Alternative Bioassay for the Detection of Saxitoxin Using the Speckled Cockroach (Nauphoeta cinerea)

Abstract

Paralytic shellfish poisoning (PSP) toxins produced by cyanobacteria pose a risk to public health as they occur in drinking water reservoirs and recreational lakes and accumulate in the food chain. One of these PSP toxins, saxitoxin (STX) is one of the most toxic nonprotein substances known. Accordingly, there is a requirement to monitor for these toxins. The standard bioassay used to detect these toxins is the mouse bioassay; however, its use is constrained by animal ethics guidelines and practical considerations. Reported here is the use of the globally distributed speckled cockroach Nauphoeta cinerea as a bioassay test organism for the selective detection of PSP toxicity of Anabaena circinalis aqueous extract and STX. N. cinerea was shown to be tolerant to pure cylindrospermopsin (CYN) and microcystin-LR (MC-LR) at doses 10-fold greater than mouse LD₅₀ values while being sensitive to STX. Similarly, N. cinerea was shown to be tolerant of toxin-containing aqueous extracts of Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Nodularia spumigena while being sensitive to A. circinalis. Peak sensitivity to STX was 60 min postinjection with a KD₅₀ of 31.2 ng/g body weight. While this was approximately 3-fold less sensitive than the mouse bioassay, the insect test organism was around 34-fold smaller in mass than a mouse (20 g); thus one-tenth the amount of toxin in absolute quantity was required to reach an ED₅₀ level. The N. cinerea bioassay presents a selective test for PSP toxicity that is rapid, economical, efficient, and simple to perform.

Acknowledgments

This study was funded by the Cooperative Research Centre for Water Quality and Treatment. The authors thank Dr. Andrew Humpage (Australian Water Quality Centre, Bolivar, South Australia [AWQC]) for kindly supplying pure CYN and the samples of A. circinalis and N. spumigena (N. spumigena originally collected by Em Prof Ian R. Falconer [University of Adelaide, South Australia]). Dr. Wasantha Wickramasinghe (National Research Centre for Environmental Toxicology [EnTox], Coopers Plains, Queensland) is also thanked for kindly supplying pure MC-LR. The contributions of three anonymous reviewers are also acknowledged for enhancing the quality of this article.