The pathogenic role of Staphylococcus epidermidis capsular polysaccharide/adhesin in a low-inoculum rabbit model of prosthetic valve endocarditis

Biomolecular mechanisms of staphylococcal biofilm formation

The multitude of biomolecular and regulatory factors involved in staphylococcal adhesion and biofilm formation owe much to their ability to colonize surfaces, allowing the biofilm form to become the preferential bacterial phenotype. Judging by total number, biomass and variety of environments colonized, bacteria can be categorized as the most successful lifeform on earth. This is due to the ability of bacteria and other microorganisms to respond phenotypically via biomolecular processes to the stresses of their surrounding environment. This review focuses on the specific pathways involved in the adhesion of the Gram-positive bacteria Staphylococcus epidermidis and Staphylococcus aureus with reference to the role of specific cell surface adhesins, the ica operon, accumulation-associated proteins and quorum-sensing systems and their significance in medical device-related infection.

How can the microbiologist help in diagnosing neonatal sepsis?

Neonatal sepsis can be classified into two subtypes depending upon whether the onset of symptoms is before 72 hours of life (early-onset neonatal sepsis—EONS) or later (late-onset neonatal sepsis—LONS). These definitions have contributed greatly to diagnosis and treatment by identifying which microorganisms are likely to be responsible for sepsis during these periods and the expected outcomes of infection. This paper focuses on the tools that microbiologist can offer to diagnose and eventually prevent neonatal sepsis. Here, we discuss the advantages and limitation of the blood culture, the actual gold standard for sepsis diagnosis. In addition, we examine the utility of molecular techniques in the diagnosis and management of neonatal sepsis.

The pgaABCD locus of Acinetobacter baumannii encodes the production of poly-beta-1-6-N-acetylglucosamine, which is critical for biofilm formation

We found that Acinetobacter baumannii contains a pgaABCD locus that encodes proteins that synthesize cell-associated poly-beta-(1-6)-N-acetylglucosamine (PNAG). Both a mutant with an in-frame deletion of the pga locus (S1Deltapga) and a transcomplemented strain (S1Deltapga-c) of A. baumannii were constructed, and the PNAG production by these strains was compared using an immunoblot assay. Deleting the pga locus resulted in an A. baumannii strain without PNAG, and transcomplementation of the S1Deltapga strain with the pgaABCD genes fully restored the wild-type PNAG phenotype. Heterologous expression of the A. baumannii pga locus in Escherichia coli led to synthesis of significant amounts of PNAG, while no polysaccharide was detected in E. coli cells harboring an empty vector. Nuclear magnetic resonance analysis of the extracellular polysaccharide material isolated from A. baumannii confirmed that it was PNAG, but notably only 60% of the glucosamine amino groups were acetylated. PCR analysis indicated that all 30 clinical A. baumannii isolates examined had the pga genes, and immunoblot assays indicated that 14 of the 30 strains strongly produced PNAG, 14 of the strains moderately to weakly produced PNAG, and 2 strains appeared to not produce PNAG. Deletion of the pga locus led to loss of the strong biofilm phenotype, which was restored by complementation. Confocal laser scanning microscopy studies combined with COMSTAT analysis demonstrated that the biovolume, mean thickness, and maximum thickness of 16-h and 48-h-old biofilms formed by wild-type and pga-complemented A. baumannii strains were significantly greater than the biovolume, mean thickness, and maximum thickness of 16-h and 48-h-old biofilms formed by the S1Deltapga mutant strain. Biofilm-dependent production of PNAG could be an important virulence factor for this emerging pathogen that has few known virulence factors.

Phenotypic antibiotic tolerance of Staphylococcus aureus in implant-related infections: relationship with in vitro colonization of artificial surfaces

Antibiotic therapy of deep-seated staphylococcal infections, especially when they are associated with foreign implants, such as orthopedic prostheses and permanently inserted catheters, is a difficult challenge. Semi-synthetic penicillins, glycopeptides and quinolones are found effective when given prophylactically in clinical and experimental trials of implant-related infections, but are frequently poorly effective after implant-related infections are established. Thus, removal of the medical devices is often required to obtain cure. The failure of antibiotic therapy to cure staphylococcal foreign body infections may arise from a broad-spectrum phenotypic tolerance to different classes of antimicrobial agents, whose molecular basis and physiological mechanisms are poorly understood.

