52 research outputs found
Bacterial chemotaxis in an optical trap
No full textAn optical trapping technique is implemented to investigate the chemotactic behavior of a marine bacterial strain Vibrio alginolyticus. The technique takes the advantage that the bacterium has only a single polar flagellum, which can rotate either in the counter-clock-wise or clock-wise direction. The two rotation states of the motor can be readily and instantaneously resolved in the optical trap, allowing the flagellar motor switching rate S(t)to be measured under different chemical stimulations. In this paper the focus will be on the bacterial response to an impulsive change of chemoattractant serine. Despite different propulsion apparati and motility patterns, cells of V. alginolyticus apparently use a similar response as Escherichia coli to regulate their chemotactic behavior. Specifically, we found that the switching rate S(t) of the bacterial motor exhibits a biphasic behavior, showing a fast initial response followed by a slow relaxation to the steady-state switching rate S0. The measured S(t)can be mimicked by a model that has been recently proposed for chemotaxis in E. coli. The similarity in the response to the brief chemical stimulation in these two different bacteria is striking, suggesting that the biphasic response may be evolutionarily conserved. This study also demonstrated that optical tweezers can be a useful tool for chemotaxis studies and should be applicable to other polarly flagellated bacteria. © 2011 Altindal et al
Determination of Bacterial Chemotaxis Response Functions by Optical Trapping
No full textA two-lobe response function is considered as a manifestation of temporal signal comparison in bacterial chemosensing. The second lobe in the response function appears as a result of adaptive behavior of the underlying signaling network, which allows bacteria to stay sensitive over a wide range of background signal levels. It has been argued that this two-lobe response reflects the dual requirements of the bacteria to taxis along a chemical gradient and to localize once the top of the gradient is reached. Calculations based on the run-tumble motility pattern of Escherichia coli showed that the second lobe improved the bacterium's localization capability. Intrigued by a recently observed run-reverse-flick motility cycle of a marine bacterium Vibrio alginolyticus, we investigate the motility-response relationship in this bacterium. Using a novel optical trapping technique, we measure the response of V. alginolyticus to an impulsive stimulus of chemoattractant serine. By exploiting an asymmetry in the rotation of the polar flagellum, we are able to determine for the first time how the bacterium responds to chemical stimuli while swimming forward or backward. Our measurements suggest that this marine bacterium regulates its forward and backward swimming intervals differently, exhibiting behaviors that is consistent with an exploration-exploitation strategy.
In our measurements, we also find that the cell-body Ω(t) and the flagellar ω(t) rotational angular frequencies oscillate in time and are in synchrony with the forward and backward swimming intervals. Unexpectedly, Ω(t) and ω(t) are found to be anticorrelated in that the cell body rotates slower in the forward direction than in the backward direction, Ωf<Ωb, but the flagellum rotates faster in the forward direction than in the backward direction. The change in the rotational load (~25%) is significantly greater than that predicted by flagellum deformation but can be accounted for by the precession of the flagellum about the body axis during the backward swimming interval. We postulate that as a result of the precession, a kink is generated at the base of the flagellum that is subsequently amplified when the flagellum motor reverses direction, leading to the flick, the direction randomization step in V. alginolyticus' motility pattern
V2O5 ince filmlerin yapısal, elektriksel ve optik özelliklerinin incelenmesi
No full textV2O5 İNCE FİLMLERİN YAPISAL, ELEKTRİKSEL ve OPTİK ÖZELLİKLERİNİN İNCELENMESİ Geçiş metal oksitleri oldukça elverişli elektriksel, optik, manyetik ve termal özellikler göstermektedir. VxOy şeklinde gösterilen vanadyum oksitleri (VO,VO2,V2O5,.v.b...), metal oksit yarıiletkenler arasında ilginç özellikler gösteren bir malzeme grubudur. Elektrokromik aygıtlar, termal aktivasyonlu optik anahtarlar, termal sensörler, saydam elektriksel iletken malzemeler, lityum piller bu filmlerin uygulama alanları içinde bulunmaktadır. Bu uygulamalar, filmlerin yapısal, elektriksel ve optik özelliklerinin araştırılmasına dayanmaktadır. Bu Yüksek Lisans tez çalışmasında, başlangıç maddesi olarak vanadyum triklorür kullanılmıştır. Katkısız ve kütlece (Ti:V) %2, %5, %10, %15, %20 oranlarında titanyum katkılı vanadyum pentaoksit (V2O5) filmler sol-jel daldırma yöntemiyle hazırlanmıştır. Filmler cam altlıklar üzerine fotolitografi yöntemiyle oluşturulan interdijital transduser yapısı