Manganese uptake, mediated by SloABC and MntH, is essential for the fitness of Streptococcus mutans

Early epidemiological studies implicated manganese (Mn) as a possible caries-promoting agent while laboratory studies have indicated that manganese stimulates the expression of virulence-related factors in the dental pathogen Streptococcus mutans . To better understand the importance of manganese homeostasis to S. mutans pathophysiology, we first used RNA sequencing to obtain the global transcriptional profile of S. mutans UA159 grown under Mn-restricted conditions. Among the most highly expressed genes were the entire sloABC operon, encoding a dual iron/manganese transporter, and an uncharacterized gene, herein mntH , that codes for a protein bearing strong similarity to Nramp-type transporters. While inactivation of sloC , which encodes the lipoprotein receptor of the SloABC system, or mntH alone had no major consequence on the overall fitness of S. mutans , simultaneous inactivation of sloC and mntH (Δ sloC Δ mntH ) impaired growth and survival under Mn-restricted conditions, including in human saliva or in the presence of calprotectin. Further, disruption of Mn transport resulted in diminished stress tolerance and reduced biofilm formation in the presence of sucrose. These phenotypes were markedly improved when cells were provided with excess Mn. Metal quantifications revealed that the single mutant strains contain similar intracellular levels of Mn as the parent strain, whereas Mn was nearly undetectable in the Δ sloCΔmntH strain. Collectively, these results reveal that SloABC and MntH work independently and cooperatively to promote cell growth under Mn-restricted conditions, and that mauitanence of Mn homeostasis is essential for the expression of major virulence attributes in S. mutans . IMPORTANCE As trace biometals such as manganese (Mn) are important for all forms of life, the ability to regulate biometals availability during infection is an essential trait of successful bacterial pathogens. Here, we showed that the caries pathogen Streptococcus mutans utilizes two Mn transport systems, namely SloABC and MntH, to acquire Mn from the environment, and that the ability to maintain the cellular levels of Mn is important for the manifestation of characteristics that associate S. mutans with dental caries. Our results indicate that the development of strategies to deprive S. mutans of Mn hold promise in the combat against this important bacterial pathogen.