Mutational analysis of the adcCBA genes in Streptococcus gordonii biofilm formation

Study of the Role of Lipoprotein Maturation Enzymes in the Pathogenesis of the Infection Caused by the Streptococcus suis Serotype 2 Sequence Type 25 North American Prototype Strain

Streptococcus suis serotype 2 is an important swine bacterial pathogen causing sudden death, septic shock, and meningitis. However, serotype 2 strains are phenotypically and genotypically heterogeneous and composed of a multitude of sequence types (STs) whose distributions greatly vary worldwide. It has been previously shown that the lipoprotein (LPP) maturation enzymes diacylglyceryl transferase (Lgt) and signal peptidase (Lsp) significantly modulate the inflammatory host response and play a differential role in virulence depending on the genetic background of the strain. Differently from Eurasian ST1/ST7 strains, the capsular polysaccharide of a North American S. suis serotype 2 ST25 representative strain only partially masks sub-capsular domains and bacterial wall components. Thus, our hypothesis is that since LPPs would be more surface exposed in ST25 strains than in their ST1 or ST7 counterparts, the maturation enzymes would play a more important role in the pathogenesis of the infection caused by the North American strain. Using isogenic Δlgt and Δlsp mutants derived from the wild-type ST25 strain, our studies suggest that these enzymes do not seem to play a role in the interaction between S. suis and epithelial and endothelial cells, regardless of the genetics background of the strain used. However, a role in the formation of biofilms (also independently of the STs) has been demonstrated. Moreover, the involvement of LPP dendritic cell activation in vitro seems to be somehow more pronounced with the ST25 strain. Finally, the Lgt enzyme seems to play a more important role in the virulence of the ST25 strain. Although some differences between STs could be observed, our original hypothesis that LPPs would be significantly more important in ST25 strains due to a better bacterial surface exposition could not be confirmed.

The Adc regulon mediates zinc homeostasis in Streptococcus mutans

Zinc (Zn²⁺ ) is an essential divalent trace metal for living cells. Intracellular zinc homeostasis is critical to the survival and virulence of bacteria. Thus, the frequent fluctuations of salivary zinc, caused by the low physiological level and the frequent exogenous zinc introduction, present a serious challenge for bacteria colonizing the oral cavity. However, the regulation strategies to keep intracellular Zn²⁺ homeostasis in Streptococcus mutans, an important causative pathogen of dental caries, are unknown. Because zinc uptake is primarily mediated by an ATP-binding ABC transporter AdcABC in Streptococcus strains, we examined the function of AdcABC and transcription factor AdcR in S. mutans in this study. The results demonstrated that deletion of either adcA or adcCB gene impaired the growth but enhanced the extracellular polymeric matrix production in S. mutans, both of which could be relieved after excessive Zn²⁺ supplementation. Using RNA sequencing analysis, quantitative reverse transcription polymerase chain reaction examination, LacZ-reporter studies, and electrophoretic mobility shift assay, we showed that a MarR (multiple antibiotic resistance regulator) family transcription factor, AdcR, negatively regulates the expression of the genes adcR, adcC, adcB, and adcA by acting on the adcRCB and adcA promoters in response to Zn²⁺ concentration in their environmental niches. The deletion of adcR increases the sensitivity of S. mutans to excessive Zn²⁺ supply. Taken together, our findings suggest that Adc regulon, which consists of a Zn²⁺ uptake transporter AdcCBA and a Zn²⁺ -responsive repressor AdcR, plays a prominent role in the maintenance of intracellular zinc homeostasis of S. mutans.

