Background: Cadmium (Cd²⁺) is one of the highly toxic heavy metals and is considered as a carcinogenic agent. Our aim was to confirm the ability of Frankia alni ACN14a to resist Cd²⁺ and to determine the genes involved in the resistance mechanism.

Results: F. alni ACN14a and Frankia casuarinae CcI3 hyphae showed up to 10 and 22 times Cd²⁺ accumulation when exposed to 1 mM Cd²⁺, respectively. Scanning electron microscopy (SEM) exhibited a stable Cd²⁺ precipitate on the cell surface, and the increase in Cd²⁺ weight level reached 16.45% when evaluated with SEM-EDAX analysis. The following two potential Cd²⁺ operons were identified: 1. cadCA operon, which encodes a copper-transporting P-type ATPase A (cadA, FRAAL0989) and an ArsR family regulator (cadC, FRAAL0988), with 37- and 70-fold increase in their expression by qRT-PCR, respectively and 2. cadB/DX, which encodes a putative cobalt-zinc-cadmium resistance protein (cadD, FRAAL3628) and heavy metal-associated domain protein (cadX, FRAAL3626), with 22- and 16-fold upregulation when exposed to Cd²⁺ stress.

Conclusions: Cd²⁺ tolerance by F. alni ACN14a involved efflux of Cd²⁺ outside the cells and binding it to the membrane surface. Our results indicate the existence of two cadmium-resistance mechanisms in Frankia strains, which support the idea of using them as a bioremediation agent.