Decorating the surface of Escherichia coli with bacterial lipoproteins: a comparative analysis of different display systems.

BACKGROUND: The display of recombinant proteins on cell surfaces has a plethora of applications including vaccine development, screening of peptide libraries, whole-cell biocatalysts and biosensor development for diagnostic, industrial or environmental purposes. In the last decades, a wide variety of surface display systems have been developed for the exposure of recombinant proteins on the surface of Escherichia coli, such as autotransporters and outer membrane proteins. RESULTS: In this study, we assess three approaches for the surface display of a panel of heterologous and homologous mature lipoproteins in E. coli: four from Neisseria meningitidis and four from the host strain that are known to be localised in the inner leaflet of the outer membrane. Constructs were made carrying the sequences coding for eight mature lipoproteins, each fused to the delivery portion of three different systems: the autotransporter adhesin involved in diffuse adherence-I (AIDA-I) from enteropathogenic E. coli, the Lpp'OmpA chimaera and a truncated form of the ice nucleation protein (INP), InaK-NC (N-terminal domain fused with C-terminal one) from Pseudomonas syringae. In contrast to what was observed for the INP constructs, when fused to the AIDA-I or Lpp'OmpA, most of the mature lipoproteins were displayed on the bacterial surface both at 37 and 25 °C as demonstrated by FACS analysis, confocal and transmission electron microscopy. CONCLUSIONS: To our knowledge this is the first study that compares surface display systems using a number of passenger proteins. We have shown that the experimental conditions, including the choice of the carrier protein and the growth temperature, play an important role in the translocation of mature lipoproteins onto the bacterial surface. Despite all the optimization steps performed with the InaK-NC anchor motif, surface exposure of the passenger proteins used in this study was not achieved. For our experimental conditions, Lpp'OmpA chimaera has proved to be an efficient surface display system for the homologous passenger proteins although cell lysis and phenotype heterogeneity were observed. Finally, AIDA-I was found to be the best surface display system for mature lipoproteins (especially heterologous ones) in the E. coli host strain with no inhibition of growth and only limited phenotype heterogeneity.

Characterization of poxtA, a novel phenicol-oxazolidinone-tetracycline resistance gene from an MRSA of clinical origin.

Objectives: To characterize a novel phenicol-oxazolidinone-tetracycline resistance gene, named poxtA, identified in a previously described MRSA strain that was highly resistant to linezolid and also carried the cfr gene. Methods: The poxtA gene was identified by bioinformatic analysis of the whole genome sequence of Staphylococcus aureus AOUC-0915. The poxtA gene was cloned in a shuttle plasmid vector and expressed in Escherichia coli, S. aureus and Enterococcus faecalis to investigate the protein function. Comparative sequence analyses at the protein and genetic levels were carried out using standard procedures. Results: The poxtA gene encodes a protein that is 32% identical to OptrA and exhibits structural features typical of the F lineage of the ATP-binding cassette (ABC) protein superfamily that cause antibiotic resistance by ribosomal protection. Expression of poxtA in E. coli, S. aureus and E. faecalis was able to decrease susceptibility to phenicols, oxazolidinones and tetracyclines. A database search identified the presence of poxtA in E. faecalis, Enterococcus faecium and Pediococcus acidilactici strains, mostly of animal origin, and revealed the presence of poxtA homologues in the genomes of some Clostridiales. Analysis of the genetic context revealed that poxtA was located in a composite transposon-like structure containing two IS1216 elements. Conclusions: A novel resistance gene, named poxtA, encoding a protein of the antibiotic resistance (ARE) ABC-F lineage, was identified in the genome of an MRSA of clinical origin. PoxtA can confer decreased susceptibility to phenicols, oxazolidinones and tetracyclines and is associated with a putative mobile element that could contribute to its horizontal dissemination.

Surface interactome in Streptococcus pyogenes.

Very few studies have so far been dedicated to the systematic analysis of protein interactions occurring between surface and/or secreted proteins in bacteria. Such interactions are expected to play pivotal biological roles that deserve investigation. Taking advantage of the availability of a detailed map of surface and secreted proteins in Streptococcus pyogenes (group A Streptococcus (GAS)), we used protein array technology to define the "surface interactome" in this important human pathogen. Eighty-three proteins were spotted on glass slides in high density format, and each of the spotted proteins was probed for its capacity to interact with any of the immobilized proteins. A total of 146 interactions were identified, 25 of which classified as "reciprocal," namely, interactions that occur irrespective of which of the two partners was immobilized on the chip or in solution. Several of these interactions were validated by surface plasmon resonance and supported by confocal microscopy analysis of whole bacterial cells. By this approach, a number of interesting interactions have been discovered, including those occurring between OppA, DppA, PrsA, and TlpA, proteins known to be involved in protein folding and transport. These proteins, all localizing at the septum, might be part, together with HtrA, of the recently described ExPortal complex of GAS. Furthermore, SpeI was found to strongly interact with the metal transporters AdcA and Lmb. Because SpeI strictly requires zinc to exert its function, this finding provides evidence on how this superantigen, a major player in GAS pathogenesis, can acquire the metal in the host environment, where it is largely sequestered by carrier proteins. We believe that the approach proposed herein can lead to a deeper knowledge of the mechanisms underlying bacterial invasion, colonization, and pathogenesis.

