Antibodies to capsular polysaccharides are not protective against experimental Staphylococcus aureus endocarditis

A genetic regulatory see-saw of biofilm and virulence in MRSA pathogenesis

Staphylococcus aureus is one of the most common opportunistic human pathogens causing several infectious diseases. Ever since the emergence of the first methicillin-resistant Staphylococcus aureus (MRSA) strain decades back, the organism has been a major cause of hospital-acquired infections (HA-MRSA). The spread of this pathogen across the community led to the emergence of a more virulent subtype of the strain, i.e., Community acquired Methicillin resistant Staphylococcus aureus (CA-MRSA). Hence, WHO has declared Staphylococcus aureus as a high-priority pathogen. MRSA pathogenesis is remarkable because of the ability of this "superbug" to form robust biofilm both in vivo and in vitro by the formation of polysaccharide intercellular adhesin (PIA), extracellular DNA (eDNA), wall teichoic acids (WTAs), and capsule (CP), which are major components that impart stability to a biofilm. On the other hand, secretion of a diverse array of virulence factors such as hemolysins, leukotoxins, enterotoxins, and Protein A regulated by agr and sae two-component systems (TCS) aids in combating host immune response. The up- and downregulation of adhesion genes involved in biofilm formation and genes responsible for synthesizing virulence factors during different stages of infection act as a genetic regulatory see-saw in the pathogenesis of MRSA. This review provides insight into the evolution and pathogenesis of MRSA infections with a focus on genetic regulation of biofilm formation and virulence factors secretion.

Molecular Targets for Antibody-Based Anti-Biofilm Therapy in Infective Endocarditis

Infective endocarditis (IE) is a heart disease caused by the infection of heart valves, majorly caused by Staphilococcus aureus. IE is initiated by bacteria entering the blood circulation in favouring conditions (e.g., during invasive procedures). So far, the conventional antimicrobial strategies based on the usage of antibiotics remain the major intervention for treating IE. Nevertheless, the therapeutic efficacy of antibiotics in IE is limited not only by the bacterial drug resistance, but also by the formation of biofilms, which resist the penetration of antibiotics into bacterial cells. To overcome these drawbacks, the development of anti-biofilm treatments that can expose bacteria and make them more susceptible to the action of antibiotics, therefore resulting in reduced antimicrobial resistance, is urgently required. A series of anti-biofilm strategies have been developed, and this review will focus in particular on the development of anti-biofilm antibodies. Based on the results previously reported in the literature, several potential anti-biofilm targets are discussed, such as bacterial adhesins, biofilm matrix and bacterial toxins, covering their antigenic properties (with the identification of potential promising epitopes), functional mechanisms, as well as the antibodies already developed against these targets and, where feasible, their clinical translation.

The Candidate Antigens to Achieving an Effective Vaccine against Staphylococcus aureus

Staphylococcus aureus (S. aureus) is an opportunistic pathogen that causes various inflammatory local infections, from those of the skin to postinfectious glomerulonephritis. These infections could result in serious threats, putting the life of the patient in danger. Antibiotic-resistant S. aureus could lead to dramatic increases in human mortality. Antibiotic resistance would explicate the failure of current antibiotic therapies. So, it is obvious that an effective vaccine against S. aureus infections would significantly reduce costs related to care in hospitals. Bacterial vaccines have important impacts on morbidity and mortality caused by several common pathogens, however, a prophylactic vaccine against staphylococci has not yet been produced. During the last decades, the efforts to develop an S. aureus vaccine have faced two major failures in clinical trials. New strategies for vaccine development against S. aureus has supported the use of multiple antigens, the inclusion of adjuvants, and the focus on various virulence mechanisms. We aimed to present a compressive review of different antigens of S. aureus and also to introduce vaccine candidates undergoing clinical trials, from which can help us to choose a suitable and effective candidate for vaccine development against S. aureus.

Pathogenicity and virulence of Staphylococcus aureus

Staphylococcus aureus is one of the most frequent worldwide causes of morbidity and mortality due to an infectious agent. This pathogen can cause a wide variety of diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis. Treatment of S. aureus infections is complicated by antibiotic resistance and a working vaccine is not available. There has been ongoing and increasing interest in the extraordinarily high number of toxins and other virulence determinants that S. aureus produces and how they impact disease. In this review, we will give an overview of how S. aureus initiates and maintains infection and discuss the main determinants involved. A more in-depth understanding of the function and contribution of S. aureus virulence determinants to S. aureus infection will enable us to develop anti-virulence strategies to counteract the lack of an anti-S. aureus vaccine and the ever-increasing shortage of working antibiotics against this important pathogen.

