Abstract
Biofilm accounts for 65–80 % of microbial infections in humans. Considerable evidence links biofilm formation by oral microbiota to oral disease and consequently systemic infections. Streptococcus sanguinis, a Gram-positive bacterium, is one of the most abundant species of the oral microbiota and it contributes to biofilm development in the oral cavity. Due to its altered biofilm formation, we investigated a biofilm mutant, ΔSSA_0351, that is deficient in type I signal peptidase (SPase) in this study. Although the growth curve of the ΔSSA_0351 mutant showed no significant difference from that of the wild-type strain SK36, biofilm assays using both microtitre plate assay and confocal laser scanning microscopy (CLSM) confirmed a sharp reduction in biofilm formation in the mutant compared to the wild-type strain and the paralogous mutant ΔSSA_0849. Scanning electron microscopy (SEM) revealed remarkable differences in the cell surface morphologies and chain length of the ΔSSA_0351 mutant compared with those of the wild-type strain. Transcriptomic and proteomic assays using RNA sequencing and mass spectrometry, respectively, were conducted on the ΔSSA_0351 mutant to evaluate the functional impact of SPase on biofilm formation. Subsequently, bioinformatics analysis revealed a number of proteins that were differentially regulated in the ΔSSA_0351 mutant, narrowing down the list of SPase substrates involved in biofilm formation to lactate dehydrogenase (SSA_1221) and a short-chain dehydrogenase (SSA_0291). With further experimentation, this list defined the link between SSA_0351-encoded SPase, cell wall biosynthesis and biofilm formation.
Highlights
A biofilm is an accumulation of uni- or polymicrobial species embedded in a protective extracellular polymeric matrix that adheres to biotic or abiotic surfaces [1] and forms a nutrient-sufficient ecosystem as a sessile microbial community
MG1655, SpsB from Staphylococcus aureus subsp. aureus NCTC 8325 and SipW from Bacillus subtilis subsp. subtilis str. 168) to SSA_0351, we constructed a phylogenic tree based on hierarchical clustering to show the evolutionary relatedness of these signal peptidase (SPase) (Fig. 1b)
Oral biofilm formation in streptococci has been shown to be involved in a variety of microbial infections in the human body, through recruiting diverse bacterial species to the site of infection and displaying an effective defence system against host immune defences [1]
Summary
A biofilm is an accumulation of uni- or polymicrobial species embedded in a protective extracellular polymeric matrix that adheres to biotic or abiotic surfaces [1] and forms a nutrient-sufficient ecosystem as a sessile microbial community. According to estimates by the US Centers for Disease Control and Prevention and the National Institutes of Health, biofilms account for 65–80 % of microbial infections in human beings [1, 2]. Many studies have shown considerable evidence linking biofilm formation in the oral cavity to oral disease and systemic infections. These systemic conditions include cardiovascular disease, diabetes mellitus, preterm births or low birth weights, rheumatoid arthritis and infective endocarditis [1, 3]. Cells in a biofilm exhibit increased tolerance to antibiotics as the biofilm matrix decreases the antibiotic diffusion rates to the physiologically dormant persister cells that are inherently resistant to antibiotics. Biofilm establishment provides a survival advantage as a defence system against host immune defences such as macrophages [4,5,6]
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