The atlA operon of Streptococcus mutans: role in autolysin maturation and cell surface biogenesis

Role of extracellular DNA in initial bacterial adhesion and surface aggregation

Extracellular DNA (eDNA) is an important component of the biofilm matrix. We show that removal of eDNA from Gram-positive bacteria reduces initial adhesion to and aggregation of bacteria on surfaces. Thermodynamic analyses indicated that eDNA introduces favorable acid-base interactions, explaining the effect of eDNA on aggregation and adhesion to the surface.

Streptococcus mutans autolysin AtlA is a fibronectin-binding protein and contributes to bacterial survival in the bloodstream and virulence for infective endocarditis

Streptococcus mutans, a commensal of the human oral cavity, can survive in the bloodstream and cause infective endocarditis (IE). However, the virulence factors associated with this manifestation of disease are not known. Here, we demonstrate that AtlA, an autolysin of S. mutans is a newly identified fibronectin (Fn) binding protein and contributes to bacterial resistance to phagocytosis and survival in the bloodstream. Interestingly, prior exposure to plasma at low concentrations was sufficient to enhance bacterial survival in the circulation. Calcium ions at physiological plasma concentrations induced maturation of AtlA from the 104-90 kDa isoform resulting in increased Fn binding and resistance to phagocytosis. An isogenic mutant strain defective in AtlA expression exhibited reduced survival and virulence when tested in a rat model of IE compared with the wild-type and complemented strains. The data presented suggest that plasma components utilized by S. mutans enhanced survival in the circulation and AtlA is a virulence factor associated with infective endocarditis.

Multiple two-component systems modulate alkali generation in Streptococcus gordonii in response to environmental stresses

The oral commensal Streptococcus gordonii must adapt to constantly fluctuating and often hostile environmental conditions to persist in the oral cavity. The arginine deiminase system (ADS) of S. gordonii enables cells to produce, ornithine, ammonia, CO(2), and ATP from arginine hydrolysis, augmenting the acid tolerance of the organism. The ADS genes are substrate inducible and sensitive to catabolite repression, mediated through ArcR and CcpA, respectively, but the system also requires low pH and anaerobic conditions for optimal activation. Here, we demonstrate that the CiaRH and ComDE two-component systems (TCS) are required for low-pH-dependent expression of ADS genes in S. gordonii. Further, the VicRK TCS is required for optimal ADS gene expression under anaerobic conditions and enhances the sensitivity of the operon to repression by oxygen. The known anaerobic activator of the ADS, Fnr-like protein (Flp), appeared to act independently of the Vic TCS. Mutants of S. gordonii lacking components of the CiaRH, ComDE, or VicRK grew more slowly in acidified media and were more sensitive to killing at lethal pH values and to agents that induce oxidative stress. This study provides the first evidence that TCS can regulate the ADS of bacteria in response to specific environmental signals and reveals some notable differences in the contribution of CiaRH, ComDE, and VicRK to viability and stress tolerance between the oral commensal S. gordonii and the oral pathogen Streptococcus mutans.

A fratricidal mechanism is responsible for eDNA release and contributes to biofilm development of Enterococcus faecalis

Extracellular DNA (eDNA), a by-product of cell lysis, was recently established as a critical structural component of the Enterococcus faecalis biofilm matrix. Here, we describe fratricide as the governing principle behind gelatinase (GelE)-mediated cell death and eDNA release. GFP reporter assays confirmed that GBAP (gelatinase biosynthesis-activating pheromone) quorum non-responders (GelE-SprE-) were a minority subpopulation of prey cells susceptible to the targeted fratricidal action of the quorum responsive predatorial majority (GelE+SprE+). The killing action is dependent on GelE, and the GelE producer population is protected from self-destruction by the co-production of SprE as an immunity protein. Targeted gene inactivation and protein interaction studies demonstrate that extracellular proteases execute their characteristic effects following downstream interactions with the primary autolysin, AtlA. Finally, we address a mechanism by which GelE and SprE may modify the cell wall affinity of proteolytically processed AtlA resulting in either a pro- or anti-lytic outcome.

