The Gilmore lab has a long established interest in antibiotic resistance, with a main focus on how microbes of the genus Enterococcus transitioned from core members of the commensal microbiota, to leading causes of multidrug resistant hospital infection in the antibiotic era. We were part of the team that described one of the first large hospital ward outbreaks of high level aminoglycoside resistant enterococcal bacteremia, and the first vancomycin-resistant Enterococcus (VRE) isolated in the US. We collaborated on the initial determination of the genome sequence of the VRE E. faecalis strain V583. Those studies led to discovery of a capsule operon and a pathogenicity island common among hospital ward endemic strains. The pathogenicity island harbors over 120 genes, including a new adhesin Esp (found in hospital isolates of both E. faecalis and E. faecium) that contributes to biofilm formation, and a novel cytolysin toxin. We showed that the post-translationally modified cytolysin is regulated by a novel system that permits the microbe to sense target cells. Further, we showed that E. faecalis can exchange large portions of their chromosomes by conjugation, allowing transmission of all of the resistance and virulence traits above to new strains.
To study the subtle pathogenesis of enterococci, staphylococci and other multidrug resistant pathogens, we developed several novel infection and colonization models. We defined the microbiome of Drosophila, which includes enterococci as a core native member. We also developed a highly tractable model for studying the pathogenesis of enterococcal and staphylococcal infection in mice, utilizing the vitreous chamber, which because the medium is clear and visible, permits direct, real time examination of an ongoing infection. This allowed us to show that in the absence of cytolysin expression, E. faecalis infection results in substantial inflammation but little direct tissue damage, and can be successfully treated. However, if the strain expressed cytolysin, no therapeutic regimen mitigated the course in this model. Similar types of analysis have been made for staphylococci.
Because microorganisms in biofilms are highly resistant to antibiotic killing, we also have investigated the molecular biology of biofilm formation by gram positive cocci. We developed a protracted biofilm model that examined gene expression over the course of 30 days. This study led us to observe that there is a dramatic bacterial population crash after about 1 week in this model, followed by the outgrowth of very stable architectural structures that supported increased numbers of bacteria for the duration of the study. These observations led us to propose a new model for biofilm formation by gram positive cocci that involves programmed fratricide and the incorporation of released DNA into the biofilm structure.
Recently we have worked with the Broad Institute to characterize the enterococcci at the genomic level, and released prior to publication over 400 new enterococcal genome sequences. To promote research nationally and internationally into these leading causes of resistant infection, we launched the International ASM Conference on Enterococci series, organized the main textbook on the subject (Gilmore MS. 2002. The Enterococci: Pathogenesis, Molecular Biology, and Antibiotic Resistance. Washington, DC: ASM Press.), have organized international conferences on streptococcal genetics and functional genomics, and promote these areas on national and international committees. A public access text on enterococci will be available March 1, 2014.