Cobalt mitigates zinc-starvation effects in Pseudomonas aeruginosa.

Bacterial pathogens must rapidly adapt to fluctuating metal availability within the host, where essential micronutrients are actively sequestered as part of nutritional immunity. Among these, zinc is a critical cofactor for a wide array of enzymes and regulatory proteins, and its availability is tightly linked to the expression of key virulence traits in Pseudomonas aeruginosa. This opportunistic pathogen employs different zinc acquisition systems transcriptionally regulated by the Zinc Uptake Regulator Zur, enabling its persistence within the host. Recently, Zur-controlled operons involved in the uptake/export of cobalt have been identified. Although cobalt is primarily associated with cobalamin-dependent reactions, its selective import under zinc-limiting conditions suggests a potential role for cobalt in bacterial adaptation to zinc scarcity. Yet, the functional relevance of this metal-based compensation remains poorly defined. This study shows that cobalt supplementation alleviates key effects of severe zinc deficiency in P. aeruginosa, including reduced pyocyanin production, impaired swarming motility, and enhanced sensitivity to oxidative stress. Furthermore, in vitro assays demonstrate that cobalt can functionally replace zinc in the proteases LasA and LasB and the transcriptional regulator Zur. Finally, we found that a P. aeruginosa strain deficient in the pyochelin-cobalt receptor PA2911 exhibits impaired colonization of Galleria mellonella larvae, supporting the hypothesis that cobalt compensatory function may be crucial during infection. Our results suggest that cobalt may play a broader biological role than previously recognized, highlighting its potential to support P. aeruginosa survival and pathogenicity in zinc-limiting environments.