Capillary Isoelectric Focusing — Useful tool for detection of the biofilm formation in Staphylococcus epidermidis

The biofilm formation is an important factor of S. epidermidis virulence. Biofilm-positive strains might be clinically more important than biofilm-negative ones. Unlike biofilm-negative staphylococci, biofilm-positive staphylococci are surrounded with an extracellular polysaccharide substance. The presence of this substance on the surface can affect physico-chemical properties of the bacterial cell, including surface charge. 73 S. epidermidis strains were examined for the presence of ica operon, for the ability to form biofilm by Christensen test tube method and for the production of slime by Congo red agar method. Isoelectric points (pI) of these strains were determined by means of Capillary Isoelectric Focusing. The biofilm negative strains focused near pI value 2.3, while the pI values of the biofilm positive strains were near 2.6. Isoelectric point is a useful criterion for the differentiation between biofilm-positive and biofilm-negative S. epidermidis strains.

Comparative antibody-mediated phagocytosis of Staphylococcus epidermidis cells grown in a biofilm or in the planktonic state

Staphylococcus epidermidis is an important cause of nosocomial infections. Virulence is attributable to elaboration of biofilms on medical surfaces that protect the organisms from immune system clearance. Even though leukocytes can penetrate biofilms, they fail to phagocytose and kill bacteria. The properties that make biofilm bacteria resistant to the immune system are not well characterized. In order to better understand the mechanisms of resistance of bacteria in biofilms to the immune system, we evaluated antibody penetration throughout the biofilm and antibody-mediated phagocytic killing of planktonic versus biofilm cells of S. epidermidis by using a rabbit antibody to poly-N-acetylglucosamine (PNAG). These antibodies are opsonic and protect against infection with planktonic cells of PNAG-positive Staphylococcus aureus and S. epidermidis. Antibody to PNAG readily penetrated the biofilm and bound to the same areas in the biofilm as did wheat germ agglutinin, a lectin known to bind to components of staphylococcal biofilms. However, biofilm cells were more resistant to opsonic killing than their planktonic counterparts in spite of producing more PNAG per cell than planktonic cells. Biofilm extracts inhibited opsonic killing mediated by antibody to PNAG, suggesting that the PNAG antigen within the biofilm matrix prevents antibody binding close to the bacterial cell surface, which is needed for efficient opsonic killing. Increased resistance of biofilm cells to opsonic killing mediated by an otherwise protective antibody was due not to a biofilm-specific phenotype but rather to high levels of antigen within the biofilm that prevented bacterial opsonization by the antibody.

Poly-N-acetylglucosamine production in Staphylococcus aureus is essential for virulence in murine models of systemic infection

The contribution of the Staphylococcus aureus surface polysaccharide poly-N-acetylglucosamine (PNAG) to virulence was evaluated in three mouse models of systemic infection: bacteremia, renal abscess formation, and lethality following high-dose intraperitoneal (i.p.) infection. Deletion of the intercellular adhesin (ica) locus that encodes the biosynthetic enzymes for PNAG production in S. aureus strains Mn8, Newman, and NCTC 10833 resulted in mutant strains with significantly reduced abilities to maintain bacterial levels in blood following intravenous or i.p. injection, to spread systemically to the kidneys following i.p. injection, or to induce a moribund/lethal state following i.p. infection. In the bacteremia model, neither growth phase nor growth medium used to prepare the S. aureus inoculum affected the conclusion that PNAG production was needed for full virulence. As the SarA regulatory protein has been shown to affect ica transcription, PNAG synthesis, and biofilm formation, we also evaluated S. aureus strains Mn8 and 10833 deleted for the sarA gene in the renal infection model. A decrease in PNAG production was seen in sarA mutants using immunoblots of cell surface extracts but was insufficient to reduce the virulence of sarA-deleted strains in this model. S. aureus strains deleted for the ica genes were much more susceptible to antibody-independent opsonic killing involving human peripheral blood leukocytes and rabbit complement. Thus, PNAG confers on S. aureus resistance to killing mediated by these innate host immune mediators. Overall, PNAG production by S. aureus appears to be a critical virulence factor as assessed in murine models of systemic infection.

Comparative opsonic and protective activities of Staphylococcus aureus conjugate vaccines containing native or deacetylated Staphylococcal Poly-N-acetyl-beta-(1-6)-glucosamine

Staphylococcus aureus and Staphylococcus epidermidis both synthesize the surface polysaccharide poly-N-acetyl-beta-(1-6)-glucosamine (PNAG), which is produced in vitro with a high level (>90%) of the amino groups substituted by acetate. Here, we examined the role of the acetate substituents of PNAG in generating opsonic and protective antibodies. PNAG and a deacetylated form of the antigen (dPNAG; 15% acetylation) were conjugated to the carrier protein diphtheria toxoid (DT) and used to immunize animals. Mice responded in a dose-dependent fashion to both conjugate vaccines, with maximum antibody titers observed at the highest dose and 4 weeks after the last of three weekly immunizations. PNAG-DT and dPNAG-DT vaccines were also very immunogenic in rabbits. Antibodies raised to the conjugate vaccines in rabbits mediated the opsonic killing of various staphylococcal strains, but the specificity of the opsonic killing was primarily to dPNAG, as this antigen inhibited the killing of S. aureus strains by both PNAG- and dPNAG-specific antibodies. Passive immunization of mice with anti-dPNAG-DT rabbit sera showed significant levels of clearance of S. aureus from the blood (54 to 91%) compared to control mice immunized with normal rabbit sera, whereas PNAG-specific antibodies were ineffective at clearing S. aureus. Passive immunization of mice with a goat antiserum raised to the dPNAG-DT vaccine protected against a lethal dose of three different S. aureus strains. Overall, these data show that immunization of animals with a conjugate vaccine of dPNAG elicit antibodies that mediated opsonic killing and protected against S. aureus infection, including capsular polysaccharide types 5 and 8 and an untypable strain.

