Molecular basis of intercellular adhesion in the biofilm-forming Staphylococcus epidermidis

Molecular mechanisms of Staphylococcus epidermidis biofilm formation

Coagulase-negative staphylococci, mainly Staphylococcus epidermidis, are the predominant cause of implanted medical-device related infections. The formation of adherent multi-layered biofilms embedded into a glycocalyx composed of exopolysaccharides on implanted devices is believed to be essential for the pathogenesis of S. epidermidis infections. Biofilm formation may be separated into primary attachment of bacteria to native or modified polymer surfaces followed by proliferation of attached bacterial cells leading to accumulation of multi-layered cell-clusters and glycocalyx formation. Recent progress in the understanding of the molecular mechanisms cooperating in S. epidermidis biofilm formation is reviewed and current thinking on the relevance of these mechanisms for S. epidermidis device-related infections is discussed.

Flagella modulate the multicellular behavior of Salmonella typhimurium on the community level

Salmonella typhimurium strains MAE52 and MAE32 display a constitutive multicellular morphotype mediated by the expression of the agfD operon. In those strains, the role of flagella in the formation of various modes of multicellular behavior was investigated. Flagella were not required for the formation of the multicellular morphotype (rdar) on plates. However, visual examination showed that the global behavior of the bacterial community on air-liquid, surface-liquid or cell-cell-liquid interfaces changed in the absence of flagella. No differences in the local cell-cell interactions were observed at the microscopic level. Using Western blot analysis, no co-regulation of flagella and thin aggregative fimbriae, an extracellular component of the multicellular morphotype, was observed either on plates or in standing culture. In a mutant lacking flagella and thin aggregative fimbriae, the contribution of the latter to the multicellular morphotype was dominant. We concluded that independently regulated genes can act in an additive fashion to confer a pronounced multicellular behavior.

The intercellular adhesion (ica) locus is present in Staphylococcus aureus and is required for biofilm formation

Nosocomial infections that result in the formation of biofilms on the surfaces of biomedical implants are a leading cause of sepsis and are often associated with colonization of the implants by Staphylococcus epidermidis. Biofilm formation is thought to require two sequential steps: adhesion of cells to a solid substrate followed by cell-cell adhesion, creating multiple layers of cells. Intercellular adhesion requires the polysaccharide intercellular adhesin (PIA), which is composed of linear beta-1,6-linked glucosaminylglycans and can be synthesized in vitro from UDP-N-acetylglucosamine by products of the intercellular adhesion (ica) locus. We have investigated a variety of Staphylococcus aureus strains and find that all strains tested contain the ica locus and that several can form biofilms in vitro. Sequence comparison with the S. epidermidis ica genes revealed 59 to 78% amino acid identity. Deletion of the ica locus results in a loss of the ability to form biofilms, produce PIA, or mediate N-acetylglucosaminyltransferase activity in vitro. Cross-species hybridization experiments revealed the presence of icaA in several other Staphylococcus species, suggesting that cell-cell adhesion and the potential to form biofilms is conserved within this genus.

Conserved domains of glycosyltransferases

Glycosyltransferases catalyze the synthesis of glycoconjugates by transferring a properly activated sugar residue to an appropriate acceptor molecule or aglycone for chain initiation and elongation. The acceptor can be a lipid, a protein, a heterocyclic compound, or another carbohydrate residue. A catalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzyme. To elucidate the structural requirements for substrate recognition and catalytic reactions of glycosyltransferases, we have searched the databases for homologous sequences, identified conserved amino acid residues, and proposed potential domain motifs for these enzymes. Depending on the configuration of the anomeric functional group of the glycosyl donor molecule and of the resulting glycoconjugate, all known glycosyltransferases can be divided into two major types: retaining glycosyltransferases, which transfer sugar residue with the retention of anomeric configuration, and inverting glycosyltransferases, which transfer sugar residue with the inversion of anomeric configuration. One conserved domain of the inverting glycosyltransferases identified in the database is responsible for the recognition of a pyrimidine nucleotide, which is either the UDP or the TDP portion of a donor sugar-nucleotide molecule. This domain is termed "Nucleotide Recognition Domain 1 beta," or NRD1 beta, since the type of nucleotide is the only common structure among the sugar donors and acceptors. NRD1 beta is present in 140 glycosyltransferases. The central portion of the NRD1 beta domain is very similar to the domain that is present in one family of retaining glycosyltransferases. This family is termed NRD1 alpha to designate the similarity and stereochemistry of sugar transfer, and it consists of 77 glycosyltransferases identified thus far. In the central portion there is a homologous region for these two families and this region probably has a catalytic function. A third conserved domain is found exclusively in membrane-bound glycosyltransferases and is termed NRD2; this domain is present in 98 glycosyltransferases. All three identified NRDs are present in archaebacterial, eubacterial, viral, and eukaryotic glycosyltransferases. The present article presents the alignment of conserved NRD domains and also presents a brief overview of the analyzed glycosyltransferases which comprise about 65% of all known sugar-nucleotide dependent (Leloir-type) and putative glycosyltransferases in different databases. A potential mechanism for the catalytic reaction is also proposed. This proposed mechanism should facilitate the design of experiments to elucidate the regulatory mechanisms of glycosylation reactions. Amino acid sequence information within the conserved domain may be utilized to design degenerate primers for identifying DNA encoding new glycosyltransferases.

