Abstract
Streptococcal glucosyltransferases (Gtf) synthesize α-glucan exopolymers which contribute to biofilm matrix. Streptococcus oralis interacts with the opportunistic pathogen Candida albicans to form hypervirulent biofilms. S. oralis 34 has a single gtf gene (gtfR). However, the role of gtfR in single and mixed species biofilms with C. albicans has never been examined. A gtfR deletion mutant, purified GtfR, and recombinant GtfR glucan-binding domain were tested in single and mixed biofilms on different substrata in vitro. A mouse oral infection model was also used. We found that in single species biofilms growing with sucrose on abiotic surfaces S. oralis gtfR increased biofilm matrix, but not bacterial biomass. In biofilms with C. albicans, S. oralis encoding gtfR showed increased bacterial biomass on all surfaces. C. albicans had a positive effect on α-glucan synthesis, and α-glucans increased C. albicans accretion on abiotic surfaces. In single and mixed infection of mice receiving sucrose S. oralis gtfR enhanced mucosal burdens. However, sucrose had a negative impact on C. albicans burdens and reduced S. oralis burdens in co-infected mice. Our data provide new insights on the GtfR-mediated interactions between the two organisms and the influence of biofilm substratum and the mucosal environment on these interactions.
Highlights
Supplementary information The online version of this article contains supplementary material, which is available to authorized users.Glucosyltransferases (Gtfs) are streptococcal extracellular or cell-associated exoenzymes that hydrolyze sucrose and synthesize glucan polymers from the resulting glucose [1, 2]
In 24 h biofilms growing in 1% sucrose we found a significant, greater than twofold increase in S. oralis counts with all three strains in mixed biofilms compared with single S. oralis biofilms, suggesting that the positive effect of C. albicans on S. oralis biomass does not require GtfR activity
Previous studies showed that C. albicans and S. oralis have a mutualistic relationship in the biofilm growth state which promotes fungal virulence [22, 23]
Summary
Glucosyltransferases (Gtfs) are streptococcal extracellular or cell-associated exoenzymes that hydrolyze sucrose and synthesize glucan polymers from the resulting glucose [1, 2] These extracellular polymers, produced in various proportions of α-1,6 and α-1,3-linkages [3], contribute to the matrix that forms the scaffold for the three-dimensional architecture of biofilms, increasing bacterial adhesion, coaggregation, and tolerance against antimicrobials [4,5,6,7,8]. The Gtfs of Streptococcus mutans have been well characterized, due to their role in dental caries development, the biological roles of Gtf from the mitis group streptococci, including S. oralis, in oral biofilm communities are less clear Streptococci of this group have been recognized as main initial colonizers in biofilms formed on tooth surfaces [9,10,11] and glucans have been implicated in facilitating biofilm accumulation [12, 13]
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