The Effect of Buffers on Weak Acid Uptake by Vesicles.

The assessment of weak acid membrane permeability (Pm) frequently involves large unilamellar vesicles. It relies on measurements of the intravesicular pH drop, ΔpHin, in response to a sudden augmentation of external acid concentration. However, ΔpHin may be primarily governed by non-instantaneous protonation and deprotonation reactions of (i) the acid itself, (ii) the buffer molecules, and (iii) the fluorescent pH reporter dye. Moreover, buffer concentration and acid gradient also serve as determinants of ΔpHin, as we show here. The uniexponential time constant (τ) of ΔpHin(t) is an invalid measure of Pm as Arrhenius plots of Pm and τ reveal different activation energies for acid influx. We calculate Pm by fitting a mathematical model to experimental stopped-flow traces. The model takes into account not only the time course of total internal buffer capacity but also (i) water self-dissociation, (ii) volume changes due to acid induced osmotic water flow, and (iii) the spontaneous membrane proton leak. It allows extracting a Pm of 30.8 ± 3.5 µm/s for formic acid for 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles.

The parasitic nematode Phasmarhabditis hermaphrodita defends its slug host from being predated or scavenged by manipulating host spatial behaviour.

Infective stages of commercially used molluscicidal rhabditide nematodes Phasmarhabditis hermaphrodita contain bacterial symbionts which kill their host by septicaemia. The nematodes feed on the multiplying bacteria and entire host tissue, develop and repeatedly reproduce. Invertebrate cadavers are rapidly (from minutes to hours) removed by scavengers. However nematodes need days to complete their life cycle inside the host. The post mortem locations of slugs killed by six different treatments (three types of molluscicides, a simulation of unsuccessful predation and two P. hermaphrodita nematode treatments) were compared. In comparison to other pathogenic states, significantly more slugs killed by the nematodes died within the soil, where the scavenging pressure is weaker than on the soil surface (where most of the slugs died regardless treatment). We suggest that this is an outcome of behavioural manipulation, which prevent the parasites from being predated or scavenged together with their host until the nematodes complete development inside the host cadaver.