Preparation and preclinical evaluation of two novel Staphylococcus aureus capsular polysaccharide 5 and 8-fusion protein (Hla-MntC-SACOL0723) immunoconjugates

Staphylococcus aureus is one of the most common pathogens in the hospital and the community. The emergence of broad-spectrum antibiotic resistance in S. aureus has made the treatment process more difficult. Therefore, it is obvious that an effective prevention strategy against the pathogen could significantly reduce costs related to care in hospitals. In this report, we describe a simple approach to conjugate S. aureus capsular polysaccharide 5 (CP5) from S. aureus Reynolds strain and 8 (CP8) from S. aureus Becker strain to a fusion protein (Hla-MntC-SACOL0723) and investigation of its bioactivity. The conjugation was done by using ADH (as a bridge) and EDAC (as a coupling agent). The immunoconjugates were characterized by routine polysaccharide/protein contents assays followed by reverse phase chromatography and FTIR spectroscopy. The groups of mice were immunized with conjugate vaccines, capsular polysaccharides, and phosphate-buffered saline (PBS) as a control group. The functional activity of the vaccine candidates was evaluated by ELISA, opsonophagocytosis tests, and determination of bacterial load in challenge study. The results showed that the specific antibody (total IgG) titers raised against conjugate molecules were higher than those of the nonconjugated capsular polysaccharides. The opsonic activity of the conjugate vaccines antisera was significantly higher than polysaccharides alone (58% reduction in the number of bacteria versus 16.3% at 1:2 dilution, p < .05), Further, the conjugate vaccine group had a significant reduction in bacterial load after challenge with S. aureus COL strain cells as compared to the PBS and nonconjugated controls. In conclusion, the immunoconjugates could be developed as a potential vaccine candidate against S. aureus.

Fighting Staphylococcus aureus Biofilms with Monoclonal Antibodies

Staphylococcus aureus (S. aureus) is a notorious pathogen and one of the most frequent causes of biofilm-related infections. The treatment of S. aureus biofilms is hampered by the ability of the biofilm structure to shield bacteria from antibiotics as well as the host's immune system. Therefore, new preventive and/or therapeutic interventions, including the use of antibody-based approaches, are urgently required. In this review, we describe the mechanisms by which anti-S. aureus antibodies can help in combating biofilms, including an up-to-date overview of monoclonal antibodies currently in clinical trials. Moreover, we highlight ongoing efforts in passive vaccination against S. aureus biofilm infections, with special emphasis on promising targets, and finally indicate the direction into which future research could be heading.

Vaccines and Immunotherapy for Staphylococcal Infections

Nosocomial or hospital-acquired infections are associated with prolonged hospitalizations and increased healthcare costs. Infections associated with surgical implants are becoming more difficult and more costly to manage, as they require repeated surgical procedures and a longer period of time to treat patients. Continued advances in the use of medical devices, an increase in the number of immunocompromised patients, and a steady rise in the prevalence of antibiotic-resistant organisms has renewed interest in the development of novel therapies that can be used to prevent and treat nosocomial infections. This review provides an overview of bacterial adhesins and focuses on novel immunological therapies developed to treat staphylococcal infections.

Murine Models of Bacteremia and Surgical Wound Infection for the Evaluation of Staphylococcus aureus Vaccine Candidates

There is an unmet need for an effective vaccine to prevent infections caused by Staphylococcus aureus. Murine models of staphylococcal infections are useful tools for evaluation of experimental vaccines and adjuvants in preclinical studies. Mice can be actively immunized with vaccines or passively immunized with antibodies prior to bacterial challenge. We described two infection models, bacteremia and surgical wound infection, that are relevant to human disease. To achieve a persistent bacteremia, mice are challenged with a sublethal inoculum of S. aureus by the intraperitoneal route. Bacteremia is assessed 2 h after challenge, and weight loss and renal infection are quantified after 4 days. Surgical wound infection can be achieved by inoculation of S. aureus directly into the sutured incision of a thigh muscle. After 3 days the tissue bacterial burden and weight loss are evaluated in this localized infection. Protective efficacy of experimental vaccines is analyzed by comparison with mice immunized with appropriate control vaccines.

