Investigation of energy transfer in terbium doped Y (2)SiO(5) phosphor particles.

The kinetics of luminescence of sol-gel synthesized terbium doped Y (2)SiO(5) (YSO) phosphor particles is investigated in detail with reference to Tb concentration in the 0.001%-10% range. By increasing the dopant concentration, the luminescence profile changes from a blue to a green peaked emission spectrum because of the energy transfer among centers. The inter-center energy transfer mechanism is well accounted for by the Inokuti-Hirayama (IH) kinetic model which is based on a statistical average of inter-center distance dependent decay modes of the donor luminescence. The distribution of the decay modes is implemented from the Förster-Dexter resonance theory of energy transfer by assuming a rate constant for the energy transfer by multipolar interactions between donors and acceptors. However, the experimental results recorded in the low concentration limit show the presence of green emission contributions in the luminescence spectrum which cannot be related to the Tb concentration; for this reason an additional internal energy transfer mechanism, occurring among levels of the same center, is proposed to account for the recorded emission properties. Thus, a new and more exhaustive model which includes both the internal and external energy transfer processes is considered; the proposed model allows a better explanation of the spectroscopic features of Tb related centers in YSO crystals and discloses the critical concentration and the quantum yields of the different energy transfer mechanisms.

Emission properties of Tb3+ ions in LYSO: evidence of a cross relaxation mechanism explained by a kinetic model.

The optical properties of Tb(3+) ions in oxyorthosilicates of lutetium and yttrium (LYSO) are reported. The introduction of a small number of terbium ions (nominal content 10 ppm) generates, in the otherwise transparent absorption spectrum of the matrix, an ultraviolet absorption band peaked at about 240 nm. By exciting within the reported UV band, line shaped emissions in the 350-600 nm range are detected. These transitions are related to the (5)D(3) and (5)D(4) levels of the Tb(3+) ions and are characterized by decay times in the millisecond time domain. Analysis of the decay time measurements allows us to individuate a cross relaxation mechanism among terbium ions even at the low dopant concentration investigated. We propose a three-level kinetic model which is able to successfully reproduce the experimental data, allowing us to discriminate among the radiative and non-radiative contributions to the observed emissions.

Proof of the thermodynamical stability of the E' center in SiO2.

The E' center is a paradigmatic radiation-induced defect in SiO2 whose peculiar EPR and hyperfine activity has been known for over 40 years. This center has been traditionally identified with a distorted, positively charged oxygen vacancy V(+)O. However, no direct proof of the stability of this defect has ever been provided, so that its identification is still largely incomplete. Here we prove directly that distorted V(+)O is metastable and that it satisfies the key requirements for its identification as E', such as thermal and optical response, and activation-deactivation mechanisms.

A limitation of 2-stage serological testing for Lyme disease: enzyme immunoassay and immunoblot assay are not independent tests.

To improve the accuracy of testing for antibody to Borrelia burgdorferi, 2-stage conditional testing has been recommended, in which sera that yield positive or equivocal results in a first-stage test (e.g., an ELISA) are then tested by immunoblot assay. The increased specificity anticipated with sequential testing, however, depends on immunoblot assays and ELISAs being independent tests. To examine whether they are independent, control serum samples were tested with 2 different commercially available IgM ELISAs and with an IgM immunoblot assay kit. The frequency of false-positive IgM immunoblot assays was significantly higher with ELISA-reactive than with ELISA-negative serum samples (P

Effects of OspA vaccination on Lyme disease serologic testing.

This study presents the effects of OspA vaccination on two-step testing for Borrelia burgdorferi antibodies. Although vaccinees developed enzyme-linked immunosorbent assay reactivity, immunoblots did not fulfill Centers for Disease Control and Prevention criteria for positivity. Furthermore, OspA reactivity did not interfere with interpretation of immunoblots with sera from patients who developed early Lyme disease despite vaccination.

Positive Lyme disease serology in patients with clinical and laboratory evidence of human granulocytic ehrlichiosis.

In 10 consecutive patients with an acute febrile illness, human granulocytic ehrlichiosis was confirmed with specific polymerase chain reaction studies, serologic conversion, or both. Although no patients had the clinical features most suggestive of early Lyme disease (eg, erythema migrans or cranial nerve palsy), tests for antibody to Borrelia burgdorferi produced a reaction in most patients. In 6 of 7 patients (86%) with evaluable results, enzyme-linked immunosorbent assay yielded positive or equivocal findings, and an immunoblot technique yielded positive findings in 60% to 90% of patients, depending on the criteria used for interpretation. Inasmuch as approximately 25% of nymphal Ixodes scapularis ticks in Westchester County, New York, are infected with B burgdorferi, the probability that at least 9 of these patients were coinfected with B burgdorferi and human granulocytic ehrlichiosis by the same tick bite is estimated to be .00003. These observations suggest that serodiagnosis is insufficient to establish the presence of coinfection with B burgdorferi.

Outbreak of vancomycin-, ampicillin-, and aminoglycoside-resistant Enterococcus faecium bacteremia in an adult oncology unit.

