Manganese transport by Streptococcus sanguinis in acidic conditions and its impact on growth in vitro and in vivo

Abstract

Streptococcus sanguinis is an oral commensal and an etiological agent of infective endocarditis. Previous studies have identified the SsaACB manganese transporter as essential for endocarditis virulence; however, the significance of SsaACB in the oral environment has never been examined. Here we report that a ΔssaACB mutant of strain SK36 exhibits reduced growth and manganese uptake under acidic conditions. Further studies revealed that these deficits resulted from the decreased activity of TmpA, shown in the accompanying paper to function as a ZIP-family manganese transporter. Transcriptomic analysis of fermentor-grown cultures of SK36 WT and ΔssaACB strains identified pH-dependent changes related to carbon catabolite repression in both strains, though their magnitude was generally greater in the mutant. In strain VMC66, which possesses a MntH transporter, loss of SsaACB did not significantly alter growth or cellular manganese levels under the same conditions. Interestingly, there were only modest differences between SK36 and its ΔssaACB mutant in competition with Streptococcus mutans in vitro and in a murine oral colonization model. Our results suggest that the heterogeneity of the oral environment may provide a rationale for the variety of manganese transporters found in S. sanguinis and point to strategies for enhancing the safety of oral probiotics.

Contribution of a ZIP‐family protein to manganese uptake and infective endocarditis virulence in Streptococcus sanguinis

“…If this is also true for SsaB, TmpA's function could become essential under the same condition. In our accompanying manuscript (Puccio et al., 2022), we found that loss of Δ ssaACB in SK36 was detrimental to growth in low‐pH media, which we hypothesize is due to acid sensitivity of TmpA. In VMC66, lack of Δ ssaACB had no effect in the same conditions, likely because MntH is still functional at the pH tested.…”

“…Likewise, we inserted the ssaACB genes into the site where tmpA had been. Based on the short distance separating tmpA and its upstream neighbors from one another and on the patterns of sequence read mapping to this region in our recent RNA‐seq studies (Puccio et al., 2022; Puccio et al., 2020), we believe the tmpA promoter lies upstream from SSA_1417 (SSA_RS06950). It is important to note, however, that by placing the ssaACB operon in the exact location of tmpA , we eliminated the need to identify the native tmpA promoter.…”

“…We have been unable to find a differential phenotype for the Δ tmpA single mutant as compared to WT, with the exception of a slight increase in endocarditis virulence (Figure 7a). In our accompanying manuscript (Puccio et al., 2022), we tested its growth in low‐pH BHI and saw no significant differences. We also assessed the growth in a biofilm assay in competition with St. mutans and observed no difference (data not shown).…”

“…Additionally, inhibitors of SsaB orthologs have been under investigation, as they could potentially be used to treat other streptococcal diseases in addition to IE (Bajaj et al., 2015; Obaidullah et al., 2018). Furthermore, given that MntH does not significantly contribute to virulence but remains a functional manganese transporter at low pH (Puccio et al., 2022), its presence could lessen the effect of anti‐SsaB drugs on the oral microbiota and may influence the selection of potential probiotic anti‐caries streptococcal strains. Although future studies will be required, we hypothesize that anti‐SsaB drugs could potentially remain effective at preventing IE without affecting growth or competitiveness of MntH‐encoding streptococcal strains in the oral cavity.…”