Toxic effects of tin compounds on microorganisms

Organotins are used for industrial and agricultural purposes and in antibiologic agents. They are significantly more toxic than inorganic tins, and eventually reach the environment where they can be toxic to a wide variety of organisms. Particular attention has been given to tributyltins which are highly toxic components of antifouling paints. Realization that the molecular species of organotin influences fate and effects of organotins led to development of sensitive methods for quantifying individual molecular species. Even though such methods are now available, little information has been obtained on the ability of microorganisms to bioaccumulate tin compounds. Trisubstituted alkyl and aryltins (R₃Sn's) are more toxic than disubstituted compounds (R₂Sn's) while monosubstituted organotins (RSn's) are still less toxic. R₄Sn's are toxic only if they are metabolized to R₃Sn's. Among trisubstituted compounds propyl-, butyl-, pentyl-, phenyl-, and cyclohexyl Sn's are generally the most toxic to microorganisms. Toxicity in the R₃Sn series is related to total molecular surface area of the tin compound and to the octanol:water partition coefficient, K_ow, which is a measure of hydrophobicity; a high K_ow indicates greater hydrophobicity and predicts greater toxicity. Care must be taken when testing the toxicity of tin compounds, for a number of biological, physical and chemical factors can influence the apparent toxicity. Although little is known of the effects of tin compounds on microbial processes, a number of bacterial processes can be inhibited by organotins and all relate to membrane functions. They include effects on energy transduction, solute transport and retention and oxidation of substrates. Very little is known of how organotins exert their toxic effects on algae and fungi; Information on effects on chloroplasts and mitochondria stems principally from animal systems and from higher plants. Triorganotins act against chloroplasts and mitochondria by causing swelling, by acting as ionophores and by acting against ATPase, while diorganotins appear to act by binding to dithiol groups on enzymes and cofactors. Nucleic acids do not seem to be affected at environmentally relevant concentrations. Virtually nothing is known of the action of tin compounds on microbial enzymes, but resistant mutants are easy to obtain and should facilitate work to understand modes of microbial interaction with tin compounds and mechanisms of resistance.