An allelic variant of the PmrB sensor kinase responsible for colistin resistance in an Escherichia coli strain of clinical origin.

We investigated the colistin resistance mechanism in an Escherichia coli strain (LC711/14) isolated in Italy in 2014, from an urinary tract infection, which was previously shown to express a colistin resistance mechanism different from mcr-1. LC711/14 was found to carry a novel mutation in the pmrB gene, resulting in a leucine to proline amino acid substitution at position 10 of the PmrB sensor kinase component of the PmrAB signal transduction system. The role of this substitution in colistin resistance was documented by expression of the wild-type and mutated alleles in a pmrB deletion derivative of the E. coli reference strain MG1655, in which expression of the mutated allele conferred colistin resistance and upregulation of the endogenous pmrHFIJKLM lipid A modification system. Complementation of LC711/14 with the wild-type pmrB allele restored colistin susceptibility and decreased expression of pmrHFIJKLM, confirming the role of this PmrB mutation. Substitution of leucine at position 10 of PmrB with other amino acids (glycine and glutamine) resulted in loss of function, underscoring a key role of this residue which is located in the cytoplasmic secretion domain of the protein. This work demonstrated that mutation in this domain of the PmrB sensor kinase can be responsible for acquired colistin resistance in E. coli strains of clinical origin.

Zn2+ Uptake in Streptococcus pyogenes: Characterization of adcA and lmb Null Mutants.

An effective regulation of metal ion homeostasis is essential for the growth of microorganisms in any environment and in pathogenic bacteria is strongly associated with their ability to invade and colonise their hosts. To gain a better insight into zinc acquisition in Group A Streptococcus (GAS) we characterized null deletion mutants of the adcA and lmb genes of Streptococcus pyogenes strain MGAS5005 encoding the orthologues of AdcA and AdcAII, the two surface lipoproteins with partly redundant roles in zinc homeostasis in Streptococcus pneumoniae. Null adcA and lmb mutants were analysed for their capability to grow in zinc-depleted conditions and were found to be more susceptible to zinc starvation, a phenotype that could be rescued by the addition of Zn2+ ions to the growth medium. Expression of AdcA, Lmb and HtpA, the polyhistidine triad protein encoded by the gene adjacent to lmb, during growth under conditions of limited zinc availability was examined by Western blot analysis in wild type and null mutant strains. In the wild type strain, AdcA was always present with little variation in expression levels between conditions of excess or limited zinc availability. In contrast, Lmb and HtpA were expressed at detectable levels only during growth in the presence of low zinc concentrations or in the null adcA mutant, when expression of lmb is required to compensate for the lack of adcA expression. In the latter case, Lmb and HtpA were overexpressed by several fold, thus indicating that also in GAS AdcA is a zinc-specific importer and, although it shares this function with Lmb, the two substrate-binding proteins do not show fully overlapping roles in zinc homeostasis.

Diversity of cwp loci in clinical isolates of Clostridium difficile.

An increased incidence of Clostridium difficile infection (CDI) is associated with the emergence of epidemic strains characterized by high genetic diversity. Among the factors that may have a role in CDI is a family of 29 paralogues, the cell-wall proteins (CWPs), which compose the outer layer of the bacterial cell and are likely to be involved in colonization. Previous studies have shown that 12 of the 29 cwp genes are clustered in the same region, named after slpA (cwp1), the slpA locus, whereas the remaining 17 paralogues are distributed throughout the genome. The variability of 14 of these 17 cwp paralogues was determined in 40 C. difficile clinical isolates belonging to six of the currently prevailing PCR ribotypes. Based on sequence conservation, these cwp genes were divided into two groups, one comprising nine cwp loci having highly conserved sequences in all isolates, and the other five loci showing low genetic conservation among isolates of the same PCR ribotype, as well as between different PCR ribotypes. Three conserved CWPs, Cwp16, Cwp18 and Cwp25, and two variable ones, Cwp26 and Cwp27, were characterized further by Western blot analysis of total cell extracts or surface-layer preparations of the C. difficile clinical isolates. Expression of genetically invariable CWPs was well conserved in all isolates, whilst genetically variable CWPs were not always expressed at comparable levels, even in strains containing identical sequences but belonging to different PCR ribotypes. This is the first report on the distribution and variability of a number of genes encoding CWPs in C. difficile.

