Structure–Activity Analysis of Peptide-Mediated Quorum Sensing in Streptococcus mitis

Antibiotic-resistant bacteria are one of the greatest challenges in modern medicine, as multidrug-resistant strains continue to outpace traditional antibiotic development. Unlike conventional antibiotics that promote resistance through selective pressure, quorum sensing (QS)-targeted therapies disrupt bacterial communication systems that regulate virulence, biofilm formation, and resistance acquirement through competence without directly killing the bacteria. In this study we examined QS modulation in Streptococcus mitis, a commensal oral bacterium that has recently been identified as an opportunistic pathogen capable of causing serious infections in immunocompromised individuals. The competence regulon QS system in S. mitis is controlled by the competence-stimulating peptide (CSP), which activates the histidine kinase receptor ComD to drive QS-regulated processes. To define the molecular interactions that drive CSP1:ComD binding and lead to ComD activation, we conducted a systematic structure-activity analysis of the S. mitis-CSP1 sequence. To this end, we synthesized and screened two S. mitis-CSP1 analog libraries: the first, an alanine scan to identify key side-chain residues responsible for activity, and the second, a D-amino acid scan to evaluate the effect of sidechain spatial orientation. Then, following the construction of a luminescence-based S. mitis NCTC 8033 QS reporter strain, we conducted reporter gene bioassays to gain insights into the structure-activity relationship between S. mitis-CSP1 and its cognate ComD receptor. Interestingly, our results revealed that the Glu1-to-Ala analog (S. mitis-CSP1-E1A) acted as an activator in S. mitis NCTC 8033, whereas the same Glu1-to-Ala substitution in CSPs from other Streptococcus species, as well as in a different S. mitis pherotype producing and responding to S. mitis-CSP2, has been reported to produce inhibitory analogs. These results suggest that the ComD receptor in S. mitis NCTC 8033 interacts with S. mitis-CSP1 differently than its homologs in other Streptococcus species, highlight the importance of species-specific studies and caution against overgeneralizing bacterial behavior within a given genus. Overall, our work offers mechanistic insight into S. mitis QS signaling and lays a foundation for the rational development of CSP-based tools to study and potentially control QS-regulated processes in streptococci.

Level thesis:degree_level

Master’s Degree

Year dc:date.issued

2026

Author dc:creator

• Yeager, Morgan

Advisor dc:contributor.advisor

• Tal-Gan, Yftah

Committee members dc:contributor.committeemember

• Alpuche, Mario A.
• Harper, Jeff

• Peptide Chemistry
• Quorum Sensing
• Streptococcus mitis

Language dc:language.iso

en_US, English

Repository record dc:identifier.uri

https://scholarwolf.unr.edu/handle/11714/11954

OAI identifier oai:identifier

oai:scholarwolf.unr.edu:11714/11954