Interconnections between Sigma B, agr, and proteolytic activity in Staphylococcus aureus biofilm maturation

Staphylococcus aureus is a proficient biofilm former on host tissues and medical implants. We mutagenized S. aureus strain SH1000 to identify loci essential for ica-independent mechanisms of biofilm maturation and identified multiple insertions in the rsbUVW-sigB operon. Following construction and characterization of a sigB deletion, we determined that the biofilm phenotype was due to a lack of sigma factor B (SigB) activity. The phenotype was conserved in a sigB mutant of USA300 strain LAC, a well-studied community-associated methicillin-resistant S. aureus isolate. We determined that agr RNAIII levels were elevated in the sigB mutants, and high levels of RNAIII expression are known to have antibiofilm effects. By introducing an agr mutation into the SH1000 or LAC sigB deletion strain, S. aureus regained biofilm capacity, indicating that the biofilm phenotype was agr dependent. Protease activity is linked to agr activity and ica-independent biofilm formation, and we observed that the protease inhibitors phenylmethylsulfonyl fluoride and alpha-macroglobulin could reverse the sigB biofilm defect. Similarly, inactivating genes encoding both the aureolysin and Spl extracellular proteases in the sigB mutant restored biofilm capacity. Due to the growing link between murein hydrolase activity and biofilm maturation, autolysin zymography was performed, which revealed an altered profile in the sigB mutant; again, the phenotype could be repaired through protease inactivation. These findings indicate that the lack of SigB activity results in increased RNAIII expression, thus elevating extracellular protease levels and altering the murein hydrolase activity profile. Altogether, our observations demonstrate that SigB is an essential regulator of S. aureus biofilm maturation.

Identification and characterization of an autolysin gene, atlh, from Streptococcus downei

An autolysin gene, atlh, was identified and sequenced from Streptococcus downei MFe28 using degenerate polymerase chain reaction (PCR) and the gene-walking method. Atlh protein encoded by atlh is composed of 879 amino acids, with a molecular weight of 95,902.26. Atlh possesses four 15-amino-acid residue repeats in the putative cell-wall-binding domain and has a catalytic domain in the C-terminus. The deduced amino acid sequence of atlh showed homology to S. mutans autolysin AtlA (68.4% similarity). Inactivation of atlh resulted in elongated chain formation compared to the parent strain. Recombinant proteins Atlh and its derivatives were constructed and analyzed by zymography. Zymographic analysis revealed that the Asp-771 residue of Atlh was essential for lytic activity and that lytic activity was not diminished by the deletion of repetitive regions in the putative cell-wall-binding domain of Atlh. Biofilm assay showed that the wild-type strain formed glucose- and sucrose-dependent biofilms, the atlh mutant diminished this ability. These results suggest that Atlh is associated with cell separation and biofilm formation.

Bacterial peptidoglycan (murein) hydrolases

Most bacteria have multiple peptidoglycan hydrolases capable of cleaving covalent bonds in peptidoglycan sacculi or its fragments. An overview of the different classes of peptidoglycan hydrolases and their cleavage sites is provided. The physiological functions of these enzymes include the regulation of cell wall growth, the turnover of peptidoglycan during growth, the separation of daughter cells during cell division and autolysis. Specialized hydrolases enlarge the pores in the peptidoglycan for the assembly of large trans-envelope complexes (pili, flagella, secretion systems), or they specifically cleave peptidoglycan during sporulation or spore germination. Moreover, peptidoglycan hydrolases are involved in lysis phenomena such as fratricide or developmental lysis occurring in bacterial populations. We will also review the current view on the regulation of autolysins and on the role of cytoplasm hydrolases in peptidoglycan recycling and induction of beta-lactamase.

Effects of oxygen on virulence traits of Streptococcus mutans

Oxygen profoundly affects the composition of oral biofilms. Recently, we showed that exposure of Streptococcus mutans to oxygen strongly inhibits biofilm formation and alters cell surface biogenesis. To begin to dissect the underlying mechanisms by which oxygen affects known virulence traits of S. mutans, transcription profiling was used to show that roughly 5% of the genes of this organism are differentially expressed in response to aeration. Among the most profoundly upregulated genes were autolysis-related genes and those that encode bacteriocins, the ClpB protease chaperone subunit, pyruvate dehydrogenase, the tricarboxylic acid cycle enzymes, NADH oxidase enzymes, and certain carbohydrate transporters and catabolic pathways. Consistent with our observation that the ability of S. mutans to form biofilms was severely impaired by oxygen exposure, transcription of the gtfB gene, which encodes one of the primary enzymes involved in the production of water-insoluble, adhesive glucan exopolysaccharides, was down-regulated in cells growing aerobically. Further investigation revealed that transcription of gtfB, but not gtfC, was responsive to oxygen and that aeration causes major changes in the amount and degree of cell association of the Gtf enzymes. Moreover, inactivation of the VicK sensor kinase affected the expression and localization the GtfB and GtfC enzymes. This study provides novel insights into the complex transcriptional and posttranscriptional regulatory networks used by S. mutans to modulate virulence gene expression and exopolysaccharide production in response to changes in oxygen availability.

