Comparison of paralytic shellfish toxin (PST) production by the dinoflagellates Alexandrium lusitanicum NEPCC 253 and Alexandrium tamarense NEPCC 407in the presence and absence of bacteria

The ability of two Alexandrium species to produce paralytic shellfish toxin s (PST) in laboratory culture following the generation of bacteria-free cul tures was investigated. The dinoflagellates Alexandrium lusitanicum NEPCC 2 53 and Alexandrium tamarense NEPCC 407 were cultured in the presence of ant ibiotics and tested for residual bacteria. After treatment with a cocktail of streptomycin, ciprofloxacin. gentamicin and penicillin G, bacteria could nor be detected in either of the treated Alexandrium cultures using 17 dif ferent solid and broth bacterial growth media, by epifluorescence microscop y with the dye Sybr green 1, or polymerase chain reaction amplification usi ng universal eubacterial primers designed to target the 16S rRNA gene. Subs equent analysis of ii. lusitanicum for PST using high performance liquid ch romatography demonstrated that the growth rate and toxin profile remained s imilar in both bacteria-free and control cultures, although the quantity of toxins produced differed with the bacteria-free culture producing generall y more of each compound and also having a greater toxin content in terms of saxitoxin equivalents. A. tamarense also retained similarities between the bacteria-free and control cultures in terms of growth rates and toxin prof ile, although in this instance, depending on the growth stage and the toxin , the control culture produced more of some toxins than the bacteria-fret c ulture. The control culture was also more toxic in terms of saxitoxin equiv alents than the axenic culture. These results suggest that bacteria can inf luence toxin production in laboratory cultures of Alexandrium species altho ugh the mechanisms remain unknown. (C) 2001 Published by Elsevier Science B .V. on behalf of the Federation of European Microbiological Societies.