Detection of Staphylococcus aureus and Staphylococcus epidermidis in clinical samples by 16S rRNA-directed in situ hybridization

Staphylococcus epidermidis and Staphylococcus aureus are the most common causes of medical device-associated infections, including septicemic loosenings of orthopedic implants. Frequently, the microbiological diagnosis of these infections remains ambiguous, since at least some staphylococci have the capacity to reduce their growth rate considerably. These strains exhibit a small-colony phenotype, and often they are not detectable by conventional microbiological techniques. Moreover, clinical isolates of S. aureus and S. epidermidis adhere to polymer and metal surfaces by the generation of thick, multilayered biofilms consisting of bacteria and extracellular polysaccharides. This study reports improved detection and identification of S. aureus and S. epidermidis by an in situ hybridization method with fluorescence-labeled oligonucleotide probes specific for staphylococcal 16S rRNA. The technique has proven to be suitable for the in situ detection of staphylococci, which is illustrated by the identification of S. epidermidis in a connective tissue sample obtained from a patient with septicemic loosening of a hip arthroplasty. We also show that this technique allows the detection of intracellularly persisting bacteria, including small-colony variants of S. aureus, and the differentiation of S. epidermidis from other clinically relevant staphylococci even when they are embedded in biofilms. These results suggest that the 16S rRNA in situ hybridization technique could represent a powerful diagnostic tool for the detection and differentiation of many other fastidious microorganisms.

Phylogenetic analysis of enteroaggregative and diffusely adherent Escherichia coli

The phylogenetics of the various pathotypes of diarrheagenic Escherichia coli are not completely understood. In this study, we identified several plasmid and chromosomal genes in the pathogenic enteroaggregative E. coli (EAEC) prototype strain 042 and determined the prevalence of these loci among EAEC and diffusely adherent E. coli strains. The distribution of these genes is analyzed within an evolutionary framework provided by the characterization of allelic variation in housekeeping genes via multilocus enzyme electrophoresis. Our data reveal that EAEC strains are heterogeneous with respect to chromosomal and plasmid-borne genes but that the majority harbor a member of a conserved family of virulence plasmids. Comparison of plasmid and chromosomal relatedness of strains suggests clonality of chromosomal markers and a limited transfer model of plasmid distribution.

Broadly protective vaccine for Staphylococcus aureus based on an in vivo-expressed antigen

Vaccines based on preferential expression of bacterial antigens during human infection have not been described. Staphylococcus aureus synthesized poly-N-succinyl beta-1-6 glucosamine (PNSG) as a surface polysaccharide during human and animal infection, but few strains expressed PNSG in vitro. All S. aureus strains examined carried genes for PNSG synthesis. Immunization protected mice against kidney infections and death from strains that produced little PNSG in vitro. Nonimmune infected animals made antibody to PNSG, but serial in vitro cultures of kidney isolates yielded mostly cells that did not produce PNSG. PNSG is a candidate for use in a vaccine to protect against S. aureus infection.

