Chemotactic responses of the fish-parasitic scuticociliate Philasterides dicentrarchi to blood and blood components of the turbot Scophthalmus maximus, evaluated using a new microplate multiassay A. Parama ´, R. Iglesias, M.F. A ´ lvarez, M.L. Sanmartı ´n, J. Leiro * Laboratorio de Parasitologı ´a, Instituto de Investigacio ´n y Ana ´lisis Alimentarios, Universidad de Santiago de Compostela, C/ Constantino Candeira, s/n, Santiago de Compostela 15782, Spain Received 1 March 2004; received in revised form 11 March 2004; accepted 28 April 2004 Available online 1 July 2004 Abstract This study describes a new capillary-type microplate multiassay for characterization of protozoal chemotactic responses, allowing up to 32 assays to be run simultaneously. We used the new multiassay to evaluate the chemoattractant activity of turbot blood components and turbot cells for the facultative parasite Philasterides dicentrarchi, which is responsible for significant losses in turbot farming. Preliminary tests indicated that the assay requires 3–4 h for detection of chemoattractant activity, that it can be performed effectively using the ciliate axenic culture medium, and that it distinguishes clearly between different concentrations of chemoattractant. Application of the assay indicated that whole blood and serum from normal turbot, and especially infected turbot, have strong chemoattractant activity for P. dicentrarchi trophozoites, whereas neither turbot blood cells nor other turbot cells nor bacteria were significant chemoattractants. These results raise the possibility that turbot serum components are involved in host detection and host invasion by P. dicentrarchi, in line with previous findings indicating that turbot with skin lesions show increased susceptibility to P. dicentrarchi infection. D 2004 Elsevier B.V. All rights reserved. Keywords: Chemotaxis bioassay; Ciliophora; Philasterides dicentrarchi; Turbot; Serum 1. Introduction Histiophagous ciliates causes severe damage in farmed turbot (Dykova ´ and Figueras, 1994; Sterud et al., 2000; Iglesias et al., 2001). The species responsible for ciliate outbreaks in turbot farming in Spain has been identified as Philasterides dicen- trarchi (Iglesias et al., 2001), a species that can also infect farmed sea bass (Dicentrarchus labrax) (Dra- gesco et al., 1995).The natural route of infection is probably through lesions in the gills and/or skin, and then via the bloodstream to other parts of the body in which the ciliate divides rapidly and feeds 0167-7012/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mimet.2004.04.018 * Corresponding author. Tel.: +34-981563100; fax: +34- 981547171. E-mail address: mpleiro@usc.es (J. Leiro). www.elsevier.com/locate/jmicmeth Journal of Microbiological Methods 58 (2004) 361 – 366