Clinical microbiology of bacterial and fungal sepsis in very-low-birth-weight infants

Twenty percent of very-low-birth-weight (<1500 g) preterm infants experience a serious systemic infection, and despite advances in neonatal intensive care and antimicrobials, mortality is as much as threefold higher for these infants who develop sepsis than their counterparts without sepsis during their hospitalization. Outcomes may be improved by preventative strategies, earlier and accurate diagnosis, and adjunct therapies to combat infection and protect the vulnerable preterm infant during an infection. Earlier diagnosis on the basis of factors such as abnormal heart rate characteristics may offer the ability to initiate treatment prior to the onset of clinical symptoms. Molecular and adjunctive diagnostics may also aid in diagnosing invasive infection when clinical symptoms indicate infection but no organisms are isolated in culture. Due to the high morbidity and mortality, preventative and adjunctive therapies are needed. Prophylaxis has been effective in preventing early-onset group B streptococcal sepsis and late-onset Candida sepsis. Future research in prophylaxis using active and passive immunization strategies offers prevention without the risk of resistance to antimicrobials. Identification of the differences in neonatal intensive care units with low and high infection rates and implementation of infection control measures remain paramount in each neonatal intensive care unit caring for preterm infants.

The fibrinogen binding protein of Staphylococcus epidermidis is a target for opsonic antibodies

Antibodies against the fibrinogen binding protein (Fbe) of Staphylococcus epidermidis significantly increased macrophage phagocytosis. Antibodies against autolysin E were opsonic but to a lesser extent. Antibodies against a novel, putatively surface-located antigen were unable to enhance phagocytosis. The severity of systemic infection of mice with S. epidermidis was reduced if the bacteria were preopsonized with anti-Fbe prior to administration. Fbe is thus a strong candidate for protein vaccination against S. epidermidis infection, and antibodies against Fbe can be used to prevent or treat infections caused by S. epidermidis.

Biologic properties and vaccine potential of the staphylococcal poly-N-acetyl glucosamine surface polysaccharide

Staphylococci have become the most common causes of nosocomial bacterial infections, and this fact, along with increasing problems associated with antimicrobial resistance, spurs the need for finding immunotherapeutic alternatives to prevent and possibly treat these infections. Most virulent, clinical isolates of both coagulase-negative staphylococci (CoNS) and Staphylococcus aureus carry the ica locus which encodes proteins that synthesize a polymer of beta-1-6 linked N-acetyl glucosamine residues (PNAG). Animal studies have shown purified PNAG can elicit protective immunity against both CoNS and S. aureus, suggesting its potential as a broadly protective vaccine for many clinically important strains of staphylococci.

Immunochemical properties of the staphylococcal poly-N-acetylglucosamine surface polysaccharide

Staphylococcus aureus and Staphylococcus epidermidis often elaborate adherent biofilms, which contain the capsular polysaccharide-adhesin (PS/A) that mediates the initial cell adherence to biomaterials. Biofilm cells produce another antigen, termed polysaccharide intercellular adhesin (PIA), which is composed of a approximately 28 kDa soluble linear beta(1-6)-linked N-acetylglucosamine. We developed a new method to purify PS/A from S. aureus MN8m, a strain hyperproducing PS/A. Using multiple analytical techniques, we determined that the chemical structure of PS/A is also beta(1-6)-N-acetylglucosamine (PNAG). We were unable to find N-succinylglucosamine residues in any of our preparations in contrast to previously reported findings (D. McKenney, K. Pouliot, Y. Wang, V. Murthy, M. Ulrich, G. Doring, J. C. Lee, D. A Goldmann, and G. B. Pier, Science 284:1523-1527, 1999). PNAG was produced with a wide range of molecular masses that could be divided into three major fractions with average molecular masses of 460 kDa (PNAG-I), 100 kDa (PNAG-II), and 21 kDa (PNAG-III). The purified antigens were not soluble at neutral pH unless first dissolved in 5 M HCl and then neutralized with 5 M NaOH. PNAG-I was very immunogenic in rabbits, but the responses of individual animals were variable. Immunization of mice with various doses (100, 50, or 10 microg) of PNAG-I, -II, and -III demonstrated that only PNAG-I was able to elicit an immunoglobulin G (IgG) immune response with the highest titers obtained with 100-microg dose. When we purified a small fraction of PNAG with a molecular mass of approximately 780 kDa (PNAG-780) from PNAG-I, significantly higher IgG titers than those in mice immunized with the same doses of PNAG-I were obtained, suggesting the importance of the molecular mass of PNAG in the antibody response. These results further clarify the chemical structure of PS/A and help to differentiate it from PIA on the basis of immunogenicity, molecular size, and solubility.