Characterization of the importance of polysaccharide intercellular adhesin/hemagglutinin of Staphylococcus epidermidis in the pathogenesis of biomaterial-based infection in a mouse foreign body infection model

The production of biofilm is thought to be crucial in the pathogenesis of prosthetic-device infections caused by Staphylococcus epidermidis. An experimental animal model was used to assess the importance of biofilm production, which is mediated by polysaccharide intercellular adhesin/hemagglutinin (PIA/HA), in the pathogenesis of a biomaterial-based infection. Mice were inoculated along the length of a subcutaneously implanted intravenous catheter with either wild-type S. epidermidis 1457 or its isogenic PIA/HA-negative mutant. The wild-type strain was significantly more likely to cause a subcutaneous abscess than the mutant strain (P < 0.01) and was significantly less likely to be eradicated from the inoculation site by host defense (P < 0.05). In addition, the wild-type strain was found to adhere to the implanted catheters more abundantly than the PIA/HA-negative mutant (P < 0.05). The reliability of the adherence assay was assessed by scanning electron microscopy. To exclude contamination or spontaneous infection, bacterial strains recovered from the experimental animals were compared to inoculation strains by analysis of restriction fragment length polymorphism patterns by pulsed-field gel electrophoresis. In vitro binding of the wild-type strain and its isogenic mutant to a fibronectin-coated surface was similar. These results confirm the importance of biofilm production, mediated by PIA/HA, in the pathogenesis of S. epidermidis experimental foreign body infection.

Characterization of Staphylococcus epidermidis polysaccharide intercellular adhesin/hemagglutinin in the pathogenesis of intravascular catheter-associated infection in a rat model

Biofilm production is thought to be a crucial factor in the ability of Staphylococcus epidermidis to produce a biomaterial-based infection. A rat central venous catheter (CVC)-associated infection model was used to assess the importance of biofilm production, mediated by polysaccharide intercellular adhesin/hemagglutinin (PIA/HA), in the pathogenesis of intravascular catheter-associated infection. PIA/HA-positive S. epidermidis 1457 was significantly more likely to cause a CVC-associated infection (71 versus 14%, P < 0.03) resulting in bacteremia and metastatic disease than its isogenic PIA/HA-negative mutant. These results confirm the importance of biofilm production, mediated by PIA/HA, in the pathogenesis of S. epidermidis experimental CVC-associated infection.

A novel mechanism of phase variation of virulence in Staphylococcus epidermidis: evidence for control of the polysaccharide intercellular adhesin synthesis by alternating insertion and excision of the insertion sequence element IS256

Biofilm formation of Staphylococcus epidermidis on smooth polymer surfaces has been shown to be mediated by the ica operon. Upon activation of this operon, a polysaccharide intercellular adhesin (PIA) is synthesized that supports bacterial cell-to-cell contacts and triggers the production of thick, multilayered biofilms. Thus, the ica gene cluster represents a genetic determinant that significantly contributes to the virulence of specific Staphylococcus epidermidis strains. PIA synthesis has been reported recently to undergo a phase variation process. In this study, biofilm-forming Staphylococcus epidermidis strains and their PIA-negative phase variants were analysed genetically to investigate the molecular mechanisms of phase variation. We have characterized biofilm-negative variants by Southern hybridization with ica-specific probes, polymerase chain reaction and nucleotide sequencing. The data obtained in these analyses suggested that in approximately 30% of the variants the missing biofilm formation was due to the inactivation of either the icaA or the icaC gene by the insertion of the insertion sequence element IS256. Furthermore, it was shown that the transposition of IS256 into the ica operon is a reversible process. After repeated passages of the PIA-negative insertional mutants, the biofilm-forming phenotype could be restored. Nucleotide sequence analyses of the revertants confirmed the complete excision of IS256, including the initially duplicated 8 bp target sites. These results elucidate, for the first time, a molecular mechanism mediating phase variation in staphylcocci, and they demonstrate that a naturally occurring insertion sequence element is actively involved in the modulation of expression of a Staphylococcus virulence factor.

