Grazing activity of Pfiesteria piscicida (Dinophyceae) and susceptibility to ciliate predation vary with toxicity status

Variability has been reported in the toxicity potential of Pfiesteria piscicida that is partly a function of the history of exposure to live fish. Grazing properties of P. piscicida and its susceptibility to ciliate predation were compared in three functional types or toxicity states of this species: actively toxic cultures, cultures with temporary loss of demonstrable toxicity, and cultures with no demonstrable toxicity. Pronounced differences in predator–prey interactions were found between actively toxic cultures and cultures with reduced toxicity. When grown with Rhodomonas sp. (Cryptophyceae) prey, specific growth rates were relatively low in actively toxic cultures under both relatively high and low irradiances. In the cultures with reduced toxicity, prey chloroplast material was apparent in nearly 100% of dinoflagellate cells 3 h after feeding, while chloroplast inclusions were found in <40% of actively toxic cells for ≤16 h (high light) and ≤23 h (low light). These results suggest a relatively high reliance on phagotrophic carbon assimilation and more rapid response to algal prey availability in Pfiesteria cells with lower toxicity. Grazing by two euplotid benthic ciliates (Euplotes vannus and E. woodruffi) on P. piscicida also varied among functional types. Grazing on actively toxic P. piscicida cells did not occur, whereas net positive ingestion rates were calculated for the other prey cultures. These results support concurrent experimental findings that a natural assemblage of microzooplankton displayed lower grazing potential on actively toxic P. piscicida than on cultures with reduced toxicity. In summary, pronounced differences in trophic interactions were found between actively toxic cultures and those with reduced or undetectable toxicity, providing additional evidence of the importance of cellular toxicity in the trophic ecology of Pfiesteria.     

In vitro efficacy of glutaraldehyde-crosslinked chitosan microspheres against the fish-pathogenic ciliate Philasterides dicentrarchi

Philasterides dicentrarchi is a protozoan ciliate which causes significant economic losses in fish aquaculture. This study investigated the effects of chitosan microspheres cross linked with glutaraldehyde and containing beta-cyclodextrin (betaCD) on the survival of this parasite in 7 d cultures. When used alone in assays, neither chitosan nor betaCD showed any activity, whereas free glutaraldehyde was strongly toxic to the parasite. Microspheres were likewise strongly toxic, at total glutaraldehyde concentrations much lower than with free glutaraldehyde: near-100% ciliate death was obtained (1) with 50 microg ml(-1) of microspheres prepared with 5% glutaraldehyde and no betaCD, or (2) with 10 microg ml(-1) of microspheres prepared with 0.15% glutaraldehyde and 0.1% betaCD. This suggests that the main active component is glutaraldehyde, but that the presence of small amounts of betaCD enhances efficacy. This high efficacy, together with the low toxicity to fish and rapid biodegradability of the individual components, suggest that these microspheres may be an attractive alternative to the formaldehyde baths traditionally used for the control of this parasite.     

A new strain of Cryptocaryon irritans from the cultured olive flounder Paralichthys olivaceus

An obligate parasite, Cryptocaryon irritans, which is responsible for the white spot disease of marine fish is known to develop in the temperature regime over 19 degrees C. Recently, however, we found white spot disease of olive flounder Paralichthys olivaceus during winter at water temperatures ranging between 12 and 16 degrees C in Korea. In the present study we isolated a C. irritans-like ciliate from the affected fish and investigated its reproductive characters to compare the newly found ciliate with typical C. irritans. The newly found ciliate had an additional process in the reproductive stage, characterized by a budding before palintomic division, and it showed a higher ability to carry out tomitogenesis at a low temperature (16 degrees C) than at a high temperature (24 degrees C). Nevertheless, the present ciliates still had much in common with typical C. irritans with respect to clinical, histopathological, and morphological characters, suggesting that it is a new strain of C. irritans, adapted to lower water temperature.     

Evolutionary Trade-Offs Underlie the Multi-faceted Virulence of Staphylococcus aureus

Bacterial virulence is a multifaceted trait where the interactions between pathogen and host factors affect the severity and outcome of the infection. Toxin secretion is central to the biology of many bacterial pathogens and is widely accepted as playing a crucial role in disease pathology. To understand the relationship between toxicity and bacterial virulence in greater depth, we studied two sequenced collections of the major human pathogen Staphylococcus aureus and found an unexpected inverse correlation between bacterial toxicity and disease severity. By applying a functional genomics approach, we identified several novel toxicity-affecting loci responsible for the wide range in toxic phenotypes observed within these collections. To understand the apparent higher propensity of low toxicity isolates to cause bacteraemia, we performed several functional assays, and our findings suggest that within-host fitness differences between high- and low-toxicity isolates in human serum is a contributing factor. As invasive infections, such as bacteraemia, limit the opportunities for onward transmission, highly toxic strains could gain an additional between-host fitness advantage, potentially contributing to the maintenance of toxicity at the population level. Our results clearly demonstrate how evolutionary trade-offs between toxicity, relative fitness, and transmissibility are critical for understanding the multifaceted nature of bacterial virulence.     

DEBtox theory and matrix population models as helpful tools in understanding the interaction between toxic cyanobacteria and zooplankton

Bioassays were performed to find out how field samples of the toxic cyanobacteria Microcystis aeruginosa affect Moina micrura, a cladoceran found in the tropical Jacarepagua Lagoon (Rio de Janeiro, Brazil). The DEBtox (Dynamic Energy Budget theory applied to toxicity data) approach has been proposed for use in analysing chronic toxicity tests as an alternative to calculating the usual safety parameters (NOEC, ECx). DEBtox theory deals with the energy balance between physiological processes (assimilation, maintenance, growth and reproduction), and it can be used to investigate and compare various hypotheses concerning the mechanism of action of a toxicant. Even though the DEBtox framework was designed for standard toxicity bioassays carried out with standard species (fish, daphnids), we applied the growth and reproduction models to M. micrura, by adapting the data available using a weight-length allometric relationship. Our modelling approach appeared to be very relevant at the individual level, and confirmed previous conclusions about the toxic mechanism. In our study we also wanted to assess the toxic effects at the population level, which is a more relevant endpoint in risk assessment. We therefore incorporated both lethal and sublethal toxic effects in a matrix population model used to calculate the finite rate of population change as a continuous function of the exposure concentration. Alongside this calculation, we constructed a confidence band to predict the critical exposure concentration for population health. Finally, we discuss our findings with regard to the prospects for further refining the analysis of ecotoxicological data.