Potential factors involved in the early pathogenesis of Streptococcus uberis mastitis: a review

Bovine mastitis is an inflammation of the mammary gland, which could be the result of allergy, physical trauma, or invasion by pathogens as Streptococcus uberis. This pathogen is an environmental pathogen associated with subclinical and clinical intramammary infection (IMI) in both lactating and non-lactating cows, which can persist in the udder and cause a chronic infection in the mammary gland. In spite of the important economic losses and increased prevalence caused by S. uberis mastitis, virulence factors involved in bacterial colonization of mammary glands and the pathogenic mechanisms are not yet clear. In the last 30 years, several studies have defined adherence and internalization of S. uberis as the early stages in IMI. S. uberis adheres to and invades into mammary gland cells, and this ability has been observed in in vitro assays. Until now, these abilities have not been determined in vivo challenges since they have been difficult to study. Bacterial surface proteins are able to bind to extracellular matrix protein components such as fibronectin, collagen and laminin, as well as proteins in milk. These proteins play a role in adhesion to host cells and have been denominated microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). This article aims to summarize our current knowledge on the most relevant properties of the potential factors involved in the early pathogenesis of S. uberis mastitis.

Genome-wide discovery of novel M1T1 group A streptococcal determinants important for fitness and virulence during soft-tissue infection

The Group A Streptococcus remains a significant human pathogen causing a wide array of disease ranging from self-limiting to life-threatening invasive infections. Epithelium (skin or throat) colonization with progression to the subepithelial tissues is the common step in all GAS infections. Here, we used transposon-sequencing (Tn-seq) to define the GAS 5448 genetic requirements for in vivo fitness in subepithelial tissue. A near-saturation transposon library of the M1T1 GAS 5448 strain was injected subcutaneously into mice, producing suppurative inflammation at 24 h that progressed to prominent abscesses with tissue necrosis at 48 h. The library composition was monitored en masse by Tn-seq and ratios of mutant abundance comparing the output (12, 24 and 48 h) versus input (T₀) mutant pools were calculated for each gene. We identified a total of 273 subcutaneous fitness (scf) genes with 147 genes (55 of unknown function) critical for the M1T1 GAS 5448 fitness in vivo; and 126 genes (53 of unknown function) potentially linked to in vivo fitness advantage. Selected scf genes were validated in competitive subcutaneous infection with parental 5448. Two uncharacterized genes, scfA and scfB, encoding putative membrane-associated proteins and conserved among Gram-positive pathogens, were further characterized. Defined scfAB mutants in GAS were outcompeted by wild type 5448 in vivo, attenuated for lesion formation in the soft tissue infection model and dissemination to the bloodstream. We hypothesize that scfAB play an integral role in enhancing adaptation and fitness of GAS during localized skin infection, and potentially in propagation to other deeper host environments.

The WHO ranks the Group A Streptococcus (GAS) in the top 10 leading causes of morbidity and mortality from infectious diseases worldwide. GAS is a strict human pathogen causing both benign superficial infections as well as life-threatening invasive diseases. All GAS infections begin by colonization of an epithelium (throat or skin) followed by propagation into subepithelial tissues. The genetic requirements for M1T1 GAS 5448 within this niche were interrogated by in vivo transposon sequencing (Tn-seq), identifying 273 subcutaneous fitness (scf) genes with 108 of those previously of “unknown function”. Two yet uncharacterized genes, scfA and scfB, were shown to be critical during GAS 5448 soft tissue infection and dissemination into the bloodstream. Thus, this study improves the functional annotation of the GAS genome, providing new insights into GAS pathophysiology and enhancing the development of novel GAS therapeutics.

Genes involved in Listeria monocytogenes biofilm formation at a simulated food processing plant temperature of 15 °C