Coagulase-negative staphylococci: role as pathogens

Coagulase-negative staphylococci have long been regarded as apathogenic but their important role as pathogens and their increasing incidence have been recognized and studied in recent years. Although specific virulence factors are not as clearly established as they are in Staphylococcus aureus, it seems clear that factors such as bacterial polysaccharide components are involved in attachment and/or persistence of bacteria on foreign materials. Coagulase-negative staphylococci are by far the most common cause of bacteremia related to indwelling devices. Most of these infections are hospital-acquired, and studies over the past several years suggest that they are often caused by strains that are transmitted among hospitalized patients. Other important infections due to coagulase-negative staphylococci include central nervous system shunt infections, native or prosthetic valve endocarditis, urinary tract infections, and endophthalmitis. Intravenous treatment of systemic infections is usually required because coagulase-negative staphylococci have become increasingly resistant to multiple antibiotics.

Organization of biogenesis genes for aggregative adherence fimbria II defines a virulence gene cluster in enteroaggregative Escherichia coli

Several virulence-related genes have been described for prototype enteroaggregative Escherichia coli (EAEC) strain 042, which has been shown to cause diarrhea in human volunteers. Among these factors are the enterotoxins Pet and EAST and the fimbrial antigen aggregative adherence fimbria II (AAF/II), all of which are encoded on the 65-MDa virulence plasmid pAA2. Using nucleotide sequence analysis and insertional mutagenesis, we have found that the genes required for the expression of each of these factors, as well as the transcriptional activator of fimbrial expression AggR, map to a distinct cluster on the pAA2 plasmid map. The cluster is 23 kb in length and includes two regions required for expression of the AAF/II fimbria. These fimbrial biogenesis genes feature a unique organization in which the chaperone, subunit, and transcriptional activator lie in one cluster, whereas the second, unlinked cluster comprises a silent chaperone gene, usher, and invasin reminiscent of Dr family fimbrial clusters. This plasmid-borne virulence locus may represent an important set of virulence determinants in EAEC strains.

Use of a modified Robbins device to directly compare the adhesion of Staphylococcus epidermidis RP62A to surfaces

Staphylococcus epidermidis is a frequent cause of infection associated with the use of biomedical devices. Flow cell studies of the interaction between bacteria and surfaces do not generally allow direct comparison of different materials using the same bacterial suspension. The use of a modified Robbins Device (MRD) to compare the adhesion to different surfaces of Staph. epidermidis RP62A grown in continuous culture was investigated. Adhesion to glass was compared with siliconized glass, plasma-conditioned glass, titanium, stainless steel and Teflon. Attachment to siliconized glass was also compared with glass under differing ionic strength, and divalent cation concentrations. Both the differences in numbers adhering and changes in adhesion (slope) through the MRD were compared. There was a trend towards higher numbers adhering to the discs at the in-flow end of the MRD than at the outflow end, probably reflecting depletion of adherent bacteria in the interacting stream. Adhesion of Staph. epidermidis RP62A to siliconized glass and Teflon was reduced when compared to glass with increasing flow rates. Adhesion to stainless steel was not affected by flow rate and titanium gave a different slope of adhesion through the MRD when compared with glass, suggesting an interaction with different sub-populations within the interacting stream. Differences between siliconized glass and glass at flow rates of 300 ml h-1 were abolished by the addition of calcium or EDTA and reduced by the addition of magnesium. Increasing ionic strength reduced the statistical significance of the differences between glass and siliconized glass. Pre-conditioning of glass with pooled human plasma reduced adhesion compared with untreated glass and again gave a different slope to glass. The MRD linked to a chemostat can be used to compare directly bacterial adhesion to potential biomaterials. Variable depletion of the interacting stream should be taken into account in the interpretation of results. Divalent cation concentration, substrate properties and flow rate were important determinants of the comparative adhesion of Staph. epidermidis RP62A to surfaces.

Essential functional role of the polysaccharide intercellular adhesin of Staphylococcus epidermidis in hemagglutination

Hemagglutination of erythrocytes is a common property of Staphylococcus epidermidis strains, which is related to adherence and biofilm formation and may be essential for the pathogenesis of biomaterial-associated infections caused by S. epidermidis. In three independent biofilm-producing, hemagglutination-positive S. epidermidis isolates, interruption of the icaADBC operon essential for polysaccharide intercellular adhesin (PIA) synthesis by Tn917 insertions led to a hemagglutination-negative phenotype. An immunoglobulin G fraction of antiserum to PIA greatly reduced hemagglutination. Purified PIA led to a 64-fold decrease of hemagglutination titers of these strains; however, it did not mediate hemagglutination by itself. These observations define PIA as the hemagglutinin of S. epidermidis or at least as its major functional component.