The Staphylococcus aureus polysaccharide capsule and Efb-dependent fibrinogen shield act in concert to protect against phagocytosis

Staphylococcus aureus has developed many mechanisms to escape from human immune responses. To resist phagocytic clearance, S. aureus expresses a polysaccharide capsule, which effectively masks the bacterial surface and surface-associated proteins, such as opsonins, from recognition by phagocytic cells. Additionally, secretion of the extracellular fibrinogen binding protein (Efb) potently blocks phagocytic uptake of the pathogen. Efb creates a fibrinogen shield surrounding the bacteria by simultaneously binding complement C3b and fibrinogen at the bacterial surface. By means of neutrophil phagocytosis assays with fluorescently labelled encapsulated serotype 5 (CP5) and serotype 8 (CP8) strains we compare the immune-modulating function of these shielding mechanisms. The data indicate that, in highly encapsulated S. aureus strains, the polysaccharide capsule is able to prevent phagocytic uptake at plasma concentrations <10 %, but loses its protective ability at higher concentrations of plasma. Interestingly, Efb shows a strong inhibitory effect on both capsule-negative and encapsulated strains at all tested plasma concentrations. Furthermore, the results suggest that both shielding mechanisms can exist simultaneously and collaborate to provide optimal protection against phagocytosis at a broad range of plasma concentrations. As opsonizing antibodies will be shielded from recognition by either mechanism, incorporating both capsular polysaccharides and Efb in future vaccines could be of great importance.

MsaB activates capsule production at the transcription level in Staphylococcus aureus

Staphylococcus aureus produces several virulence factors that allow it to cause a variety of infections. One of the major virulence factors is the capsule, which contributes to the survival of the pathogen within the host as a way to escape phagocytosis. The production of the capsular polysaccharide is encoded in a 16 gene operon, which is regulated in response to several environmental stimuli including nutrient availability. For instance, the capsule is produced in the late- and post-exponential growth phases, but not in the early- or mid-exponential growth phase. Several regulators are involved in capsule production, but the regulation of the cap operon is still poorly understood. In this study, we show that MsaB activates the cap operon by binding directly to a 10 bp repeat in the promoter region. We show that despite the fact that MsaB is expressed throughout four growth phases, it only activates capsule production in the late- and post-exponential growth phases. Furthermore, we find that MsaB does not bind to its target site in the early and mid-exponential growth phases. This correlates with decreased nutrient availability and capsule production. These data suggest either that MsaB binding ability changes in response to nutrients or that other cap operon regulators interfere with the binding of MsaB to its target site. This study increases our understanding of the regulation of capsule production and the mechanism of action of MsaB.

Phenotypic heterogeneity and temporal expression of the capsular polysaccharide in Staphylococcus aureus

Bacteria respond to ever-changing environments through several adaptive strategies. This includes mechanisms leading to a high degree of phenotypic variability within a genetically homogeneous population. In Staphylococcus aureus, the capsular polysaccharide (CP) protects against phagocytosis, but also impedes adherence to endothelial cells and/or matrix proteins. We analysed the regulation of core biosynthesis genes (capA-P) necessary for CP synthesis using single-cell assays (immunofluorescence and promoter-activity). In persistent human carriers, we found a distinct subpopulation of nasal S. aureus to be CP positive. In vitro, cap expression is also heterogeneous and strongly growth-phase dependent. We asked whether this peculiar expression pattern (earlyOff/lateHeterogen) is orchestrated by the quorum system Agr. We show that the Agr-driven effector molecule RNAIII promotes cap expression largely via inactivation of the repressor Rot. High NaCl, deletion of CodY or Sae also resulted in higher cap expression but did not change the earlyOFF/lateHeterogen expression pattern. Activity of the quorum system itself is largely homogenous and does not account for the observed heterogeneity of cap expression or the strictly growth phase dependent expression. Our findings are in contrast to the prevailing view that quorum sensing is the main driving force for virulence gene expression when bacterial cell densities increase.