Lack of fbe, the gene for a fibrinogen-binding protein from Staphylococcus epidermidis, reduces its adherence to fibrinogen coated surfaces

The significance of Fbe, a fibrinogen-binding protein in Staphylococcus epidermidis, was investigated. A fbe mutant was constructed by allelic replacement, where a Gentamicin resistance gene replaced a portion of the A region of fbe. Adherence assay to immobilized fibrinogen on polyethylene surfaces and peripheral venous catheters from patients showed that the fibrinogen binding ability of the mutant was reduced compared to its parental strain. This shows that Fbe is a major factor involved in adherence of S. epidermidis to fibrinogen. No difference was found between the wild-type and mutant in their affinity to immobilized fibronectin.

Vaccine potential of poly-1-6 beta-D-N-succinylglucosamine, an immunoprotective surface polysaccharide of Staphylococcus aureus and Staphylococcus epidermidis

Staphylococcus aureus and S. epidermidis are among the most common causes of nosocomial infection, and S. aureus is also of major concern to human health due to its occurrence in community-acquired infections. These staphylococcal species are also major pathogens for domesticated animals. We have previously identified poly-N-succinyl beta-1-6 glucosamine (PNSG) as the chemical form of the S. epidermidis capsular polysaccharide/adhesin (PS/A) which mediates adherence of coagulase-negative staphylococci (CoNS) to biomaterials, serves as the capsule for strains of CoNS that express PS/A, and is a target for protective antibodies. We have recently found that PNSG is made by S. aureus as well, where it is an environmentally regulated, in vivo-expressed surface polysaccharide and similarly serves as a target for protective immunity. Only a minority of fresh human clinical isolates of S. aureus elaborate PNSG in vitro but most could be induced to do so under specific in vitro growth conditions. However, by immunofluorescence microscopy, S. aureus cells in infected human sputa and lung elaborated PNSG. The ica genes, previously shown to encode proteins in CoNS that synthesize PNSG, were found by PCR in all S. aureus strains examined, and immunogenic and protective PNSG could be isolated from S. aureus. Active and passive immunization of mice with PNSG protected them against metastatic kidney infections after intravenous inoculation with eight phenotypically PNSG-negative S. aureus. Isolates recovered from kidneys expressed PNSG, but expression was lost with in vitro culture. Strong antibody responses to PNSG were elicited in S. aureus infected mice, and a PNSG-capsule was observed by electron microscopy on isolates directly plated from infected kidneys. PNSG represents a previously unidentified surface polysaccharide of S. aureus that is elaborated during human and animal infection and is a prominent target for protective antibodies.

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.

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.

The inhibitory effect of Staphylococcus epidermidis slime on the phagocytosis of murine peritoneal macrophages is interferon-independent

The extracellular slime produced by Staphylococcus epidermidis has been shown to interfere with several human neutrophil functions in vitro, such as chemotaxis, degranulation and phagocytosis. Slime production has been suggested as a useful marker for clinically significant infections with coagulase-negative Staphylococcus. Since the main role of macrophages in defense mechanisms is phagocytosis, the effect of slime on the phagocytic activity of macrophages was investigated. The phagocytic activity of murine peritoneal macrophages treated with slime in vitro decreased in a dose-dependent fashion. A similar decrease was also observed in macrophages isolated from mice that had previously received intraperitoneal injection of slime. To investigate whether interferon also plays a role in this process, mice were treated with interferon or an interferon inducer, polyinosinic-polycytidylic acid (poly I:C), together with slime before macrophage isolation. The slime-suppressed phagocytic activity of macrophages was partially relieved by both agents, and the recovery effect of poly I:C in slime-suppressed phagocytosis of macrophages in vivo might be attributed to the increased interferon level in peritoneal fluid and sera. However, when slime was given to poly I:C-pretreated mice, the phagocytic activity remained suppressed. Thus, it appears that slime is able to suppress the phagocytic activity of macrophages regardless of the state of macrophage activation by poly I:C. The results suggest that the inhibition of phagocytosis by S. epidermidis slime may be independent from the activation of interferon.