Listeria monocytogenes is a pathogenic foodborne bacterium whose persistence in food processing environments is in part attributed to its biofilm formation. Most biofilm studies have been carried out at 30-37 °C rather than at temperatures found in the food processing plants (i.e., 10-20 °C). The objective of the present study was to mine for novel genes that contribute to L. monocytogenes biofilm formation at 15 °C using the random insertional mutagenesis approach. A library of 11,024 L. monocytogenes 568 (serotype 1/2a) Himar1 insertional mutants was created. Mutants with reduced or enhanced biofilm formation at 15 °C were detected in microtiter plate assays with crystal violet and safranin staining. Fourteen mutants expressed enhanced biofilm phenotypes, and harbored transposon insertions in genes encoding cell wall biosynthesis, motility, metabolism, stress response, and cell surface associated proteins. Deficient mutants (n=5) contained interruptions in genes related to peptidoglycan, teichoic acid, or lipoproteins. Enhanced mutants produced significantly (p<0.05) higher cell densities in biofilm formed on stainless steel (SS) coupons at 15 °C (48 h) than deficient mutants, which were also more sensitive to benzalkonium chloride. All biofilm deficient mutants and four enhanced mutants in the microtiter plate assay (flaA, cheR, lmo2563 and lmo2488) formed no biofilm in a peg lid assay (Calgary biofilm device) while insertions in lmo1224 and lmo0543 led to excess biofilm in all assays. Two enhanced biofilm formers were more resistant to enzymatic removal with DNase, proteinase K or pectinase than the parent strain. Scanning electron microscopy of individual biofilms made by five mutants and the parent on SS surfaces showed formation of heterogeneous biofilm with dense zones by immotile mutants, while deficient mutants exhibited sparse growth. In conclusion, interruptions of 9 genes not previously linked to biofilm formation in L. monocytogenes (lmo2572, lmo2488 (uvrA), lmo1224, lmo0434 (inlB), lmo0263 (inlH), lmo0543, lmo0057 (EsaA), lmo2563, lmo0453), caused enhanced biofilm formation in the bacterium at 15 °C. The remaining mutants harbored interruptions in 10 genetic loci previously associated with biofilm formation at higher temperatures, indicating some temperature driven differences in the formation of biofilm by L. monocytogenes.

Quorum sensing in plaque biofilms: challenges and future prospects

AIM

This review intends to provide a brief overview regarding quorum sensing among bacteria in biofilms and also attempts to throw light on the new research focusing on interference with the quorum sensing.

BACKGROUND

Dental plaque is an example of microbial biofilm leading to periodontal disease and dental caries. Quorum sensing is widely employed by a variety of gram-positive and gram-negative bacterial species to coordinate various activities in biofilms. Quorum-sensing-interfering compounds have either a positive or a negative effect on the expression of bacterial phenotypes regulated by quorum sensing. These studies of bacterial quorum sensing have also suggested several ideal targets for drug design which can be promising in preventive and therapeutic aspects of periodontal diseases and dental caries.

RESULTS

Studies have shown that periodontal disease and dental caries is caused by plaque biofilm bacteria. Quorum sensing is the means of communication between these bacteria to regulate a wide range of behavior patterns among them. The in vitro studies reviewed here have a vital role in opening up this field, because they reveal the basic machinery of cell--cell signaling in microbial communities. The signal machinery bacteria use to coordinate a variety of their activities is identified by these studies. Further, this review aims to discuss several natural and synthetic methods which were used for manipulating bacterial quorum sensing.

CONCLUSION

The future challenge lies in the ability of the dental research to develop additional mechanisms for interfering with bacterial quorum sensing which can be used as preventive and therapeutic tools for combating oral polymicrobial diseases.

CLINICAL SIGNIFICANCE

This article aims at reviewing the literature and helping us to understand the ways of communication among bacteria in biofilms, which further open up the prospects in the treatment of diseases caused by biofilms.

