A semi-fluid gellan gum medium improves nematode toxicity testing.

This study examined an alternative test medium for nematodes that use gellan gum as the gelling agent instead of agar. The semi-fluid consistency of the gel-like component nematode growth gellan gum (CNGG) supports three-dimensional distribution of the nematodes and food bacteria, but still allows free movement of the former. Moreover, flexible preparation of the medium and easy recovery of the test organisms are possible. Here, the effects of the nematicides ivermectin (pharmaceutical) and aldicarb (pesticide) and of the metal cadmium on the growth and reproduction of the free-living nematodes Caenorhabditis elegans and Panagrolaimus cf. thienemanni were studied in CNGG media. Results were compared to those obtained with the standard liquid test media in order to evaluate the applicability of CNGG for nematode toxicity testing. The sensitivity of P. cf. thienemanni to all three substances was found to be higher than that of C. elegans, but both nematodes showed the highest sensitivity to ivermectin exposure. This raises concerns about the risk posed by the pharmaceutical to non-target nematodes. In contrast to ivermectin bioassays carried out in CNGG medium, those conducted in liquid medium resulted in wide-ranging variability between and within replicates. Thus, CNGG seems to be particularly valuable for testing hydrophobic substances with a high sorption affinity as it favors their sorption to food bacteria and minimizes contact with the surfaces of the test vessels. However, the medium was less suitable for deriving toxicity thresholds for cadmium and may likewise not be an appropriate choice for testing other metals. The medium introduced herein was shown to be appropriate for sublethal nematode toxicity testing and likely provides a convenient environment for testing other nematode species. Besides improved testing of hydrophobic substances, CNGG also offers advantages for long-term studies, such as full life-cycle experiments, in which fresh medium is regularly needed. Moreover it may be beneficial for testing other poorly soluble or insoluble substances, such as nanoparticles.