Molecular basis of bacterial virulence and survival within infected hosts and the environment

The EMBO–FEBS lecture course on the ‘Molecular Basis of Bacterial Virulence and Survival Within Infected Hosts and in the Environment’, organized by Pascale Cossart, Efstathios Gonos and Roberto Kolter, was held on the island of Spetsai, Greece, September 3–13, 2002.     

Learning and memory in disease vectors

All animals can learn to some extent and it should not be surprising to discover that important vectors can also be influenced by experience. The potential effect of memory on vector behaviour, particularly vectorial capacity, has barely been investigated. Yet, how a population of blood-feeding insects distributes between available resources has important epidemiological consequences. Several recent studies have shown that behaviour during oviposition site-selection, host location and even host choice can be influenced by the environment or by experience after eclosion. The significance of these studies and their consequences for epidemiology and control are considered here.     

Three Types of Membrane Excitations in the Marine Diatom Coscinodiscus wailesii

Three types of electrical excitation have been investigated in the marine diatom Coscinodiscus wailesii. I: Depolarization-triggered, transient Cl⁻ conductance, G _Cl (t), followed by a transient, voltage-gated K⁺ conductance, G _K , with an active state a and two inactive states i ₁ and i ₂ in series (a-i ₁-i ₂). II: Similar G _Cl (t) as in Type-I but triggered by hyperpolarization; a subsequent increase of G _K in this type is indicated but not analyzed in detail. III: Hyperpolarization-induced transient of a voltage-gated activity of an electrogenic pump (i ₂-a-i ₂), followed by G _Cl (t) as in Type-II excitations. Type-III with pump gating is novel as such. G _Cl (t) in all types seems to reflect the mechanism of InsP⁻ ₃ and Ca²⁺-mediated G _Cl (t) in the action potential in Chara (Biskup et al., 1999). The nonlinear current-voltage-time relationships of Type-I and Type-III excitations have been recorded under voltage-clamp using single saw-tooth command voltages (voltage range: −200 to +50 mV, typical slope: ±1 Vs⁻¹). Fits of the corresponding models to the experimental data provided numerical values of the model parameters. The statistical significance of these solutions is investigated. We suggest that the original function of electrical excitability of biological membranes is related to osmoregulation which has persisted through evolution in plants, whereas the familiar and osmotically neutral action potentials in animals have evolved later towards the novel function of rapid transmission of information over long distances. Received: 2 December 1999/Revised: 3 March 2000     

Identification of gibberellins in leaf exudates of strawberry (Fragaria ×ananassaDuch.)

Leaf exudates from the short-day (SD) strawberry (Fragaria × ananassa Duch.) cv. Elsanta were analysedfor gibberellin (GA) content using combined gaschromatography – mass spectrometry (GC-MS). Comparison of full-scan mass spectra and Kovatsretention indices (KRIs) with those of standardsrevealed the presence of GA₁, GA₃, 3-epiGA₁ and GA₃-isolactone.     

Invasion of West African cattle by the tick Amblyomma variegatum

Studies in Cameroon and Burkina Faso examined the invasion process of cattle by adult Amblyomma variegatum Fabricius (Acari: Ixodidae) ticks. Nearly all the ticks picked up in the pasture during grazing were found on the limb ends, near the hooves, where they temporarily attached. Then when the cattle lay down, the ticks moved from the feet towards the predilection sites, where they attached definitively. Many ticks disappeared during this displacement. All the female ticks and approximately 70% of the males were thus unable to attach to the predilection sites as long as the pioneer males had not attached and started to produce attraction-aggregation-attachment pheromones. Nevertheless, A. variegatum females and males attached to the feet in similar numbers during grazing, whether the cattle were already infested or not, indicating that stimuli originating from the hosts are powerful enough to attract both sexes. After attachment of the pioneer males, the number of ticks that successfully reached the predilection sites increased. However, even on infested animals, 40-50% of A. variegatum ticks found near the hooves after the grazing periods disappeared during the night following their capture. When moving from the temporary attachment sites towards the final ones, one-third of the ticks changed the individual host. Considering this two-stage infestation process, it is suggested that a targeted tick control, using a foot-bath, might greatly reduce cattle infestation. In particular, it could be effective in traditional herds, with animals grazing permanently during the day, lying down only once back in the night pen.     

Field evidence for apparent competition mediated via the shared parasites of two gamebird species

Although apparent competition mediated via shared parasites is potentially an important force influencing community structure, there is limited evidence to demonstrate its occurrence in the field. Here we show that the intensity of infection by the caecal nematode Heterakis gallinarum picked up by naive grey partridges, both maintained in pens and released on six gamebird estates in the UK, is significantly correlated with the intensity of infection recorded in the previous year from pheasants on those estates. Furthermore, the worm burdens picked up appeared to be sufficient to negatively influence host condition. These results provide evidence that infection from pheasants determines the worm burdens of partridges in the field, supporting the hypothesis that parasite-mediated apparent competition with the pheasant may be a factor influencing the decline and subsequent recovery of wild grey partridges.     