Transcriptional response of Streptococcus pneumoniae to Zn2+) limitation and the repressor/activator function of AdcR

Zinc (Zn(2+)) is an important trace metal ion that has been shown to regulate the expression of several (virulence) genes in streptococci. Previously, we analyzed the genome-wide response of S. pneumoniae to Zn(2+)-stress. In this work, we have performed a transcriptomic analysis to identify genes that are differentially expressed under intracellular Zn(2+) limitation. This revealed a number of genes that are highly upregulated in the absence of extracellular Zn(2+), amongst which the genes belonging to the regulon of the Zn(2+)-responsive repressor AdcR, like adcBCA, encoding a Zn(2+)-dependent ABC-uptake system, adcAII, encoding a Zn(2+)-binding lipoprotein, and also virulence genes belonging to the Pht family (phtA, phtB, phtD and phtE). Using transcriptome analysis, lacZ-reporter studies, in vitro DNA binding experiments, and in silico operator predictions, we show that AdcR directly represses the promoters of adcRCBA, adcAII-phtD, phtA, phtB and phtE in the presence of Zn(2+). AdcR can also function as an activator, since in the presence of Zn(2+) it directly induces expression of adh that encodes a Zn(2+)-containing alcohol dehydrogenase. In conclusion, the genome-wide transcriptional response of S. pneumoniae to Zn(2+) limitation was established, which is mainly mediated via direct regulation by the Zn(2+)-dependent regulator AdcR.

The metalloregulatory zinc site in Streptococcus pneumoniae AdcR, a zinc-activated MarR family repressor

Streptococcus pneumoniae D39 AdcR (adhesin competence repressor) is the first metal-sensing member of the MarR (multiple antibiotic resistance repressor) family to be characterized. Expression profiling with a ΔadcR strain grown in liquid culture (brain-heart infusion) under microaerobic conditions revealed upregulation of 13 genes, including adcR and adcCBA, encoding a high-affinity ABC uptake system for zinc, and genes encoding cell-surface zinc-binding pneumococcal histidine triad (Pht) proteins and AdcAII (Lmb, laminin binding). The ΔadcR, H108Q and H112Q adcR mutant allelic strains grown in 0.2 mM Zn(II) exhibit a slow-growth phenotype and an approximately twofold increase in cell-associated Zn(II). Apo- and Zn(II)-bound AdcR are homodimers in solution and binding to a 28-mer DNA containing an adc operator is strongly stimulated by Zn(II) with K(DNA-Zn)=2.4 × 10(8) M(-1) (pH 6.0, 0.2 M NaCl, 25 °C). AdcR binds two Zn(II) per dimer, with stepwise Zn(II) affinities K(Zn1) and K(Zn2) of ≥10(9) M(-1) at pH 6.0 and ≥10(12) M(-1) at pH 8.0, and one to three lower affinity Zn(II) depending on the pH. X-ray absorption spectroscopy of the high-affinity site reveals a pentacoordinate N/O complex and no cysteine coordination, the latter finding corroborated by wild type-like functional properties of C30A AdcR. Alanine substitution of conserved residues His42 in the DNA-binding domain, and His108 and His112 in the C-terminal regulatory domain, abolish high-affinity Zn(II) binding and greatly reduce Zn(II)-activated binding to DNA. NMR studies reveal that these mutants adopt the same folded conformation as dimeric wild type apo-AdcR, but fail to conformationally switch upon Zn(II) binding. These studies implicate His42, His108 and H112 as metalloregulatory zinc ligands in S. pneumoniae AdcR.

Opposite effects of Mn2+ and Zn2+ on PsaR-mediated expression of the virulence genes pcpA, prtA, and psaBCA of Streptococcus pneumoniae

Homeostasis of Zn(2+) and Mn(2+) is important for the physiology and virulence of the human pathogen Streptococcus pneumoniae. Here, transcriptome analysis was used to determine the response of S. pneumoniae D39 to a high concentration of Zn(2+). Interestingly, virulence genes encoding the choline binding protein PcpA, the extracellular serine protease PrtA, and the Mn(2+) uptake system PsaBC(A) were strongly upregulated in the presence of Zn(2+). Using random mutagenesis, a previously described Mn(2+)-responsive transcriptional repressor, PsaR, was found to mediate the observed Zn(2+)-dependent derepression. In addition, PsaR is also responsible for the Mn(2+)-dependent repression of these genes. Subsequently, we investigated how these opposite effects are mediated by the same regulator. In vitro binding of purified PsaR to the prtA, pcpA, and psaB promoters was stimulated by Mn(2+), whereas Zn(2+) destroyed the interaction of PsaR with its target promoters. Mutational analysis of the pcpA promoter demonstrated the presence of a PsaR operator that mediates the transcriptional effects. In conclusion, PsaR is responsible for the counteracting effects of Mn(2+) and Zn(2+) on the expression of several virulence genes in S. pneumoniae, suggesting that the ratio of these metal ions exerts an important influence on pneumococcal pathogenesis.

