Involvement of the adc Operon and Manganese Homeostasis in Streptococcus gordonii Biofilm Formation

Abstract

Pioneer oral bacteria, including Streptococcus gordonii, initiate the formation of oral biofilms on tooth surfaces, which requires differential expression of genes that recognize unique environmental cues. An S. gordonii::Tn917-lac biofilm-defective mutant was isolated by using an in vitro biofilm formation assay. Subsequent inverse PCR and sequence analyses identified the transposon insertion to be near the 3 end of an open reading frame (ORF) encoding a protein homologous to a Streptococcus pneumoniae repressor, AdcR. The S. gordonii adc operon, consisting of the four ORFs adcR, adcC, adcB, and adcA, is homologous to the adc operon of S. pneumoniae, which plays a role in zinc and/or manganese transport and genetic competence in S. pneumoniae. AdcR is a metal-dependent repressor protein containing a putative metal-binding site, AdcC contains a consensus-binding site for ATP, AdcB is a hydrophobic protein with seven hydrophobic membranespanning regions, and AdcA is a lipoprotein permease with a putative metal-binding site. The three proteins (AdcC through -A) are similar to those of the binding-lipoprotein-dependent transport system of gram-positive bacteria. Reverse transcriptase PCR confirmed that adcRCBA are cotranscribed as an operon in S. gordonii and that the transposon insertion in S. gordonii adcR::Tn917-lac had resulted in a polar mutation. Expression of adcR, measured by the ␤-galactosidase activity of the adcR::Tn917-lac mutant, was growth phase dependent and increased when the mutant was grown in media with high levels of manganese (>1 mM) and to a lesser extent in media with zinc, indicating that AdcR may be a regulator at high levels of extracellular manganese. A nonpolar inactivation of adcR generated by allelic replacement resulted in a biofilm-and competence-defective phenotype. The biofilm-defective phenotype observed suggests that AdcR is an active repressor when synthesized and acts at a distant site(s) on the chromosome. Thus, the adc operon is involved in manganese acquisition in S. gordonii and manganese homeostasis and appears to modulate sessile growth in this bacterium.Oral streptococci (viridans streptococci) initiate the formation of oral biofilms known as dental plaque and are ubiquitous primary colonizers that are the predominant cultivable bacteria found in supragingival plaque (45). Viridans streptococci are also frequent etiologic agents of bacterial endocarditis (21, 64) and are becoming significant opportunistic pathogens and a major cause of bacteremia in immunocompromised patients, accounting for 40% of infections in neutropenic patients (25,32). Fully developed, surface-attached oral biofilms are highly structured, with distinct architectural and physiochemical properties commonly observed with other biofilm communities (12). Sessile bacteria found in biofilms represent a distinct mode of growth that differs from planktonic cells, characterized by differences in gene expression and cellular physiology and an increased resistance to antibiotics.The multistep process of biofil...

Variation in the Group BStreptococcusCsrRS Regulon and Effects on Pathogenicity

“…These included the oppA1-F operon (SAG_0148 to -0152), encoding an oligopeptide ABC transporter that has been implicated in modulating the attachment of GBS to host cells and the adc operon (SAG_0154 to -0156), homologs of which encode a zinc/manganese transporter in Streptococcus pneumoniae and a manganese acquisition and homeostasis system in Streptococcus gordonii (6,16,25). Also regulated by CsrRS are homologs of a system involved in iron transport (SAG_1007-1010).…”

Physiological impact of transposable elements encoding DDE transposases in the environmental adaptation of Streptococcus agalactiae

“…The addition of zinc to the medium almost totally restores the transformability of the mutants and is required soon after contact of cells with the CSP competence-stimulating peptide (Dintilhac et al, 1997). In Streptococcus gordonii, a transpositional adcR polar mutant is also defective in competence and impaired in biofilm formation (Loo et al, 2003). Finally, in B. subtilis, transpositional mutants of the genes encoding the ZnuABC high-affinity zinc transporter and of zosA encoding a P-type ATPase for zinc incorporation exhibit low transformability (Ogura, 2011).…”

“…Several studies based on the search for transformation-deficient mutants revealed the involvement of genes encoding highaffinity zinc transporters in the genetic competence of streptococci and other Gram-positive bacteria (Dintilhac et al, 1997;Loo et al, 2003;Ogura, 2011). The adcCBA operon (called adc for adhesin competence repressor) was identified as being indispensable for the competence of S. pneumoniae (Dintilhac et al, 1997).…”

Quorum Sensing in Plaque Biofilms: Challenges and Future Prospects

“…Mutations in the adc operon, which shows high similarity to a metal uptake system of Streptococcus pneumoniae, resulted in a S. gordonii strain defective in biofilm formation and competence. 18,19 Since trace elements are essential for growth and survival, it is not surprising that sensing levels of available metals and biofilm development are linked. However, the authors propose an intriguing hypothesis that high levels of available metals, such as zinc or manganese, may be a signal for the cells to switch from surface-attached growth to planktonic growth and dissemination.…”