üzerine kaplanmıştır. Filmlerin yapısal özellikleri, X-ışını kırınımı (XRD) tekniği ile araştırılmıştır. Filmlerin d.c ve a.c elektriksel özellikleri sıcaklığa bağlı olarak (295-523 K), vakum ortamında (~3x10-2 mbar) ve karanlıkta incelenmiştir. A.c elektriksel özellikler, 40-105 Hz frekans aralığında incelenmiştir. Filmlerin optik özellikleri, spektrofotometre kullanılarak 250-800 nm dalgaboyu arasındaki soğurma ve geçirgenlik değerleri hava ortamında ölçülmek suretiyle belirlenmiştir. XRD sonuçları, filmlerin kristal yapıda olduğunu göstermiştir. D.c ölçüm sonuçlarından, hazırlanan filmlerin yarıiletken özellik gösterdiği ve aktivasyon enerjilerinin katkı oranı artarken arttığı belirlenmiştir. A.c özelliklerin incelenmesinden, yük iletim mekanizmasının sıcaklığa ve frekans bölgesine bağlı olarak hoplama ve Reider modeliyle açıklanabileceği görülmüştür. Optik özelliklerin incelenmesinden, optik bant aralıklarının, katkı oranı artarken arttığı belirlenmiştir.INVESTIGATION of STRUCTURAL, ELECTRICAL and OPTICAL PROPERTIES of V2O5 THIN FILMS Transition metal-oxides exhibit unique electrical, optical, magnetic and thermal properties. Vanadium oxides in VxOy form (VO, VO2, V2O5, etc.) also a class of materials exhibiting lots of interesting properties. There are many application areas such as electrochromic devices, thermally activated optic switches, thermal sensors, transparent conductive materials and lithium batteries. These applications are based on investigation of structural, electrical and optical properties of the films. In this master thesis, vanadium (III) chloride was used as starting materials. Undoped and doped with titanium (wt%, Ti: V, 2%, 5%, 10%, 15%, 20%) vanadium pentoxide (V2O5) films were prepared using sol-gel dipping technique. The films were coated on interdigital transducer which is patterned on glass substrate using photolithography technique. Structural investigations of the films were performed by means of X-ray diffraction method (XRD). D.c and a.c electrical properties of the films were investigated as a function of temperature (295-523 K) in vacuum (~3x10-2 mbar) and in dark environment. A.c electrical properties were performed in the frequency range of 40-105 Hz. Optical properties were determined by means of absorption and transmission measurements between the wavelengths of 250-800 nm in air using spectrophotometer. XRD results revealed that the films were crystal. From d.c electrical measurements, we can say that the films behave as semiconductor and activation energies of the films increase with increasing doping concentration. A.c charge transport mechanism can be explained by hopping and Reider model depending on temperature and frequency region. It was found from optical measurements that, optical band gap of the films increases with increasing doping concentrations
Implications of Three-Step Swimming Patterns in Bacterial Chemotaxis
No full textAbstractWe recently found that marine bacteria Vibrio alginolyticus execute a cyclic three-step (run-reverse-flick) motility pattern that is distinctively different from the two-step (run-tumble) pattern of Escherichia coli. How this novel, to our knowledge, swimming pattern is regulated by cells of V. alginolyticus is not currently known, but its significance for bacterial chemotaxis is self-evident and will be delineated herein. Using a statistical approach, we calculated the migration speed of a cell executing the three-step pattern in a linear chemical gradient, and found that a biphasic chemotactic response arises naturally. The implication of such a response for the cells to adapt to ocean environments and its possible connection to E. coli's response are also discussed
An Element of Determinism in a Stochastic Flagellar Motor Switch.
No full textMarine bacterium Vibrio alginolyticus uses a single polar flagellum to navigate in an aqueous environment. Similar to Escherichia coli cells, the polar flagellar motor has two states; when the motor is counter-clockwise, the cell swims forward and when the motor is clockwise, the cell swims backward. V. alginolyticus also incorporates a direction randomization step at the start of the forward swimming interval by flicking its flagellum. To gain an understanding on how the polar flagellar motor switch is regulated, distributions of the forward Δf and backward Δb intervals are investigated herein. We found that the steady-state probability density functions, P(Δf) and P(Δb), of freely swimming bacteria are strongly peaked at a finite time, suggesting that the motor switch is not Poissonian. The short-time inhibition is sufficiently strong and long lasting, i.e., several hundred milliseconds for both intervals, which is readily observed and characterized. Treating motor reversal dynamics as a first-passage problem, which results from conformation fluctuations of the motor switch, we calculated P(Δf) and P(Δb) and found good agreement with the measurements
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