Effect of multiple antibiotic treatments on a paralytic shellfish toxin-producing culture of the dinoflagellate Alexandrium minutum

E.W. Maas, R.M. Latter, J. Thiele, Anya Waite, H.J.L. Brooks

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    ABSTRACT: Alexandrium minutum is one of several dinoflagellate species capable of producing paralyticshellfish toxins. Previous work suggests that toxin levels are influenced by a number of parameters,including dinoflagellate-associated bacteria. In the present study, a toxin-producing cultureof A. minutum isolated from Anakoha Bay in the Marlborough Sounds of New Zealand was subjectedto an antibiotic treatment regimen designed to eliminate the associated bacteria. Antibiotics usedincluded penicillin, streptomycin, kanamycin and tetracycline (Treatment 1); ciprofloxacin andgentamicin (Treatment 2); and penicillin, streptomycin and ciprofloxacin (Treatment 3). Enzymeimmunoassay showed that saxitoxin levels in the A. minutum culture fell significantly following thefirst round of antibiotic treatment, and this coincided with a large reduction in the associated copiotrophicbacterial population. HPLC data indicated that there was also a reduction in gonyautoxins(GTX1–3). The oligotrophic population was more difficult to eliminate and required 2 additionalrounds of antibiotic treatment, but saxitoxin levels did not change any further. Scanning laser confocalmicroscopy following acridine orange staining was used to observe intracellular bacteria-like particles,which were considerably reduced by the end of the treatments, probably due to the inclusionof antibiotics that penetrate eukaryotic cells. Algal mean generation times were not significantlyaffected by the antibiotic treatments. Qualitative and quantitative changes in toxin production coincidedwith a reduction in the culturable, copiotrophic and/or intracellular bacteria in the A. minutumAnakoha A culture. The premise that bacteria can exert a strong influence on algal toxicity wassupported by this study, although the mechanisms remain unknown.
    Original languageEnglish
    Pages (from-to)255-260
    JournalAquatic Microbial Ecology
    Issue number3
    Publication statusPublished - 2007

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