Enhancement of clavulanic acid production by Streptomyces sp MU-NRC77 via mutation and medium optimization

Purpose: To enhance clavulanic acid production using UV-mutagenesis on Streptomyces sp. NRC77.Methods: UV-mutagenesis was used to study the effect of Streptomyces sp. NRC77 on CA production. Phenotypic and genotypic identification methods of the promising mutant strain were characterized. Optimization of the fermentation medium and culture conditions were investigatedResults: Out of the screened mutants, 120A3 mutant isolate was selected as promising. The phenotypic properties of 120A3 mutant showed culture characteristics similar to those of Streptomyces species. Phylogenetic analysis of 16S rRNA gene sequence indicate that this strain has similarity (99 %) to Streptomyces sp.T2-7; therefore it was suggested as Streptomyces sp. MU-NRC77 and has Gen Bank accession no. KT953342.Conclusion: Improvement of CA yield by 48 % was obtained from fermentation medium and culture condition optimization. Further optimization by addition of H2O2 and activated charcoal to the production medium increased CA yield to 646.12 and 682.94 mg/L respectively, i.e., 83 % more than that obtained prior to addition.Keywords: Clavulanic acid, Medium optimization, Phenotypic and Genotypic identification, Streptomyces sp. MU-NRC77, UV-Mutagenesi

Structural and Optical Properties of Znse1−Xtex Nanocrystalline Thin Films in Terms of Optical Spectroscopic Ellipsometry

Polycrystalline thin films of ZnSe1−xTex (0.0 ≤ x ≤ 1.0) were deposited on glass substrate using electron beam deposition technique. The structure of the prepared films was examined using X-ray diffraction technique and revealed that the deposited films have polycrystalline zinc blend structure. The optical constants and film thicknesses of nanocrystalline ZnSe1−xTex films were obtained by fitting the spectroscopic ellipsometric data (ψ, Δ) using a three-layer model system in the wavelength range from 400 to 1100 nm. The refractive index was observed to increase with increasing Te concentration. This increase in the refractive index with increasing Te content may be attributed to the increase in the polarizability due tothe large ionic radius of Te compared to the ionic radius of Zn. The optical studies of the polycrystalline ZnSe1−xTex films showed that the refractive index increases and fundamental band gap opt g E decreases from 2.58 to 2.21 eV as the tellurium concentration increases from 0 to 1. Furthermore, it was also found that the variation of optical band gap with compositionshows quadratic behavior.Keywords: ZnSe1−xTex thin film, nanocrystalline, Spectroscopic ellipsometry, bandgap

Structural and Optical Properties of Znse1−Xtex Nanocrystalline Thin Films in Terms of Optical Spectroscopic Ellipsometry

Polycrystalline thin films of ZnSe1−xTex (0.0 ≤ x ≤ 1.0) were deposited on glass substrate using electron beam deposition technique. The structure of the prepared films was examined using X-ray diffraction technique and revealed that the deposited films have polycrystalline zinc blend structure. The optical constants and film thicknesses of nanocrystalline ZnSe1−xTex films were obtained by fitting the spectroscopic ellipsometric data (ψ, Δ) using a three-layer model system in the wavelength range from 400 to 1100 nm. The refractive index was observed to increase with increasing Te concentration. This increase in the refractive index with increasing Te content may be attributed to the increase in the polarizability due to the large ionic radius of Te compared to the ionic radius of Zn. The optical studies of the polycrystalline ZnSe1−xTex films showed that the refractive index increases, and fundamental band gap opt g E decreases from 2.58 to 2.21 eV as the tellurium concentration increases from 0 to 1. Furthermore, it was also found that the variation of optical band gap with composition shows quadratic behavior

Relaxation bottleneck and its suppression in semiconductor microcavities

A polariton relaxation bottleneck is observed in angle-resolved measurements of photoluminescence emission from a semiconductor microcavity. For low power laser excitation, low k polariton states are found to have a very small population relative to those at high k. The bottleneck is found to be strongly suppressed at higher powers in the regime of superlinear emission of the lower polariton states. Evidence for the important role of carrier-carrier scattering in suppression of the bottleneck is presented

Continuous wave observation of massive polariton redistribution by stimulated scattering in semiconductor microcavities

A massive redistribution of the polariton occupancy to two specific wave vectors is observed under conditions of continuous wave excitation of a semiconductor microcavity. The “condensation” of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure

Factors Affecting the Reluctance to Pursue Teaching as a Career from the Perspective of Students in the Sultanate of Oman: A mixed-approach Investigation

