Making Fluorescent Streptococci and Enterococci for Live Imaging

Structural and functional insights into a novel two-component endolysin encoded by a single gene in Enterococcus faecalis phage

Using bacteriophage-derived endolysins as an alternative strategy for fighting drug-resistant bacteria has recently been garnering renewed interest. However, their application is still hindered by their narrow spectra of activity. In our previous work, we demonstrated that the endolysin LysIME-EF1 possesses efficient bactericidal activity against multiple strains of Enterococcus faecalis (E. faecalis). Herein, we observed an 8 kDa fragment and hypothesized that it contributes to LysIME-EF1 lytic activity. To examine our hypothesis, we determined the structure of LysIME-EF1 at 1.75 Å resolution. LysIME-EF1 exhibits a unique architecture in which one full-length LysIME-EF1 forms a tetramer with three additional C-terminal cell-wall binding domains (CBDs) that correspond to the abovementioned 8 kDa fragment. Furthermore, we identified an internal ribosomal binding site (RBS) and alternative start codon within LysIME-EF1 gene, which are demonstrated to be responsible for the translation of the truncated CBD. To elucidate the molecular mechanism for the lytic activity of LysIME-EF1, we combined mutagenesis, lytic activity assays and in vivo animal infection experiments. The results confirmed that the additional LysIME-EF1 CBDs are important for LysIME-EF1 architecture and its lytic activity. To our knowledge, this is the first determined structure of multimeric endolysin encoded by a single gene in E. faecalis phages. As such, it may provide valuable insights into designing potent endolysins against the opportunistic pathogen E. faecalis.

LysIME-EF1, an endolysin that lyses E. faecalis, displays the prospect of treating E. faecalis infection. We find that the C-terminal cell-wall binding domain (CBD) is important for the lytic activity of LysIME-EF1. By determining the crystal structures of wild type (WT) LysIME-EF1 and its C-terminal CBD, this study reveals how the holoenzyme is organized to carry out its highly efficient lytic activity. Our finding provides structural and functional evidence that LysIME-EF1 belongs to a unique two-component multimeric endolysin encoded by a single gene.

Fluorescence Tools Adapted for Real-Time Monitoring of the Behaviors of Streptococcus Species

Tagging of bacteria with fluorescent proteins has become an essential component of modern microbiology. Fluorescent proteins can be used to monitor gene expression and biofilm growth and to visualize host-pathogen interactions. Here, we developed a collection of fluorescent protein reporter plasmids for Streptococcus mutans UA159 and other oral streptococci. Using superfolder green fluorescent protein (sfGFP) as a reporter for transcriptional activity, we were able to characterize four strong constitutive promoters in S. mutans These promoter-sfgfp fusions worked both for single-copy chromosomal integration and on a multicopy plasmid, with the latter being segregationally stable in the absence of selective pressure under the conditions tested. We successfully labeled S. mutans UA159, Streptococcus gordonii DL1, and Streptococcus sp. strain A12 with sfGFP, DsRed-Express2 (red), and citrine (yellow). To test these plasmids under more challenging conditions, we performed mixed-species biofilm experiments and separated fluorescent populations using fluorescence-activated cell sorting (FACS). This allowed us to visualize two streptococci at a time and quantify the amounts of each species simultaneously. These fluorescent reporter plasmids add to the genetic toolbox available for the study of oral streptococci.IMPORTANCE Oral streptococci are the most abundant bacteria in the mouth and have a major influence on oral health and disease. In this study, we designed and optimized the expression of fluorescent proteins in Streptococcus mutans and other oral streptococci. We monitored the levels of expression and noise (the variability in fluorescence across the population). We then created several fluorescent protein delivery systems (green, yellow, and red) for use in oral streptococci. The data show that we can monitor bacterial growth and interactions in situ, differentiating between different bacteria growing in biofilms, the natural state of the organisms in the human mouth. These new tools will allow researchers to study these bacteria in novel ways to create more effective diagnostic and therapeutic tools for ubiquitous infectious diseases.