27 research outputs found
Rotational dynamics of a superhelix towed in a Stokes fluid
Motivated by the intriguing motility of spirochetes (helically-shaped
bacteria that screw through viscous fluids due to the action of internal
periplasmic flagella), we examine the fundamental fluid dynamics of
superhelices translating and rotating in a Stokes fluid. A superhelical
structure may be thought of as a helix whose axial centerline is not straight,
but also a helix. We examine the particular case where these two superimposed
helices have different handedness, and employ a combination of experimental,
analytic, and computational methods to determine the rotational velocity of
superhelical bodies being towed through a very viscous fluid. We find that the
direction and rate of the rotation of the body is a result of competition
between the two superimposed helices; for small axial helix amplitude, the body
dynamics is controlled by the short-pitched helix, while there is a cross-over
at larger amplitude to control by the axial helix. We find far better, and
excellent, agreement of our experimental results with numerical computations
based upon the method of Regularized Stokeslets than upon the predictions of
classical resistive force theory
Neutralizing Antibody Responses to Human Immunodeficiency Virus Type 1 in Primary Infection and Long-Term-Nonprogressive Infection
The role of neutralizing antibodies in human immunodeficiency virus type 1 (HIV-1) infection is poorly understood and was assessed by evaluating responses at different stages of infection. Undiluted sera from long-term nonprogressors (LTNP) had broad neutralizing antibodies against heterologous primary isolates and were more likely to neutralize the contemporaneous autologous isolate than were sera from short-term nonprogressors and progressors. In primary infection, envelopespecific IgG was detected before the initial decline in plasma viremia, but neutralizing antibodies developed more slowly. Here, neutralizing antibodies against strains SF-2 and MN were sometimes the first to be detected, but titers were low for at least 17 weeks from onset of symptoms. Neutralizing antibodies against the early autologous isolate were detected for 4 patients by 5-40 weeks but were undetectable in 2 additional patients for 27-45 weeks. The results indicate that neutralizing antibody responses are slow to develop during primary infection and are uniquely broad in LTN
Blood Parasites in Owls with Conservation Implications for the Spotted Owl (Strix occidentalis)
The three subspecies of Spotted Owl (Northern, Strix occidentalis caurina; California, S. o. occidentalis; and Mexican, S. o. lucida) are all threatened by habitat loss and range expansion of the Barred Owl (S. varia). An unaddressed threat is whether Barred Owls could be a source of novel strains of disease such as avian malaria (Plasmodium spp.) or other blood parasites potentially harmful for Spotted Owls. Although Barred Owls commonly harbor Plasmodium infections, these parasites have not been documented in the Spotted Owl. We screened 111 Spotted Owls, 44 Barred Owls, and 387 owls of nine other species for haemosporidian parasites (Leucocytozoon, Plasmodium, and Haemoproteus spp.). California Spotted Owls had the greatest number of simultaneous multi-species infections (44%). Additionally, sequencing results revealed that the Northern and California Spotted Owl subspecies together had the highest number of Leucocytozoon parasite lineages (n = 17) and unique lineages (n = 12). This high level of sequence diversity is significant because only one Leucocytozoon species (L. danilewskyi) has been accepted as valid among all owls, suggesting that L. danilewskyi is a cryptic species. Furthermore, a Plasmodium parasite was documented in a Northern Spotted Owl for the first time. West Coast Barred Owls had a lower prevalence of infection (15%) when compared to sympatric Spotted Owls (S. o. caurina 52%, S. o. occidentalis 79%) and Barred Owls from the historic range (61%). Consequently, Barred Owls on the West Coast may have a competitive advantage over the potentially immune compromised Spotted Owls
Author's personal copy Coupling biochemistry and hydrodynamics captures hyperactivated sperm motility in a simple flagellar model
This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. a b s t r a c t Hyperactivation in mammalian sperm is characterized by highly asymmetrical waveforms and an increase in the amplitude of flagellar bends. It is important for the sperm to be able to achieve hyperactivated motility in order to reach and fertilize the egg. Calcium (Ca 2 þ ) dynamics are known to play a large role in the initiation and maintenance of hyperactivated motility. Here we present an integrative model that couples the CatSper channel mediated Ca 2 þ dynamics of hyperactivation to a mechanical model of an idealized sperm flagellum in a 3-d viscous, incompressible fluid. The mechanical forces are due to passive stiffness properties and active bending moments that are a function of the local Ca 2 þ concentration along the length of the flagellum. By including an asymmetry in bending moments to reflect an asymmetry in the axoneme's response to Ca 2 þ , we capture the transition from activated motility to hyperactivated motility. We examine the effects of elastic properties of the flagellum and the Ca 2 þ dynamics on the overall swimming patterns. The swimming velocities of the model flagellum compare well with data for hyperactivated mouse sperm