Disruption of a Novel Iron Transport System Reverses Oxidative Stress Phenotypes of a dpr Mutant Strain of Streptococcus mutans

ABSTRACT

The Dps-like peroxide resistance protein (Dpr) is essential for H ₂O ₂ stress tolerance and aerobic growth of the oral pathogen Streptococcus mutans. Dpr accumulates during oxidative stress, protecting the cell by sequestering iron ions and thereby preventing the generation of toxic hydroxyl radicals that result from the interaction of iron with H ₂O ₂. Previously, we reported that the SpxA1 and SpxA2 regulators positively regulate expression of dpr in S. mutans. Using an antibody raised against S. mutans Dpr, we confirmed at the protein level the central and cooperative nature of SpxA1 and SpxA2 regulation in Dpr production. During phenotypic characterization of the S. mutans Δ dpr strain, we observed the appearance of distinct colony variants, which sometimes lost the oxidative stress sensitivity typical of Δ dpr strains. Whole-genome sequencing of these phenotypically distinct Δ dpr isolates revealed that a putative iron transporter operon, smu995-smu998, was a genomic hot spot with multiple single nucleotide polymorphisms identified within the different isolates. Deletion of smu995 or the entire smu995-smu998 operon in the Δ dpr background strain completely reversed the oxidative stress-sensitive phenotypes associated with dpr inactivation. Conversely, inactivation of genes encoding the ferrous iron transport system FeoABC did not alleviate phenotypes of the Δ dpr strain. Preliminary characterization of strains lacking smu995-smu998, feoABC, and the iron/manganese transporter gene sloABC revealed the interactive nature of these three systems in iron transport but also indicated that there may be additional iron uptake systems in S. mutans.

IMPORTANCE The dental caries-associated pathogen Streptococcus mutans routinely encounters oxidative stress within the human plaque biofilm. Previous studies revealed that the iron-binding protein Dpr confers protection toward oxidative stress by limiting free iron availability, which is associated with the generation of toxic hydroxyl radicals. Here, we report the identification of spontaneously occurring mutations within Δ dpr strains. Several of those mutations were mapped to the operon smu995-smu998, revealing a previously uncharacterized system that appears to be important in iron acquisition. Disruption of the smu995-smu998 operon resulted in reversion of the stress-sensitive phenotype typical of a Δ dpr strain. Our data suggest that the Smu995-Smu998 system works along with other known metal transport systems of S. mutans, i.e., FeoABC and SloABC, to coordinate iron uptake.