Phosphorylation of the synthetic hexasaccharide repeating unit is essential for the induction of antibodies to Clostridium difficile PSII cell wall polysaccharide.

Clostridium difficile is emerging worldwide as a major cause of nosocomial infections. The negatively charged PSII polysaccharide has been found in different strains of C. difficile and, thereby, represents an important target molecule for a possible carbohydrate-based vaccine. In order to identify a synthetic fragment that after conjugation to a protein carrier could be able to induce anti-PSII antibodies, we exploited a combination of chemical synthesis with immunochemistry, confocal immunofluorescence microscopy, and solid state NMR. We demonstrate that the phosphate group is crucial in synthetic glycans to mimic the native PSII polysaccharide; both native PSII and a phosphorylated synthetic hexasaccharide repeating unit conjugated to CRM(197) elicit comparable immunogenic responses in mice. This finding can aid design and selection of carbohydrate antigens to be explored as vaccine candidates.

The LMW surface-layer proteins of Clostridium difficile PCR ribotypes 027 and 001 share common immunogenic properties.

The aim of this study was to investigate the S-layer proteins (SLPs) of the hypervirulent Clostridium difficile PCR ribotype 027 and compare them with those of PCR ribotype 001 and other PCR ribotypes involved in C. difficile infection and outbreaks, by molecular analysis and immunological assays. It has been demonstrated previously that PCR ribotype 027 SlpA is conserved in C. difficile strains belonging to this PCR ribotype and that it is a new variant, showing 88 % identity with SlpA of PCR ribotype 001. As the low-molecular-weight (LMW) SLPs of C. difficile are immunodominant antigens, attention was focused on this region of the genome. Sequencing of strains of different PCR ribotypes (001, 012, 014, 017, 027 and 078) showed that SlpA was conserved among strains belonging to the same PCR ribotype. Comparison of the LMW SLP region among these strains identified ten regions with sequence identity between PCR ribotypes 027 and 001, and low conservation with the other PCR ribotypes. In particular, two of these regions corresponded to areas predicted to be surface exposed. Three specific peptides, including those of the two surface-exposed regions, were recognized by human sera against PCR ribotypes 027 and 001 and by a rabbit polyclonal serum against the SLPs of PCR ribotype 027. In contrast, these peptides were not recognized by a polyclonal serum against the SLPs of PCR ribotype 012 used as a control. These results confirm the antigenic role of the LMW SLP and suggest that it may have a role in evasion of the host immune response.

Specific involvement of pilus type 2a in biofilm formation in group B Streptococcus.

Streptococcus agalactiae is the primary colonizer of the anogenital mucosa of up to 30% of healthy women and can infect newborns during delivery and cause severe sepsis and meningitis. Persistent colonization usually involves the formation of biofilm and increasing evidences indicate that in pathogenic streptococci biofilm formation is mediated by pili. Recently, we have characterized pili distribution and conservation in 289 GBS clinical isolates and we have shown that GBS has three pilus types, 1, 2a and 2b encoded by three corresponding pilus islands, and that each strain carries one or two islands. Here we have investigated the capacity of these strains to form biofilms. We have found that most of the biofilm-formers carry pilus 2a, and using insertion and deletion mutants we have confirmed that pilus type 2a, but not pilus types 1 and 2b, confers biofilm-forming phenotype. We also show that deletion of the major ancillary protein of type 2a did not impair biofilm formation while the inactivation of the other ancillary protein and of the backbone protein completely abolished this phenotype. Furthermore, antibodies raised against pilus components inhibited bacterial adherence to solid surfaces, offering new strategies to prevent GBS infection by targeting bacteria during their initial attachment to host epithelial cells.

BibA induces opsonizing antibodies conferring in vivo protection against group B Streptococcus.

We recently characterized the function of BibA, an immunogenic surface-associated antigen expressed by group B Streptococcus (GBS) that is involved in virulence. In this study, we performed a bibA gene variability analysis on a panel of 72 clinical isolate strains. The bibA gene was present in all the strains analyzed, and 4 allelic variants, correlating with serotype, were identified. Moreover, although the BibA protein was expressed in all strains tested, only 54% of strains demonstrated surface exposure of the antigen in vitro. Importantly, expression of BibA on the bacterial surface was correlated with protection, because mice immunized with BibA were protected against challenge by a GBS strain with high levels of surface exposure of the antigen. In agreement with these findings, serum samples from mice immunized with recombinant BibA induced neutrophil-mediated in vitro opsonophagocytic killing of GBS. In conclusion, we propose BibA as a novel GBS vaccine candidate.

