Yung-Hua Li received his Doctorate in Molecular Microbiology at University of Manitoba. Following his Post-doctoral fellowships in the University of British Columbia and University of Rochester, NY, he worked as a Scientist in the University of Toronto, with his research focus on molecular dissection of microbial biofilms. In 2004, he joined the Faculties of Dentistry and Medicine at Dalhousie University, where he has been directing a research team on genetic analyses of bacterial biofilms, biofilm ecology and pathogenesis.
Streptococcus mutans is a primary etiological agent of dental caries worldwide. Natural life of S. mutans in dental biofilms often faces life-threatening insults, such as killing by antibiotics or innate defense molecules produced by competing species or by the host. How such insults affect physiology and virulence of S. mutans is poorly understood. In this study, we explored this question by analyzing the effects of sub-MIC concentrations of bacitracin and -defensin 3 on S. mutans. Microarray analysis showed that both bacitracin and -defensin 3 induced differential expression of subsets of genes that were largely regulated by the BceABRS four-component system. The results were further confirmed by examining gene expression profiles of selected genes or genetic loci using qRT-PCR. We then examined the effects of gene deletion of bceRS on the peptide antibiotics and virulence. The results showed that a deletion of bceRS resulted in a mutant that was sensitive to bacitracin or -defensin 3. Introduction of a wild copy of becRS in trans (complementation) restored the wild type phenotype of the mutant. In particular, both peptide antibiotics at a sub-MIC induced biofilm formation in the parent but not in the mutant. A competitive fitness analysis showed that the mutant was unable to compete with the parent for co-existence in duel-strain mixed cultures in the presence of bacitracin. In conclusion, the BceABRS four-component system controls a regulon that is required for sensing, response and resistance to bacitracin and -defensin. This system may play an important role in adaptation and virulence expression of S. mutans in dental biofilms.
Xiao-Lin Tian has received her MD from Shanghai Medical University. Since 1993, she worked as a Research Technician in Novopharm Biotech Inc. in Winnepeg for six years. She then worked in the Mount Sinai Hospital Lunenfeld Research Institute, Toronto, for another six years. Since 2006, she has been working as a Researcher at Dalhousie University, with expertise in Moleculr Biology, Bacterial Biofilms and Pathogenesis.
Streptococcus mutans is a leading cariogenic pathogen of dental caries worldwide. Clinically, eliminating S. mutans from dental biofilms using antibiotics is not practical, because these agents indiscriminately kill other members of the resident microflora, leading to ecological disruption and other negative clinical consequences. To develop target-specific antimicrobials, we evaluated several fusion peptides and identified a new peptide HP30 that showed a high selectivity for targeted killing of S. mutans. In the dual-species cultures, 80% of S. mutans cells were killed, but only 20% of S. sanguinis were killed following exposure to HP30 (5.0 M) for 15 min. Similarly, 80% of S. mutans cells were killed but only 5% of Actinomyces naeuslundii were killed following the same exposure. The peptide-guided killing was also confirmed in the dual-species biofilms and the killing increased with increasing concentrations of HP30. However, a combination of low concentrations of HP30 with EDTA well maintained the killing activity against S. mutans in the biofilms. A S. mutans mutant lacking the ComD receptor only showed 20% of killing, while a ComD overexpression strain showed 90% of killing, suggesting that HP30 predominantly binds to the ComD receptor before triggering the selective killing. New peptide HP30 displays a high selectivity for targeted killing of S. mutans due to an improved binding of the peptide to the ComD receptor.