An outbreak of bacteremia caused by Enterococcus faecium with high-level resistance to vancomycin (MIC of > or = 256 micrograms/ml), ampicillin (MIC of > or = 64 micrograms/ml), and gentamicin or streptomycin (MIC of > or = 2,000 micrograms/ml) occurred in an adult oncology unit from June 1991 to May 1992. Active surveillance for the presence of this organism in stool or perianal cultures was begun in September 1991. Between June 1991 and May 1992, seven patients with bacteremia and 22 noninfected carriers of the organism in stool were identified. The vanA gene, tested for by PCR and gene probe, was present in all isolates evaluated. All bacteremic patients also had resistant E. faecium present in a stool or perianal culture; the stool isolates tested were closely related to the respective blood isolates as determined by pulsed-field gel electrophoresis. Antibiotic regimens using high-dose ampicillin and an aminoglycoside were ineffective with four patients. Five patients (71%) had multiple positive blood cultures; four of these patients died. Following a multiple logistic regression analysis, it was found that bacteremic patients received a significantly greater number of total antibiotic days compared with noninfected stool carriers (P = 0.019). The emergence of E. faecium with high-level resistance to vancomycin, ampicillin, and aminoglycosides underscores the importance of performing susceptibility testing on all clinically significant isolates. In the neutropenic adult oncology patient, bacteremia with this organism is of probable gastrointestinal origin, is often persistent, and is refractory to treatment with ampicillin in combination with an aminoglycoside. Prolonged use of antibiotics may predispose patients with gastrointestinal colonization to develop bacteremia.

Serodiagnosis in early Lyme disease.

Using a commercially available enzyme-linked immunosorbent assay (ELISA) and an immunoblot assay (IB), we tested sera from 100 patients with erythema migrans (EM) seen in 1991 a the Westchester County Medical Center Lyme Disease Diagnostic Center. Convalescent-phase sera were available from 59 patients. Fifty-five patients had EM of < 7 days' duration, 31 had EM of 7 to 14 days' duration, and 14 had EM of > 14 days' duration. During the acute phase of infection, 35 patients had a positive ELISA result and 43 had a positive IB result by the recently published criteria of Dressler et al. (F. Dressler, J. A. Whalen, B. N. Reinhardt, and A. C. Steere, J. Infect. Dis. 167:392-400, 1993) for interpretation of IB in patients with Lyme disease. A greater sensitivity of IB was observed in patients with EM of < 7 days' duration, as follows: 14 of 55 (25%) for IB versus 7 of 55 (13%) for ELISA (P = 0.144). Sera of all 14 patients with EM of > 14 days' duration were reactive by both tests, as follows: 13 positive and 1 equivocal by ELISA and 12 positive and 2 indeterminate by the IB. The band reactivity most frequently observed in the IB was to the 41- and 25-kDa antigens, the latter being the most frequent band observed in immunoglobulin M blots. Seroconversion was observed in 74 and 64% of evaluable patients by ELISA and IB, respectively, despite the use of antibiotic therapy.

A bacterial pathogenicity determinant associated with necrotizing enterocolitis.

Predominant enterobacteria from infants with necrotizing enterocolitis (NEC) were examined for an unusual ability to ferment lactose. One such isolate, a Klebsiella pneumoniae strain, was partially induced for lactose operon expression in tryptone containing media, and was also pathogenic in a rabbit ileal loop model for NEC. A spontaneous segregant of this strain was no longer partially induced for lactose operon expression, and was no longer pathogenic in the rabbit model. The gene responsible for this phenotype was cloned. The resulting plasmid was shown to cause both partially induced lactose operon expression and pathogenicity when introduced into a laboratory K. pneumoniae strain. A K. pneumoniae mutant deficient in lactose repressor synthesis was also pathogenic in the rabbit model. These results and previous studies on the intraluminal biochemistry of infants with NEC support the hypothesis that an increased ability for lactose fermentation may be a bacterial pathogenic trait with respect to NEC.

Effect of castration on prostaglandin-mediated changes in membrane potential and prostaglandin synthesis in guinea-pig seminal vesicle tissue.

Arachidonic acid stimulated an increase in transmural electrical potential difference (p.d.) in guinea-pig seminal vesicle tissue in vitro. Pretreatment with indomethacin abolished this response. Indomethacin pretreatment did not prevent the p.d. from increasing in response to theophylline. Changes in p.d. in response to arachidonic acid were greatly attenuated, and the response to theophylline was abolished in seminal vesicle tissue taken from castrated guinea-pigs. Seminal vesicles, aorta and ileum taken from castrated guinea-pigs synthesized and released more prostaglandins than did those from control animals. It is concluded that the effects of arachidonic acid on p.d. are mediated by its metabolism to prostaglandins; the inability of seminal vesicles from castrated animals to respond to arachidonic acid is not a result of a decrease in prostaglandin production, but is more likely a result of other degenerative changes attendant upon castration; and androgens appear to have some regulatory function on prostaglandin synthesis in a variety of tissues.