Preventing bacterial infections with pilus-based vaccines: the group B streptococcus paradigm.

We recently described the presence of 3 pilus variants in the human pathogen group B streptococcus (GBS; also known as Streptococcus agalactiae), each encoded by a distinct pathogenicity island, as well as the ability of pilus components to elicit protection in mice against homologous challenge. To determine whether a vaccine containing a combination of proteins from the 3 pilus types could provide broad protection, we analyzed pili distribution and conservation in 289 clinical isolates. We found that pilus sequences in each island are conserved, all strains carried at least 1 of the 3 islands, and a combination of the 3 pilus components conferred protection against all tested GBS challenge strains. These data are the first to indicate that a vaccine exclusively constituted by pilus components can be effective in preventing infections caused by GBS, and they pave the way for the use of a similar approach against other pathogenic streptococci.

A Helicobacter pylori vacuolating toxin mutant that fails to oligomerize has a dominant negative phenotype.

Most Helicobacter pylori strains secrete a toxin (VacA) that causes massive vacuolization of target cells and which is a major virulence factor of H. pylori. The VacA amino-terminal region is required for the induction of vacuolization. The aim of the present study was a deeper understanding of the critical role of the N-terminal regions that are protected from proteolysis when VacA interacts with artificial membranes. Using a counterselection system, we constructed an H. pylori strain, SPM 326-Delta49-57, that produces a mutant toxin with a deletion of eight amino acids in one of these protected regions. VacA Delta49-57 was correctly secreted by H. pylori but failed to oligomerize and did not have any detectable vacuolating cytotoxic activity. However, the mutant toxin was internalized normally and stained the perinuclear region of HeLa cells. Moreover, the mutant toxin exhibited a dominant negative effect, completely inhibiting the vacuolating activity of wild-type VacA. This loss of activity was correlated with the disappearance of oligomers in electron microscopy. These findings indicate that the deletion in VacA Delta49-57 disrupts the intermolecular interactions required for the oligomerization of the toxin.

The signal peptide sequence of a lytic transglycosylase of Neisseria meningitidis is involved in regulation of gene expression.

The 60 nucleotides encoding the signal peptide of the Neisseria meningitidis membrane-bound lytic transglycosylase (MltA) homologue GNA33 were found to exert a negative regulatory effect on expression of GNA33 from either a T7- or a P(lac)-driven system in Escherichia coli. Down-regulation was observed to occur at the transcriptional/post-transcriptional level and could possibly be ascribed to the formation of a stem-loop secondary structure within the signal peptide sequence. Slowing down the transcription rate through inhibition/titration of the RNA polymerase resulted in a considerable increase in mRNA accumulation, suggesting that a better coupling of translation to transcription would impede the formation of the putative secondary structure. Screening of synonymous mutations in the signal peptide sequence that showed high-level expression of an in-frame fusion to a reporter resulted in the isolation of several deletion mutants lacking most of the sequence participating in the putative secondary structure. Interestingly, the increase in the steady-state mRNA level observed in deletion mutants was higher, reaching a 300-fold increment, than that found in substitution mutants. Our results support the hypothesis that the rate of transcription controls the formation of a secondary structure in the region of the GNA33 transcript corresponding to the signal peptide sequence and this, when formed, negatively regulates expression.

Yeast coexpression of human papillomavirus types 6 and 16 capsid proteins.

The L1 and L2 capsid proteins of animal and human papillomaviruses (HPVs) can self-assemble into virus-like particles (VLPs) that closely resemble native virions. The use of different animal models shows that VLPs can be very efficient at inducing a protective immune response. However, studies with infectious HPV virions and VLPs of different HPV types indicate that the immune response is predominantly type-specific. We have generated a diploid yeast strain that coexpresses the L1 and L2 capsid proteins of both HPV-6b and HPV-16, and we have purified fully assembled VLPs banding in a cesium chloride gradient at the expected density of 1.29-1.3 mg/ml. Experimental evidence strongly indicated that the four proteins coassembled into VLPs. Western blot analysis, using anti-HPV-6 and anti-HPV-16 L1-specific monoclonal antibodies and type-specific L2 antisera, demonstrated that all four proteins copurified. Most importantly, immunoprecipitation experiments, carried out using type-specific anti-L1 monoclonals and either total yeast cell extracts or purified VLPs, confirmed the interaction and the formation of covalent disulfide bonds between the two L1 proteins. Finally, HPV-6/16 VLPs administered to mice induced conformational antibodies against both L1 protein types. These results suggest that coexpression of different capsid proteins may provide new tools for the induction of antibodies directed against multiple HPV types.