The ica locus of Staphylococcus epidermidis encodes production of the capsular polysaccharide/adhesin

Clinical isolates of coagulase-negative staphylococci often elaborate a biofilm involved in adherence to medical devices and resistance to host defenses. The biofilm contains the capsular polysaccharide/adhesin (PS/A), which mediates cell adherence to biomaterials, and another antigen, termed polysaccharide intercellular adhesin (PIA), which is thought to mediate bacterial accumulation into cellular aggregates. PIA is a polymer of beta-1, 6-linked N-acetyl glucosamine residues with a molecular mass of <30, 000 kDa. We found that recombinant Staphylococcus carnosus and Staphylococcus aureus carrying a plasmid with genes of the ica locus, which was reported to encode the biosynthetic proteins for production of PIA, were also able to synthesize PS/A. PS/A and a chemically and immunologically identical polysaccharide isolated from S. carnosus carrying the ica genes on plasmid pCN27 were found to be high-molecular-mass (>250,000 kDa), acid-stable polymers of beta-1,6-linked glucosamine substituted on the amino group primarily with succinate, although some preparations also contained acetate. Moreover, all recombinant staphylococcal strains with the ica genes had the biologic properties previously attributed to PS/A. ica-positive strains readily formed an in vitro biofilm on plastic, adhered 3- to 10-fold more to catheters during a 30-min assay compared with control strains carrying only the cloning vector, adsorbed out antibodies to PS/A from immune serum, and elaborated a capsule visualized by immunoelectron microscopy with antisera to PS/A. These properties were also seen with PS/A-producing strains of Staphylococcus epidermidis, but not with transposon mutants lacking PS/A. An antiserum raised to PIA contained high-titer antibody to PS/A that was readily adsorbed out by PS/A-positive strains of S. epidermidis and recombinant strains of staphylococci carrying the ica genes. We conclude that the ica locus encodes production of PS/A and that the properties of S. epidermidis associated with initial bacterial adherence, biofilm formation, and intercellular adhesion can be correlated with elaboration of PS/A.

Characterization of the N-acetylglucosaminyltransferase activity involved in the biosynthesis of the Staphylococcus epidermidis polysaccharide intercellular adhesin

The polysaccharide intercellular adhesin (PIA) is an important factor in the colonization of medical devices by Staphylococcus epidermidis. The genes encoding PIA production are organized in the icaADBC (intercellular adhesion) operon. To study the function of the individual genes, we have established an in vitro assay with UDP-N-acetylglucosamine, the substrate for PIA biosynthesis, and analyzed the products by thin-layer chromatography and mass spectrometry. IcaA alone exhibited a low N-acetylglucosaminyltransferase activity and represents the catalytic enzyme. Coexpression of icaA with icaD led to a significant increase in activity. The newly identified icaD gene is located between icaA and icaB and overlaps both genes. N-Acetylglucosamine oligomers produced by IcaAD reached a maximal length of 20 residues. Only when icaA and icaD were expressed together with icaC were oligomer chains that react with PIA-specific antiserum synthesized. IcaA and IcaD are located in the cytoplasmic membrane, and IcaC also has all the structural features of an integral membrane protein. These results indicate a close interaction between IcaA, IcaD, and IcaC. Tunicamycin and bacitracin did not affect the in vitro synthesis of PIA intermediates or the complete PIA biosynthesis in vivo, suggesting that a undecaprenyl phosphate carrier is not involved. IcaAD represents a novel protein combination among beta-glycosyltransferases.

Molecular genetics of bacterial attachment and biofouling

Microbial adhesion to animate or inert surfaces is potentially mediated by nonspecific physical or specific ligand-receptor interactions. Growth and survival of the microbial community or biofilm then depends on adaptation to a series of changing environmental milieux. Within the realm of cell-cell interaction, recent advances suggest that flagella, fimbriae and other protein receptors are essential for bacterial attachment to surfaces. There has also been profound progress in the elucidation of genes and molecules necessary for bacterial attachments to surfaces and subsequent biofilm formation.