هدفت الدراسة الحالية إلى الكشف عن العوامل المؤثرة في عزوف الطلبة عن مهنة التدريس بسلطنة عمان. استخدمت الدراسة المنهج المختلط، حيث تم جمع البيانات الكمية من خلال مقياس العوامل المؤثرة على اختيار مهنة التدريس بعد تعريبه وتكييفه لمجتمع الدراسة المكون من طلبة الصفوف 10-12 (ن= 3455)، أما البيانات النوعية فقد تم جمعها من خلال مجموعات النقاش البؤرية مع طلبة هذه الصفوف (ن= 253). أشارت النتائج إلى أن العوامل المؤثرة في عزوف الطلبة عن مهنة التدريس تمحورت حول: صعوبة المهنة، وقلة حوافزها المادية، وتدني المكانة الاجتماعية لها مقارنة بالمهن الأخرى، بينما بينت نتائج مجموعات النقاش البؤرية وجود آراء إيجابية وسلبية للمهنة، ووجود تحديات أبرزها: صعوبة التعامل مع عدد كبير من الطلبة، وتوقف ترقيات المعلمين، وتعيين المعلمين بعيدا عن أماكن إقامتهم. كما بين أغلبية طلبة المدارس من الجنسين بأنه ليس لديهم الرغبة والاتجاه الإيجابي لاختيار التدريس كمهنة المستقبل. كما أشارت نتائج الدراسة المتعلقة بالفروق بين الجنسين إلى وجود فروق دالة إحصائيا لصالح الذكور في العزوف عن مهنة التدريس

Uncoupled excitons in semiconductor microcavities detected in resonant Raman scattering

We present an outgoing resonant Raman-scattering study of a GaAs/AlGaAs based microcavity embedded in a p-i-n junction. The p-i-n junction allows the vertical electric field to be varied, permitting control of exciton-photon detuning and quenching of photoluminescence which otherwise obscures the inelastic light scattering signals. Peaks corresponding to the upper and lower polariton branches are observed in the resonant Raman cross sections, along with a third peak at the energy of uncoupled excitons. This third peak, attributed to disorder activated Raman scattering, provides clear evidence for the existence of uncoupled exciton reservoir states in microcavities in the strong-coupling regime

Relaxation bottleneck and its suppression in semiconductor microcavities

A polariton relaxation bottleneck is observed in angle-resolved measurements of photoluminescence emission from a semiconductor microcavity. For low power laser excitation, low k polariton states are found to have a very small population relative to those at high k. The bottleneck is found to be strongly suppressed at higher powers in the regime of superlinear emission of the lower polariton states. Evidence for the important role of carrier-carrier scattering in suppression of the bottleneck is presented. Semiconductor microcavities ͑MC's͒ with embedded quantum wells ͑QW's͒ have attracted great interest recently. In these structures, two-dimensional ͑2D͒ confinement of photon and exciton modes is realized. In high finesse MC's, as a result of exciton-photon coupling, mixed 2D polariton states occur 1 which exhibit a strong dependence of their properties on their relative exciton and photon contents. 2 In particular, due to the very small in-plane photon mass in MC's (M p Ϸ10 Ϫ4 m ex ), polaritons with a significant photon fraction have a very steep E -k dispersion curve, Of particular relevance to the present work, energy relaxation in the polariton system is expected to be strongly modified relative to that of uncoupled excitons. Acoustic phonon scattering rates from high k exciton states into polariton states with wave vectors corresponding to the strong coupling regime (kр5 -6ϫ10 4 cm Ϫ1 ) are expected to be small due to the large energy transfer required compared to typical acoustic phonon energies of 1 meV. Furthermore, the strongly coupled states have very short radiative lifetime ͑ϳ1 psec͒ due to their high photon fraction, thus leading to a nonthermal polariton population and the occurrence of a relaxation bottleneck. Such bottleneck effects have been much discussed theoretically for the cavity polariton system. 7 A manifestation of the bottleneck has been observed in time-resolved measurements of the polariton emission in II-VI based microcavities. In this paper, we present a definitive observation of the relaxation bottleneck for lower branch polaritons ͑LP's͒ from angle resolved photoluminescence ͑PL͒. Nonresonant cw laser excitation creates excitons at high energy which relax to form a reservoir of high k (Ͼ10 5 cm Ϫ1 ) excitons. These excitons scatter into the observable polariton states and give rise to the PL. For low density excitation, low k (Ͻ1.5 ϫ10 4 cm Ϫ1 ) occupancies nearly an order of magnitude smaller than those at higher k are found. Strong redistribution of the polariton population, and suppression of the bottleneck, is found in the regime where the LP emission exhibits superlinear behavior with increasing excitation intensity The sample studied is a high quality single MC structure grown by metal organic vapor phase epitaxy. The Bragg reflectors are composed of 17 ͑20͒ repeats of /4 Al 0.13 Ga 0.87 As/AlAs layers in the top ͑bottom͒ mirrors. The 3/2 GaAs cavity contains six 10-nm In 0.06 Ga 0.94 As/GaAs QW's. The vacuum Rabi splitting was ⍀Ϸ6 meV, with linewidths of Ͻ1 meV. Nonuniformities of the cavity width permit tuning of the photon mode energy over a wide range of negative and positive detunings (⌬ϭE cav ϪE ex ). Measurements were carried out with the sample immersed either in superfluid He or in He gas. A Ti-sapphire laser was used for nonresonant ͑1.56 eV͒ excitation. The PL signal was collected by a lens which imaged the excited sample region ͑collection angle Ͻ2°) on to a fiber. Both the lens and the fiber were mounted on a rotating rail permitting precise angle tuning. The signal was detected by a single monochromator/ CCD system. PL spectra for a detuning of ⌬ϭϪ4.3 meV and temperature Tϭ34 K over a wide range of collection angles (⌽, RAPID COMMUNICATION