Identification of Streptococcus sanguinis genes required for biofilm formation and examination of their role in endocarditis virulence

Streptococcus sanguinis is one of the pioneers in the bacterial colonization of teeth and is one of the most abundant species in the oral biofilm called dental plaque. S. sanguinis is also the most common viridans group streptococcal species implicated in infective endocarditis. To investigate the association of biofilm and endocarditis, we established a biofilm assay and examined biofilm formation with a signature-tagged mutagenesis library of S. sanguinis. Four genes that have not previously been associated with biofilm formation in any other bacterium, purB, purL, thrB, and pyrE, were putatively identified as contributing to in vitro biofilm formation in S. sanguinis. By examining 800 mutants for attenuation in the rabbit endocarditis model and for reduction in biofilm formation in vitro, we found some mutants that were both biofilm defective and attenuated for endocarditis. However, we also identified mutants with only reduced biofilm formation or with only attenuation in the endocarditis model. This result indicates that the ability to form biofilms in vitro is not associated with endocarditis virulence in vivo in S. sanguinis.

Genome-wide transcriptional changes in Streptococcus gordonii in response to competence signaling peptide

Streptococcus gordonii is a primary colonizer of the multispecies biofilm on tooth surfaces forming dental plaque and a potential agent of endocarditis. The recent completion of the genome sequence of the naturally competent strain Challis allowed the design of a spotted oligonucleotide microarray to examine a genome-wide response of this organism to environmental stimuli such as signal peptides. Based on temporal responses to synthetic competence signaling peptide (CSP) as indicated by transformation frequencies, the S. gordonii transcriptome was analyzed at various time points after CSP exposure. Microarray analysis identified 35 candidate early genes and 127 candidate late genes that were up-regulated at 5 and 15 min, respectively; these genes were often grouped in clusters. Results supported published findings on S. gordonii competence, showing up-regulation of 12 of 16 genes that have been reported to affect transformation frequencies in this species. Comparison of CSP-induced S. gordonii transcriptomes to results published for Streptococcus pneumoniae strains identified both conserved and species-specific genes. Putative intergenic regulatory sites, such as the conserved combox sequence thought to be a binding site for competence sigma factor, were found preceding S. gordonii late responsive genes. In contrast, S. gordonii early CSP-responsive genes were not preceded by the direct repeats found in S. pneumoniae. These studies provide the first insights into a genome-wide transcriptional response of an oral commensal organism. They offer an extensive analysis of transcriptional changes that accompany competence in S. gordonii and form a basis for future intra- and interspecies comparative analyses of this ecologically important phenotype.

Manganese transport and the role of manganese in virulence

Two areas of research have recently converged to highlight important roles for Mn(2+) in pathogenesis: the recognition that both bacterial Nramp homologs and members of LraI family of proteins are Mn(2+) transporters. Their mutation is associated with decreased virulence of various bacterial species. Thus, Mn(2+) appears to be essential for bacterial virulence. This review describes what is currently known about Mn(2+) transport in prokaryotes and how prokaryotic Mn(2+) transport is regulated. Some of the phenotypes that arise when microorganisms lack Mn(2+) are then discussed, with an emphasis on those phenotypes involving pathogenesis. The concluding section describes possible enzymatic roles for Mn(2+) that might help explain why Mn(2+) is necessary for virulence.