Staphylococcal lipases: molecular characterisation, secretion, and processing

Up to date five different staphylococcal lipase genes, two of Staphylococcus aureus (sal-1 and sal-2), two of Staphylococcus epidermidis (sel-1 and sel-2) and one of Staphylococcus hyicus (sh1) have been cloned and sequenced. All corresponding proteins are organised as pre-pro-enzymes: the pre-region represents the signal peptide, the pro-region has a length between 207 and 267 amino acids, and the mature part comprises 380 to 400 amino acids. We found that the lipases are secreted in the pro-lipase form. The processing of the pro-form to the mature enzyme occurs extracellular by a specific protease. Interestingly the pro-lipase reveals not much less activity compared to the mature lipase. There are evidences that the pro-region acts as an intramolecular chaperone which facilitates translocation not only of the native lipase but also of a number of completely unrelated proteins fused to the pro-peptide. It was also observed that the pro-region protects the proteins from proteolytic degradation. While the Staphylococcus aureus and Staphylococcus epidermidis lipases have only lipase (esterase) activity, the related Staphylococcus hyicus enzyme (SHL) is distinguished by both lipase and phospho-lipase activity. The biochemical and catalytic properties of these lipases are described in the accompanying article (Simons, J.W., Götz, F., Egmont, M.R. and Verheij, H.M., 1998. Staphylococcal lipases: Biochemical properties. Accompanying article).

Differential patterns of acquired virulence genes distinguish Salmonella strains

Analysis of several Salmonella typhimurium in vivo-induced genes located in regions of atypical base composition has uncovered acquired genetic elements that cumulatively engender pathogenicity. Many of these regions are associated with mobile elements, encode predicted adhesin and invasin-like functions, and are required for full virulence. Some of these regions distinguish broad host range from host-adapted Salmonella serovars and may contribute to inherent differences in host specificity, tissue tropism, and disease manifestation. Maintenance of this archipelago of acquired sequence by selection in specific hosts reveals a fossil record of the evolution of pathogenic species.

Impact of bacterial biofilm formation on in vitro and in vivo activities of antibiotics

The impact of bacterial adherence on antibiotic activity was analyzed with two isogenic strains of Staphylococcus epidermidis that differ in the features of their in vitro biofilm formation. The eradication of bacteria adhering to glass beads by amikacin, levofloxacin, rifampin, or teicoplanin was studied in an animal model and in a pharmacokinetically matched in vitro model. The features of S. epidermidis RP62A that allowed it to grow on surfaces in multiple layers promoted phenotypic resistance to antibiotic treatment, whereas strain M7 failed to accumulate, despite initial adherence on surfaces and growth in suspension similar to those for RP62A. Biofilms of S. epidermidis M7 were better eradicated than those of strain RP62A in vitro (46 versus 31%; P < 0.05) as well as in the animal model (39 versus 9%; P < 0.01).

Multicellular and aggregative behaviour of Salmonella typhimurium strains is controlled by mutations in the agfD promoter

A colony morphology type is described in which cells of Salmonella typhimurium form a rigid multicellular network with expression of thin aggregative fimbriae that mediate tight intercellular bonds. Surface translocation of cells on plates and adherence to glass and polystyrene surfaces in biofilm assays are further characteristics of the morphotype. This morphotype (rdar) is normally expressed only at low temperature. However, in two unrelated S. typhimurium strains, spontaneous mutants were found forming rdar colonies independent of temperature. Allelic replacement proved a single point mutation in the promoter region of PagfD in each of the two mutants to be responsible for the constitutive phenotype of a multicellular behaviour. Transcription levels of the two divergently transcribed agf operons required for biogenesis of thin aggregative fimbriae by Northern blot analysis with gene probes for agfA and agfD as well as expression levels of AgfA by Western blotting were compared in the wild type, the constitutive mutants and their respective ompR and rpoS- derivatives. In the wild type the rdar morphotype and expression of thin aggregative fimbriae are restricted to low temperature on plates containing rich medium of low osmolarity, but biogenesis of thin aggregative fimbriae occurs upon iron starvation at 37 degrees C. In the upregulated mutants biogenesis of thin aggregative fimbriae is only abolished at high osmolarity at 37 degrees C and in the exponential phase in broth culture. Control of expression of thin aggregative fimbriae and rdar morphology takes place at the transcriptional level at the agfD promoter. A functional ompR allele is required, however an rpoS mutation abolishes transcription only in the wild type, but has no influence on expression of thin aggregative fimbriae in the constitutive mutants.

