Endosymbiotic bacteria were identified in the parasitic ciliate Ichthyophthirius multifiliis, a common pathogen of freshwater fish. PCR amplification of DNA prepared from two isolates of I. multifiliis, using primers that bind conserved sequences in bacterial 16S rRNA genes, generated an ϳ1,460-bp DNA product, which was cloned and sequenced. Sequence analysis demonstrated that 16S rRNA gene sequences from three classes of bacteria were present in the PCR product. These included Alphaproteobacteria (Rickettsiales), Sphingobacteria, and Flavobacterium columnare. DAPI (4,6-diamidino-2-phenylindole) staining showed endosymbionts dispersed throughout the cytoplasm of trophonts and, in most, but not all theronts. Endosymbionts were observed by transmission electron microscopy in the cytoplasm, surrounded by a prominent, electron-translucent halo characteristic of Rickettsia. Fluorescence in situ hybridization demonstrated that bacteria from the Rickettsiales and Sphingobacteriales classes are endosymbionts of I. multifiliis, found in the cytoplasm, but not in the macronucleus or micronucleus. In contrast, F. columnare was not detected by fluorescence in situ hybridization. It likely adheres to I. multifiliis through association with cilia. The role that endosymbiotic bacteria play in the life history of I. multifiliis is not known.

No abstract

Applied and basic research on the ciliate Ichthyophthirius multifiliis, an obligate parasite of freshwater fishes, requires passage on fish hosts to maintain laboratory stocks. However, continual repeated passage results in senescence of parasite clones over time. Because growth and development are directly correlated to water temperature, our objective was to grow the parasite at low temperature in order to extend the period that the organism remains on the fish, thus reducing: (1) the number of passages and (2) the number of fish required to maintain the parasite over time. Lowering of water temperature from 25 C to 9 C resulted in significant slowing of growth on channel catfish (parasites remained on fish for 20.4 days at 9 C, as compared to 5-6 days at 25 C), with no discernible changes in viability, antigenicity, or infectivity. Low-temperature growth is proposed as an improved method for stable maintenance of I. multifiliis cultures in the laboratory.

No abstract

Naïve channel catfish Ictalurus punctatus were infected by 2 isolates of the parasitic ciliate Ichthyophthirius multifiliis that differed in virulence. The isolates, NY1 and G5, Serotypes A and D, respectively, express different surface immobilization-antigens. The virulence of the 2 isolates was compared using tail-fin infections to quantitate parasite numbers and by analysis of the survival of infected fish. Although NY1 infected fish at a lower level than G5, all NY1-infected fish died, but 51% of G5-infected fish survived. The greater virulence of NY1 is apparently a consequence of its shorter life cycle, which results in overwhelming reinfection of fish before they can develop a protective immune response. This report represents the first experimental evidence for differences in virulence between serotypes of I. multifiliis. KEY WORDS: Fish parasite virulence · Ichthyophthirius multifiliis · Immobilization antigen · Ictalurus punctatus · Channel catfishResale or republication not permitted without written consent of the publisher