The Alternative Role of Enterobactin as an Oxidative Stress Protector Allows Escherichia coli Colony Development

Abstract

Numerous bacteria have evolved different iron uptake systems with the ability to make use of their own and heterologous siderophores. However, there is growing evidence attributing alternative roles for siderophores that might explain the potential adaptive advantages of microorganisms having multiple siderophore systems. In this work, we show the requirement of the siderophore enterobactin for Escherichia coli colony development in minimal media. We observed that a strain impaired in enterobactin production (entE mutant) was unable to form colonies on M9 agar medium meanwhile its growth was normal on LB agar medium. Given that, neither iron nor citrate supplementation restored colony growth, the role of enterobactin as an iron uptake-facilitator would not explain its requirement for colony development. The absence of colony development was reverted either by addition of enterobactin, the reducing agent ascorbic acid or by incubating in anaerobic culture conditions with no additives. Then, we associated the enterobactin requirement for colony development with its ability to reduce oxidative stress, which we found to be higher in media where the colony development was impaired (M9) compared with media where the strain was able to form colonies (LB). Since oxyR and soxS mutants (two major stress response regulators) formed colonies in M9 agar medium, we hypothesize that enterobactin could be an important piece in the oxidative stress response repertoire, particularly required in the context of colony formation. In addition, we show that enterobactin has to be hydrolyzed after reaching the cell cytoplasm in order to enable colony development. By favoring iron release, hydrolysis of the enterobactin-iron complex, not only would assure covering iron needs, but would also provide the cell with a molecule with exposed hydroxyl groups (hydrolyzed enterobactin). This molecule would be able to scavenge radicals and therefore reduce oxidative stress.

Bacterial Siderophores Hijack Neutrophil Functions

“…Accordingly, it was presumed that Ent may exert a plethora of key physiological functions that could account for its high metabolic cost (38), aside from acquiring iron to support bacterial growth. Such notion has been demonstrated in studies whereby Ent was shown to be fundamental for E. coli to establish colonization in the mammalian gut (39), develop mature biofilms (40, 41), alleviate oxidative stress (42, 43), and neutralize the anti-microbial activity of MPO (21). The multifaceted property of Ent is further exemplified by our findings that it could inhibit an array of neutrophil functions, including ROS generation, NETs formation, degranulation, phagocytosis and bacterial killing activity.…”

Influence of Niche-Specific Nutrients on Secondary Metabolism in Vibrionaceae

“…4). Siderophores similar to fluvibactin can inhibit bacterial and fungal growth (20,56), and catechol iron chelators have also been suggested to protect bacteria from oxidative stress (57,58). Hence, beside the competitive advantage during surface colonization due to the antibacterial activity of fluvibactin, producers of this compound might as well be protected from oxidative stress, which is a prevalent phenomenon in the marine environment (59).…”

Beyond iron: non-classical biological functions of bacterial siderophores

“…Notably, this function is associated with the redox activity of the enterobactin catecholates rather than iron transport. 204 Moreover, E. coli strains that cannot produce enterobactin are unable to form colonies on minimal medium. This effect could not be reversed with iron supplementation; however, addition of enterobactin or a reducing agent such as ascorbic acid to the agar allowed the colonies to form, as did anaerobic incubation.…”

“…On the basis of these results, the catecholate moieties of the siderophore are proposed to scavenge radicals. 204 …”