Physical and genetic map of the genome of Staphylococcus carnosus TM300

A genome map of Staphylococcus carnosus TM300, an important micro-organism in the food industry and long used as a starter culture, was constructed by pulsed-field gel electrophoresis of DNA fragments obtained after digestion with NotI, SfiI and ApaI. The size of the chromosome was estimated to be 2590 kb. The fragments were assembled into a physical map using a combination of complementary methods including multiple and partial digests of genomic DNA, hybridization with homologous gene probes, and cross-Southern hybridization. Fifteen genes or gene clusters were positioned on the physical map by Southern hybridization analysis. The map provides a basis for further analysis of the S. carnosus chromosome.

Further characterization of Staphylococcus epidermidis transposon mutants deficient in primary attachment or intercellular adhesion

Biofilm formation is suggested to be the result of primary attachment of Staphylococcus epidermidis cells to a polymer surface followed by accumulation in multilayered cell clusters. Here we describe the further characterization of transposon (Tn917) mutants of Staphylococcus epidermidis O-47 having been biofilm-negative in a polystyrene microtiter plate adhesion assay. Among 5000 Tn917 insertion strains, we isolated four biofilm-negative mutants, each carrying one copy of Tn917. The mutants could be divided into two phenotypic classes: class A (mut1 and mut1a) and class B (mut2 and mut2a). Mutants of phenotype class A lacked four cell surface proteins and were affected in the primary attachment to polystyrene, but remained able to form multilayered cell clusters and to produce PIA. Mutants of phenotype class B were able to attach to polystyrene, but did not form multilayered cell clusters nor produce PIA. The cell surface protein pattern relative to the wild type was unchanged in class B mutants. On Congo red agar, the wild type and class A mutants formed black colonies (positive reaction on Congo red agar) while class B mutant colonies were red (negative reaction). The initial binding of cells to polystyrene and the ability to form multilayered cell clusters were found to be phenotypically and genetically distinct traits.

Molecular biology of S-layers

In this chapter we report on the molecular biology of crystalline surface layers of different bacterial groups. The limited information indicates that there are many variations on a common theme. Sequence variety, antigenic diversity, gene expression, rearrangements, influence of environmental factors and applied aspects are addressed. There is considerable variety in the S-layer composition, which was elucidated by sequence analysis of the corresponding genes. In Corynebacterium glutamicum one major cell wall protein is responsible for the formation of a highly ordered, hexagonal array. In contrast, two abundant surface proteins from the S-layer of Bacillus anthracis. Each protein possesses three S-layer homology motifs and one protein could be a virulence factor. The antigenic diversity and ABC transporters are important features, which have been studied in methanogenic archaea. The expression of the S-layer components is controlled by three genes in the case of Thermus thermophilus. One has repressor activity on the S-layer gene promoter, the second codes for the S-layer protein. The rearrangement by reciprocal recombination was investigated in Campylobacter fetus. 7-8 S-layer proteins with a high degree of homology at the 5' and 3' ends were found. Environmental changes influence the surface properties of Bacillus stearothermophilus. Depending on oxygen supply, this species produces different S-layer proteins. Finally, the molecular bases for some applications are discussed. Recombinant S-layer fusion proteins have been designed for biotechnology.

Detection of the intercellular adhesion gene cluster (ica) and phase variation in Staphylococcus epidermidis blood culture strains and mucosal isolates

Staphylococcus epidermidis is a common cause of catheter-associated infections and septicemia in immunocompromised patients. To answer the question whether S. epidermidis skin isolates differ from isolates causing septicemic diseases, 51 strains obtained from blood cultures, 1 strain from shunt-associated meningitis, and 36 saprophytic isolates were characterized. The study demonstrates that most of the blood culture strains formed a multilayered biofilm on plastic material, whereas skin and mucosal isolates did not. Moreover, biofilm-producing strains were found to generate large bacterial autoaggregates in liquid culture. Autoaggregation and biofilm formation on polymer surfaces was associated with the presence of a DNA sequence encoding an intercellular adhesion gene cluster (ica) that mediates the production of a polysaccharide intercellular adhesin. The presence of the intercellular adhesion genes in blood culture isolates was also found to be correlated with the exhibition of black colonies on Congo red agar, whereas the adhesin-negative strains formed red colonies. Upon subcultivation on Congo red agar, the black colony forms of the blood culture strains exhibited red colony variants which were biofilm and autoaggregation negative and occurred at a frequency of 10(-5). The DNA analysis of these S. epidermidis variants by pulsed-field gel electrophoresis and Southern hybridization with an ica-specific gene probe revealed no detectable difference between the black and red colony types. Moreover, after repeated passage, the phenotype of the parent strain could be restored. Therefore, these colony forms were regarded as phase variants. This phenotypic change was observed exclusively in adhesin-positive clinical isolates and not in adhesin-negative saprophytic strains of S. epidermidis.

