the additional advantages of a stress-free non-invasive sampling procedure.

Adult↗
PubMed1978

Relation between serum pancreatic isoamylase concentration and pancreatic exocrine function.

Serum pancreatic isoamylase concentrations were compared to secretory and clinical evidence of pancreatic insufficiency in 19 consecutive alcoholic patients undergoing pancreatic stimulation testing for suspected pancreatic insufficiency. In patients with normal total serum amylase levels, there was a good correlation (r = 0.83) between serum pancreatic isoamylase activity and stimulated pancreatic secretion of amylase and the 8 patients with a low pancreatic isoamylase concentration had markedly diminished pancreatic secretion of amylase, lipase, and bicarbonate. However, patients with elevated total serum amylase activity frequently had extremely poor pancreatic exocrine function despite normal or elevated levels of pancreatic serum isoamylase. Thus, the finding of a subnormal serum concentration of pancreatic isoamylase provides strong evidence for pancreatic exocrine insufficiency; however, a normal or elevated serum pancreatic isoamylase activity cannot be used as evidence for normal pancreatic exocrine function.

Adult↗