Development of sensitive bacterial tests, exemplified by two mycotoxins

Evaluation of health aspects of kojic acid in food

Kojic acid is a fungal metabolite commonly produced by many species of Aspergillus, Acetobacter, and Penicillium. The Aspergillus flavus group has traditionally been used in the production of a number of foods, including miso (soybean paste), shoyu (soy sauce), and sake. Kojic acid is widely used as a food additive for preventing enzymatic browning, and in cosmetic preparations as a skin-lightening or bleaching agent. Because kojic acid is often produced during the fermentation of historically used dietary staples, it has a long history of consumption. Various types of compounds, such as glucose, sucrose, acetate, ethanol, arabinose, and xylose, have been used as carbon sources for kojic acid production. Different Aspergillus species are known to produce variable amounts of kojic acid. The mechanism of action of kojic acid is well defined and it has been shown to act as a competitive and reversible inhibitor of animal and plant polyphenol oxidases, xanthine oxidase, and D- and some L-amino acid oxidases. The structure of kojic acid indicates a relatively simple route of metabolism much like dietary hexoses. Acute or subchronic toxicity resulting from an oral dose has not been reported, but convulsions may occur if kojic acid is injected. Results of mutagenicity studies are mixed, but in the in vivo mammalian dominant lethal assay, kojic acid was proven negative. Continuous administration of high doses of kojic acid in mice resulted in induction of thyroid adenomas in both sexes. Kojic acid reversibly affects thyroid function primarily by inhibiting iodine uptake, leading to decreases in T3 and T4 and increase in TSH. Increased TSH from pituitary gland in turn stimulates thyroid hyperplasia. Several lines of evidence indicate that the proliferative effects of kojic acid on thyroid are not related to a genotoxic pathway. The risk of functional inhibition of iodine uptake and its metabolism (organification) and thyroid tumor induction by kojic acid in humans appears to be extremely low. Based on the literature reviewed and discussed here, consumption of kojic acid at levels normally found in food does not present a concern for safety.

Development of an optimal bacterial medium based on the growth inhibition assay with Vibrio fischeri

Chronic toxicity level of chemicals to bacteria can depend on composition and concentration of medium ingredients. This was demonstrated by means of a growth rate inhibition test with the marine bacterium V. fischeri. In a minimal medium (following the validity criterion of achieving a least cell multiplication rate) containing only yeast extract as organic nutrient component, V. fischeri was to Cu2+ 11.9 times, to Hg2+ 3 times and to Zn2+ 2.8 times more sensitive than in the complete defined medium. Obviously yeast extract did not interfere greatly in the complexation mechanisms. The detection limits of the toxicity of Cd2+, 3,5-dichorophenol and nitrobenzene were not influenced by the substrate.

A highly sensitive bacterial assay for toxins based on swarming inhibition, and comparison with the cup plate assay based on growth inhibition

The motility inhibition of the swarming bacteria Proteus mirabilis and Azospirillum brasilense was found to be an appropriate parameter to indicate toxic effects caused by some mycotoxins, lactones and anhydrides of dicarboxylic acids. If these substances are in contact with the motile bacteria the following phenomena can be observed: at a certain toxin concentration the swarming of the bacteria is inhibited. If the concentration is increased the swarming ceases, and at still higher concentrations the bacteria are inactivated. In some instances swarming is stimulated at very low toxin doses. The sensitivity of this assay is comparable to the cup plate assay based on growth inhibition with Bacillus thuringiensis [Lenz, P. et al., Toxicology, 40 (1986) 199, Boutibonnes et al., Pharmacology, 11 (1983) 430] and in some cases it is even higher.