New insights into iron uptake in Streptococcus mutans: evidence for a role of siderophore-like molecules

Streptococcus mutans, a gram-positive coccus commonly found in the human oral cavity, is the primary causative agent of dental caries as well as infective endocarditis. Bacteria produce potent iron chelators called siderophores to absorb iron. Because, there are few studies on siderophore-mediated iron transport in S. mutans, the current study investigates the presence of such a mechanism in S. mutans GS-5. Deferration of culture medium and different concentrations of 2, 2'-Bipyridyl has been used to simulate iron-restricted conditions. Iron restriction alters the colony morphology and slows bacterial growth. Cross-feeding conditioned medium into an iron-restricted medium promotes bacterial growth, indicating the presence of siderophore-like molecules. This was further confirmed by Chrome Azurol S (CAS) assay and Modified CAS-agar assay. Cśaky's and Arnow's assays detected the presence of hydroxamate and catecholate-type molecules in optimal and iron-restricted conditions, respectively. Further, the siderophore-like molecules were extracted and purified with thin layer chromatography (TLC). TLC elutes were also found to be positive for iron-chelation in CAS-agar assay and aided growth of S. mutans under iron-restricted conditions. LC-MS analysis of culture supernatants under iron-restricted conditions identified iron-binding small molecules, including a catechol structural motif. Computational analysis utilizing KEGG and BLASTp suggested homologues of siderophore biosynthesis and transport proteins, including genes associated with mutanobactin production. These findings indicate a possible siderophore-mediated iron uptake mechanism in S. mutans GS-5, warranting further molecular studies and advanced spectroscopic characterization of this unidentified siderophore. Once confirmed, this mechanism can be used as a potential drug target to control streptococcal infection.