Variation in pea aphid population development in three different habitats

Abstract.

• 1 Seasonal population growth rates for the pea aphid, Acyrthosiphon pisum Harris, were determined in three different host plant habitats; alfalfa, Medicago sativa (L.), clover, Trifolium pratense (L.), and peas, Pisum sativum (L.); over four years and eight places. It was possible to estimate a common intrinsic rate of increase for each host plant habitat.
• 2 An analysis of the relative influence of temporal, spatial and host plant habitat variation showed that the host plant habitat was most important in determining the growth rates of the populations, both in rate of build-up and decline.
• 3 Patterns of alate production in the three different habitats differed substantially between the annual peas and the two perennial legumes. During the summer, alate production was large and rapid in peas and remained low and constant in clover and alfalfa
• 4 Parasitism was highest in peas. The species composition of parasitoids differed between crops.
• 5 Aphids in annual peas had a higher intrinsic rate of increase and a faster rate of decline than in the two perennial legumes. This explains the presence of both migratory and sedentary forms among pea aphids.

     

Influence of rearing hosts on host size acceptance by Trichogramma australicum

We investigated the acceptance of different sized host models by Trichogramma australicum in the laboratory. We used isoline cultures of T. australicum reared in hosts of different sizes. Helicoverpa armigera represent relatively large hosts, and those of Sitotroga cerealella are small, termed the Ha and Sc biotypes, respectively. Five sizes of glass beads were tested for acceptance: diameter 0.5, 0.75, 1.0, 1.5 and 2.0 mm. The acceptance of a host model was determined by persistent attempted drilling of a glass bead by a female. The relationship between host egg size and number of eggs laid by a female was also investigated. We used three sizes of artificial egg (diameter 0.75, 1.00, and 1.50 mm of hemispherical cupules) each containing artificial diet. Ha biotype wasps accepted host models in the range 0.75--1.50 mm dia. (vol. 0.166--1.325 l), whereas those of the Sc biotype accepted host models in the range 0.50--1.00 mm dia. (vol. 0.05--0.393 l). This evidence suggests a lineal (possibly genetic) influence to host size acceptance for T. australicum, dependent on the size of the host in which the wasp has been reared. Also T. australicum lay fewer eggs in smaller artificial eggs than in larger ones. The role of host size in host acceptance and number of eggs delivered, and its implications, is discussed.     

Stock Identification of Gag, Mycteroperca microlepis, Along the Southeast Coast of the United States

The gag grouper Mycteroperca microlepis is an important component of commercial and recreational fisheries along the South Atlantic coast of the United States and in the Gulf of Mexico. Over the past two decades, this species has experienced significant declines in abundance and an increasing skew in sex ratios. Analysis of microsatellite DNA variation in this species shows mosaic patterns of population subdivision and significant departures from Hardy-Weinberg equilibrium in all sampling locations. Given the length of the pelagic stage (egg and larvae), the prevailing current patterns, and the migratory capabilities of the adults, it is unlikely that these observations are the result of restricted gene flow among genetically differentiated populations. The apparent structure of gag populations most likely reflects inbreeding in size-limited populations. Population declines, skewed sex ratios, and perhaps variance in female fecundity appear to have acted in concert to limited the number of individuals that contribute to a given year class. These data are reinforced by studies of other fish stocks that have experienced precipitous declines over the past two decades. Received April 13, 1998; accepted September 30, 1998.     

The genetic control of seven isozymic loci in Vicia faba L. Identification of lines and estimates of outcrossing rates between plants pollinated by bumble bees

A simplified and non-destructive method using starch gel electrophoresis has been developed on seeds to identify inbred lines of Vicia faba and assess outcrossing rates and gene dispersal in pollination experiments. Six enzyme systems (Alcohol dehydrogenase, Aspartate aminotransferase, Glucose-6-phosphate isomerase, Isocitrate dehydrogenase, Phosphogluconate dehydrogenase and Shikimate dehydrogenase) were analysed from parental lines, crosses performed between lines bearing dissimilar isozyme patterns in pollination cages with Bombus and F₂ progenies obtained from manual selfing of F₁ hybrids. The allozymes at each of the seven studied loci segregated in the expected Mendelian fashion and behaved in a co-dominant manner except for the Adh-2 locus where the only variant was a null allele. No evidence of genetic linkage was observed between at least 13 of the 15 pairs of the studied loci. Percentage of cross fertilisation by Bombus between seven pairs of inbred lines ranged between 1.7% and 28.3%. Pollen transfer between a donor line and a recipient line by two species of Bombus did not lead to differences in outcrossing rates (both about 8%). The new PGD marker with two loci at three alleles each is particularly discriminating and valuable in pollination studies and breeding of V. faba.