Anomalous Dynamics of Translocation

We study the dynamics of the passage of a polymer through a membrane pore (translocation), focusing on the scaling properties with the number of monomers $N$. The natural coordinate for translocation is the number of monomers on one side of the hole at a given time. Commonly used models which assume Brownian dynamics for this variable predict a mean (unforced) passage time $\tau$ that scales as $N^2$, even in the presence of an entropic barrier. However, the time it takes for a free polymer to diffuse a distance of the order of its radius by Rouse dynamics scales with an exponent larger than 2, and this should provide a lower bound to the translocation time. To resolve this discrepancy, we perform numerical simulations with Rouse dynamics for both phantom (in space dimensions $d=1$ and 2), and self-avoiding (in $d=2$) chains. The results indicate that for large $N$, translocation times scale in the same manner as diffusion times, but with a larger prefactor that depends on the size of the hole. Such scaling implies anomalous dynamics for the translocation process. In particular, the fluctuations in the monomer number at the hole are predicted to be non-diffusive at short times, while the average pulling velocity of the polymer in the presence of a chemical potential difference is predicted to depend on $N$.Comment: 9 pages, 9 figures. Submitted to Physical Review

Anomalous Dynamics of Forced Translocation

We consider the passage of long polymers of length N through a hole in a membrane. If the process is slow, it is in principle possible to focus on the dynamics of the number of monomers s on one side of the membrane, assuming that the two segments are in equilibrium. The dynamics of s(t) in such a limit would be diffusive, with a mean translocation time scaling as N^2 in the absence of a force, and proportional to N when a force is applied. We demonstrate that the assumption of equilibrium must break down for sufficiently long polymers (more easily when forced), and provide lower bounds for the translocation time by comparison to unimpeded motion of the polymer. These lower bounds exceed the time scales calculated on the basis of equilibrium, and point to anomalous (sub-diffusive) character of translocation dynamics. This is explicitly verified by numerical simulations of the unforced translocation of a self-avoiding polymer. Forced translocation times are shown to strongly depend on the method by which the force is applied. In particular, pulling the polymer by the end leads to much longer times than when a chemical potential difference is applied across the membrane. The bounds in these cases grow as N^2 and N^{1+\nu}, respectively, where \nu is the exponent that relates the scaling of the radius of gyration to N. Our simulations demonstrate that the actual translocation times scale in the same manner as the bounds, although influenced by strong finite size effects which persist even for the longest polymers that we considered (N=512).Comment: 13 pages, RevTeX4, 16 eps figure

Prognostic factors for nosocomial bacteraemia outcome: a prospective study in a Greek teaching hospital

Epidemiological and clinical features were studied as potential prognostic factors for outcomes of bacteraemic patients in a tertiary care teaching hospital in Greece. The prospective study was conducted over 12 months and enrolled 153 consecutive hospital-acquired bacteraemic episodes in 137 patients. The crude in-hospital mortality rate was 27% (37/137). The mean lengths of stay and of antimicrobial treatment were significantly longer for patients with a fatal outcome than for survivors (P<0.0001 and P=0.001, respectively). Needs for mechanical ventilation, urinary catheters and parenteral nutrition before or during the onset of episodes were significantly associated with fatalities [odds ratio (OR) = 5.54, 95% confidence intervals (CI) 2.22-14.0, P<0.0001; OR 0.70, 95% CI 0.63-0.79, P=0.021; OR=5.03, 95% CI 1.88-13.95, P<0.0001, respectively]. Also, in logistic regression analysis, mechanical ventilation (OR=5.5, 95% CI 1.2-7.9, P=0.02) and parenteral nutrition (OR=8.8, 95% CI 3.8-11.4, P=0.003) were independent predictors of mortality. No differences between fatalities and survivors were found in sex, age, smoking habit, intensive care unit hospitalization, need for a nasogastric catheter and previous surgery. Neutropenia was associated with a fatal outcome (OR=3.65, 95% Cl 1.24-10.91, P=0.006). None of the bacterial pathogens were significantly associated with an adverse outcome, whereas Staphylococcus aureus was recovered more frequently from survivors (P=0.02). Fatalities were more often associated with an intravascular catheter origin (P=0.002), whereas bacteraemias in survivors were associated with a skin/soft tissue origin (P=0.02). Various prognostic factors were associated with outcome in our bacteraemic population, and can be employed to identify bacteraemic patients at risk of death and to develop local strategies for its prevention. (c) 2005 The Hospital Infection Society. Published by Elsevier Ltd. All rights reserved