Population changes induced in Candida albicans by nalidixic acid

Abstract

Cells of Candida albicans plated on media containing nalidixic acid (Nal) either die, adapt physiologically to Nal-tolerance or mutate to Nal-resistance. The fraction of a population exhibiting each response depends on the growth phase of cells when plated and their nitrogen and carbon nutrition and growth temperatures before and after plating. Nal induces Nal-resistant mutants in very high frequency but only at 37 C on plates containing i) glucose as primary carbon source and ii) adenine, a sulfur amino acid or a representative of the glutamic acid family of amino acids. Nal does not affect either forward mutation to caffeine-resistance or reverse mutation from histidine auxotrophy to prototrophy. Nal-resistant mutants produce minute colonies on Nal-free medium, respire oxidatively and are unusually sensitive to inhibitors of oxidative phosphorylation. They revert spontaneously to wild type at very high rates but can be propagated indefinitely in the absence of Nal by serial selection and replating of minute colonies. Cellular inactivation and induction of Nal-resistant mutants are greatly affected by specific inhibitors of mitochondrial macromolecular syntheses. The presence of chloramphenicol or erythromycin during exposure to Nal prevents cell death and mutation but has no effect on adaptation to Nal-tolerance. Growth on acriflavin or ethidium bromide enhances resistance of cells to inactivation when subsequently plated on Nal containing media. It is concluded that Nal-induced cellular inactivation and mutation to Nal-resistance, but not adaptation to Nal-tolerance, result from damages to the mitochondrion which are fixed or promoted by macromolecular syntheses within the mitochondrion. Implications of these findings for the therapeutic use of Nal are discussed.