A 140-kilodalton extracellular protein is essential for the accumulation of Staphylococcus epidermidis strains on surfaces

Two distinct pathogenic mechanisms, adhesion to polymer surfaces and subsequent accumulation of sessile bacterial cells, are considered important pathogenic steps in foreign body infections caused by Staphylococcus epidermidis. By using mitomycin mutagenesis, we have recently generated a mutant, strain M7, from S. epidermidis RP62A which is unaffected in adhesion but deficient in accumulation on glass or polystyrene surfaces and lacks a 115-kDa extracellular protein (designated the 140-kDa antigen; F. Schumacher-Perdreau, C. Heilmann, G. Peters, F. Götz, and G. Pulverer, FEMS Microbiol. Lett. 117:71-78, 1994). To evaluate the role of this protein in accumulation, we harvested extracellular proteins from S. epidermidis RP62A grown on dialysis membranes placed over chemically defined medium, purified the protein by using ion-exchange chromatography, determined its N-terminal amino acid sequence, and raised antiserum in rabbits. The antibody recognized only a single band in a Western immunoblot of the crude extracellular extract. With the microtiter biofilm test, antiserum at a dilution of < or =1:1,000 blocked accumulation of RP62A up to 98% whereas preimmune serum did not. The 140-kDa antigen was found only in extracellular products from bacteria grown under sessile conditions. Of 58 coagulase-negative clinical isolates, 32 strains were 140-kDa antigen positive and produced significantly larger amounts of biofilm than the 26 strains that were 140-kDa antigen negative. The 140-kDa protein appears to be biochemically and functionally unrelated to any previously described factors associated with biofilm formation. Thus, the 140-kDa antigen, referred to as accumulation-associated protein, may be a factor essential in S. epidermidis accumulation and, due to its immunogenicity, may allow the development of novel immunotherapeutic strategies for prevention of foreign body infection.

A novel pathway for O-polysaccharide biosynthesis in Salmonella enterica serovar Borreze

The plasmid-encoded gene cluster for O:54 O-polysaccharide synthesis in Salmonella enterica serovar Borreze (rfbO:54) contains three genes that direct synthesis of a ManNAc homopolymer with alternating beta1,3 and beta1,4 linkages. In Escherichia coli K-12, RfbAO:54 adds the first ManNAc residue to the Rfe (UDP-GlcpNAc::undecaprenylphosphate GlcpNAc-1-phosphate transferase)- modified lipopolysaccharide core. Hydrophobic cluster analysis of RfbAO:54 indicates this protein belongs to the ExoU family of nonprocessive beta-glycosyltransferases. Two putative catalytic residues and a potential substrate-binding motif were identified in RfbAO:54. Topological analysis of RfbBO:54 predicts four transmembrane domains and a large central cytoplasmic domain. The latter shares homology with a similar domain in the processive beta-glycosyltransferases Cps3S of Streptococcus pneumoniae and HasA of Streptococcus pyogenes. Hydrophobic cluster analysis of RfbBO:54 and Cps3S indicates both possess the structural features characteristic of the HasA family of processive beta-glycosyltransferases. Four potential catalytic residues and a putative substrate-binding motif were identified in RfbBO:54. In Deltarfb E. coli K-12, RfbAO:54 and RfbBO:54 direct synthesis of smooth O:54 lipopolysaccharide, indicating that this O-polysaccharide involves a novel pathway for O-antigen transport. Based on sequence and structural conservation, 15 new ExoU-related and 17 new HasA-related transferases were identified.