Structure and Function of Surface Polysaccharides of Staphylococcus aureus

The major surface polysaccharides of Staphylococcus aureus include the capsular polysaccharide (CP), cell wall teichoic acid (WTA), and polysaccharide intercellular adhesin/poly-β(1-6)-N-acetylglucosamine (PIA/PNAG). These glycopolymers are important components of the staphylococcal cell envelope, but none of them is essential to S. aureus viability and growth in vitro. The overall biosynthetic pathways of CP, WTA, and PIA/PNAG have been elucidated, and the functions of most of the biosynthetic enzymes have been demonstrated. Because S. aureus CP and WTA (but not PIA/PNAG) utilize a common cell membrane lipid carrier (undecaprenyl-phosphate) that is shared by the peptidoglycan biosynthesis pathway, there is evidence that these processes are highly integrated and temporally regulated. Regulatory elements that control glycopolymer biosynthesis have been described, but the cross talk that orchestrates the biosynthetic pathways of these three polysaccharides remains largely elusive. CP, WTA, and PIA/PNAG each play distinct roles in S. aureus colonization and the pathogenesis of staphylococcal infection. However, they each promote bacterial evasion of the host immune defences, and WTA is being explored as a target for antimicrobial therapeutics. All the three glycopolymers are viable targets for immunotherapy, and each (conjugated to a carrier protein) is under evaluation for inclusion in a multivalent S. aureus vaccine. Future research findings that increase our understanding of these surface polysaccharides, how the bacterial cell regulates their expression, and their biological functions will likely reveal new approaches to controlling this important bacterial pathogen.

MgrA activates expression of capsule genes, but not the α-toxin gene in experimental Staphylococcus aureus endocarditis

BACKGROUND

Staphylococcus aureus produces numerous virulence factors but little is known about their in vivo regulation during an infection.

METHODS

The production of capsule and α-toxin, and the expression of their respective genes, cap5 and hla, were analyzed by comparing CYL11481 (derivative of Newman) and its isogenic regulatory mutants in vitro. The temporal expression of cap5 and hla and the regulatory genes in vivo was carried out using a rat infective endocarditis model.

RESULTS

In vitro analyses showed that capsule was positively regulated by MgrA, Agr, Sae, ArlR, and ClpC, and negatively by CodY and SbcDC. The α-toxin was positively regulated by MgrA, Agr, Sae, ArlR, and SbcDC but negatively by ClpC and CodY. In vivo analyses showed that cap5 expression correlated best with mgrA expression, whereas hla expression correlated best with sae expression. Mutation in mgrA drastically reduced cap5 expression in vivo.

CONCLUSIONS

Our results suggest that, in vitro, Agr is the most important regulator for capsule and α-toxin production, as well as for cap5 transcription, but SaeR is the most critical for hla transcription. However, in vivo, MgrA is the major transcriptional regulator of capsule, but not α-toxin, whereas saeR expression correlates best with hla expression.

Natural antibodies in normal human serum inhibit Staphylococcus aureus capsular polysaccharide vaccine efficacy

BACKGROUND

Vaccines against Streptococcus pneumoniae, Neisseria meningitidis, and Hemophilus influenzae type b induce functional opsonic or bactericidal antibodies to surface capsular polysaccharides (CP). Targeting the comparable Staphylococcus aureus CP seems logical, but to date such efforts have failed in human trials. Studies using immunization-induced animal antibodies have documented interference in opsonic and protective activities of antibodies to CP by antibodies to another S. aureus cell surface polysaccharide, poly-N-acetyl glucosamine (PNAG). Here we evaluated whether natural antibody to PNAG in normal human serum (NHS) had a similar deleterious effect.

METHODS

Functional and/or protective activities of antibody to S. aureus CP and PNAG antigens in patients with bacteremia, in mice immunized with combinations of CP and PNAG conjugate vaccines, and in serum samples of healthy subjects with natural antibody to PNAG, to which immunization-induced animal antibodies to CP antigens were added, were evaluated.

RESULTS

Antibodies to PNAG and CP that mutually interfered with opsonic killing of S. aureus were detected in 9 of 15 bacteremic patients. Active immunization of mice with combinations of PNAG and CP conjugate antigens always induced antibodies that interfered with each other's functional activity. Non-opsonic natural antibodies to PNAG found in NHS interfered with the functional and protective activities of immunization-induced antibody to CP antigens during experimental infection with S. aureus.

CONCLUSIONS

Both immunization-induced animal antibodies and natural antibodies to PNAG in NHS interfere with the protective activities of immunization-induced antibody to S. aureus CP5 and CP8 antigens, representing potential barriers to successful use of CP-specific vaccines.

Progress in the development of effective vaccines to prevent selected gram-positive bacterial infections

Infections caused by virulent Gram-positive bacteria, such as Staphylococcus aureus, group B streptococci and group A streptococci, remain significant causes of morbidity and mortality despite progress in antimicrobial therapy. Despite significant advances in the understanding of the pathogenesis of infection caused by these organisms, there are only limited strategies to prevent infection. In this article, we review efforts to develop safe and effective vaccines that would prevent infections caused by these 3 pathogens.

