Enterococcus faecalis produces abundant extracellular structures containing DNA in the absence of cell lysis during early biofilm formation.

UNLABELLED: Enterococcus faecalis is a common Gram-positive commensal bacterium of the metazoan gastrointestinal tract capable of biofilm formation and an opportunistic pathogen of increasing clinical concern. Dogma has held that biofilms are slow-growing structures, often taking days to form mature microcolonies. Here we report that extracellular DNA (eDNA) is an integral structural component of early E. faecalis biofilms (≤4 h postinoculation). Combining cationic dye-based biofilm matrix stabilization techniques with correlative immuno-scanning electron microscopy (SEM) and fluorescent techniques, we demonstrate that--in early E. faecalis biofilms--eDNA localizes to previously undescribed intercellular filamentous structures, as well as to thick mats of extruded extracellular matrix material. Both of these results are consistent with previous reports that early biofilms are exquisitely sensitive to exogenous DNase treatment. High-resolution SEM demonstrates a punctate labeling pattern in both structures, suggesting the presence of an additional, non-DNA constituent. Notably, the previously described fratricidal or lytic mechanism reported as the source of eDNA in older (≥24 h) E. faecalis biofilms does not appear to be at work under these conditions; extensive visual examination by SEM revealed a striking lack of lysed cells, and bulk biochemical assays also support an absence of significant lysis at these early time points. In addition, some cells demonstrated eDNA labeling localized at the septum, suggesting the possibility of DNA secretion from metabolically active cells. Overall, these data are consistent with a model in which a subpopulation of viable E. faecalis cells secrete or extrude DNA into the extracellular matrix. IMPORTANCE: This paper reports the production of extracellular DNA during early biofilm formation in Enterococcus faecalis. The work is significant because the mechanism of eDNA (extracellular DNA) production is independent of cell lysis and the DNA is confined to well-defined structures, suggesting a novel form of DNA secretion by viable cells. Previous models of biofilm formation in enterococci and related species propose cell lysis as the mechanism of DNA release.

Functional genomics of Enterococcus faecalis: multiple novel genetic determinants for biofilm formation in the core genome.

The ability of Enterococcus faecalis to form robust biofilms on host tissues and on abiotic surfaces such as catheters likely plays a major role in the pathogenesis of opportunistic antibiotic-resistant E. faecalis infections and in the transfer of antibiotic resistance genes. We have carried out a comprehensive analysis of genetic determinants of biofilm formation in the core genome of E. faecalis. Here we describe 68 genetic loci predicted to be involved in biofilm formation that were identified by recombinase in vivo expression technology (RIVET); most of these genes have not been studied previously. Differential expression of a number of these determinants during biofilm growth was confirmed by quantitative reverse transcription-PCR, and genetic complementation studies verified a role in biofilm formation for several candidate genes. Of particular interest was genetic locus EF1809, predicted to encode a regulatory protein of the GntR family. We isolated 14 independent nonsibling clones containing the putative promoter region for this gene in the RIVET screen; EF1809 also showed the largest increase in expression during biofilm growth of any of the genes tested. Since an in-frame deletion of EF1809 resulted in a severe biofilm defect that could be complemented by the cloned wild-type gene, we have designated EF1809 ebrA (enterococcal biofilm regulator). Most of the novel genetic loci identified in our studies are highly conserved in gram-positive bacterial pathogens and may thus constitute a pool of uncharacterized genes involved in biofilm formation that may be useful targets for drug discovery.

Identification and analysis of a siderophore biosynthetic gene cluster from Agrobacterium tumefaciens C58.

Using the complete genome sequence from Agrobacterium tumefaciens C58, the authors identified a secondary metabolite gene cluster that encodes the biosynthesis of a metabolite with siderophore activity. Support for this conclusion came from genetic and regulatory analysis of the gene cluster, along with the purification of a metabolite from A. tumefaciens C58 with iron-chelating activity. Genetic analysis of mutant strains disrupted in this gene cluster showed that these strains grew more slowly than the wild-type strain in medium lacking iron. Additionally, the mutant strains failed to produce a chrome-azurol-S-reactive material in liquid or solid medium, and failed to produce the metabolite with iron-chelating characteristics that was identified in the wild-type strain. Addition of this purified metabolite to the growth medium of a mutant strain restored its ability to grow in iron-deficient medium. Furthermore, expression of this gene cluster was induced by growth under iron-limiting conditions, suggesting that expression of this gene cluster occurs when iron is scarce. These data are all consistent with the proposal that the proteins encoded by this gene cluster are involved in the production of a siderophore. Interestingly, these proteins show the highest level of amino acid similarity to proteins from a gene cluster found in the filamentous cyanobacterium Nostoc sp. PCC7120, rather than to known siderophore biosynthetic enzymes. Given these properties, it is proposed that the siderophore produced by A. tumefaciens C58 will have a unique chemical structure. Production of the siderophore was not required for virulence of A. tumefaciens when tested with a standard stem inoculation assay.