Genetic characterization of the hdrRM operon: a novel high-cell-density-responsive regulator in Streptococcus mutans

Many species of bacteria can adhere to surfaces and grow as sessile communities. The continued accumulation of bacteria can eventually lead to the extremely high-cell-density environment characteristic of many biofilms or cell colonies. This is the normal habitat of the cariogenic species Streptococcus mutans, which normally resides in the high-cell-density, multispecies community commonly referred to as dental plaque. Previous work has demonstrated that the transcription of two separate bacteriocins can be activated by the high-cell-density conditions created through the centrifugation and incubation of cell pellets. In this study, we identified an uncharacterized two-gene operon that was induced >10-fold by conditions of high cell density. The genes of the operon encode a putative transcription regulator and a membrane protein, which were renamed as hdrR and hdrM, respectively. A transcription fusion to the hdrRM operon confirmed its induction by high cell density. Mutation of hdrM abolished bacteriocin production, greatly increased natural competence, reduced the growth rate, and severely affected biofilm formation. Interestingly, no obvious phenotypes were observed from a non-polar mutation of hdrR or mutations affecting the entire operon. These data suggest that the hdrRM operon may constitute a novel regulatory system responsible for mediating a cellular response to a high-cell-density environment.

Effects of oxygen on biofilm formation and the AtlA autolysin of Streptococcus mutans

The Streptococcus mutans atlA gene encodes an autolysin required for biofilm maturation and biogenesis of a normal cell surface. We found that the capacity to form biofilms by S. mutans, one of the principal causative agents of dental caries, was dramatically impaired by growth of the organism in an aerated environment and that cells exposed to oxygen displayed marked changes in surface protein profiles. Inactivation of the atlA gene alleviated repression of biofilm formation in the presence of oxygen. Also, the formation of long chains, a characteristic of AtlA-deficient strains, was less evident in cells grown with aeration. The SMu0629 gene is immediately upstream of atlA and encodes a product that contains a C-X-X-C motif, a characteristic of thiol-disulfide oxidoreductases. Inactivation of SMu0629 significantly reduced the levels of AtlA protein and led to resistance to autolysis. The SMu0629 mutant also displayed an enhanced capacity to form biofilms in the presence of oxygen compared to that of the parental strain. The expression of SMu0629 was shown to be under the control of the VicRK two-component system, which influences oxidative stress tolerance in S. mutans. Disruption of vicK also led to inhibition of processing of AtlA, and the mutant was hyperresistant to autolysis. When grown under aerobic conditions, the vicK mutant also showed significantly increased biofilm formation compared to strain UA159. This study illustrates the central role of AtlA and VicK in orchestrating growth on surfaces and envelope biogenesis in response to redox conditions.

The cidA murein hydrolase regulator contributes to DNA release and biofilm development in Staphylococcus aureus

The Staphylococcus aureus cidA and lrgA genes have been shown to affect cell lysis under a variety of conditions during planktonic growth. It is hypothesized that these genes encode holins and antiholins, respectively, and may serve as molecular control elements of bacterial cell lysis. To examine the biological role of cell death and lysis, we studied the impact of the cidA mutation on biofilm development. Interestingly, this mutation had a dramatic impact on biofilm morphology and adherence. The cidA mutant (KB1050) biofilm exhibited a rougher appearance compared with the parental strain (UAMS-1) and was less adherent. Propidium iodide staining revealed that KB1050 accumulated more dead cells within the biofilm population relative to UAMS-1, indicative of reduced cell lysis. In agreement with this finding, quantitative real-time PCR experiments demonstrated the presence of 5-fold less genomic DNA in the KB1050 biofilm relative to UAMS-1. Furthermore, treatment of the UAMS-1 biofilm with DNase I caused extensive cell detachment, whereas similar treatment of the KB1050 biofilm had only a modest effect. These results demonstrate that cidA-controlled cell lysis plays a significant role during biofilm development and that released genomic DNA is an important structural component of S. aureus biofilm.