Protection from Staphylococcus aureus mastitis associated with poly-N-acetyl beta-1,6 glucosamine specific antibody production using biofilm-embedded bacteria

Staphylococcus aureus vaccines based on bacterins surrounded by slime, surface polysaccharides coupled to protein carriers and polysaccharides embedded in liposomes administered together with non-biofilm bacterins confer protection against mastitis. However, it remains unknown whether protective antibodies are directed to slime-associated known exopolysaccharides and could be produced in the absence of bacterin immunizations. Here, a sheep mastitis vaccination study was carried out using bacterins, crude bacterial extracts or a purified exopolysaccharide from biofilm bacteria delivered in different vehicles. This polysaccharide reacted specifically with antibodies to poly-N-acetyl-beta-1,6-glucosamine (PNAG) and not with antibodies to other capsular antigens or bacterial components. Following intra-mammary challenge with biofilm-producing bacteria, antibody production against the polysaccharide, milk bacterial counts and mastitis lesions were determined. Bacterins from strong biofilm-producing bacteria triggered the highest production of antibodies to PNAG and conferred the highest protection against infection and mastitis, compared with weak biofilm-producing bacteria and non-cellular inocula. Thus, bacterins from strong biofilm bacteria, rather than purified polysaccharide, are proposed as a cost-efficient vaccination against S. aureus ruminant mastitis.

Immunization with Staphylococcus aureus clumping factor B, a major determinant in nasal carriage, reduces nasal colonization in a murine model

Staphylococcus aureus is responsible for a wide range of infections, including soft tissue infections and potentially fatal bacteremias. The primary niche for S. aureus in humans is the nares, and nasal carriage is a documented risk factor for staphylococcal infection. Previous studies with rodent models of nasal colonization have implicated capsule and teichoic acid as staphylococcal surface factors that promote colonization. In this study, a mouse model of nasal colonization was utilized to demonstrate that S. aureus mutants that lack clumping factor A, collagen binding protein, fibronectin binding proteins A and B, polysaccharide intercellular adhesin, or the accessory gene regulator colonized as well as wild-type strains colonized. In contrast, mutants deficient in sortase A or clumping factor B (ClfB) showed reduced nasal colonization. Mice immunized intranasally with killed S. aureus cells showed reduced nasal colonization compared with control animals. Likewise, mice that were immunized systemically or intranasally with a recombinant vaccine composed of domain A of ClfB exhibited lower levels of colonization than control animals exhibited. A ClfB monoclonal antibody (MAb) inhibited S. aureus binding to mouse cytokeratin 10. Passive immunization of mice with this MAb resulted in reduced nasal colonization compared with the colonization observed after immunization with an isotype-matched control antibody. The mouse immunization studies demonstrate that ClfB is an attractive component for inclusion in a vaccine to reduce S. aureus nasal colonization in humans, which in turn may diminish the risk of staphylococcal infection. As targets for vaccine development and antimicrobial intervention are assessed, rodent nasal colonization models may be invaluable.

The pathogenesis of Staphylococcus aureus infection in the diabetic NOD mouse

Although Staphylococcus aureus is a major pathogen implicated in diabetic foot infections, little is known about the pathogenesis of this disease. A model of S. aureus infection in the hindpaw of nonobese diabetic (NOD) mice was developed. The experimental infection was exacerbated in diabetic mice (blood glucose levels > or =19 mmol/l) compared with nondiabetic mice, and the diabetic animals were unable to clear the infection over a 10-day period. Insulin-mediated control of glycemia in diabetic mice resulted in enhanced clearance of S. aureus from the infected tissue. Diabetic mice showed reduced tissue inflammation in response to bacterial inoculation compared with nondiabetic NOD animals, and this was consistent with the novel finding of significantly decreased tissue levels of the chemokines KC and MIP-2 in diabetic mice. Blood from nondiabetic and diabetic NOD mice killed S. aureus in vitro, whereas the bacteria multiplied in blood from diabetic mice with severe hyperglycemia. The impaired killing of S. aureus by diabetic mice was correlated with a diminished leukocytic respiratory burst in response to S. aureus in blood from diabetic animals. This animal model of hindpaw infection may be useful for the analysis of host defects in innate immunity that contribute to recalcitrant diabetic foot infections.

Immunotherapeutics for nosocomial infections

Nosocomial or hospital-acquired infections are associated with prolonged hospitalisation and increased healthcare costs and resource utilisation. Continued advances in sophisticated medical procedures, an increase in the number of immunocompromised patients and a steady rise in the prevalence of antibiotic-resistant organisms has renewed interest in the development of novel therapies that can treat nosocomial infections. This review focuses on novel immunological approaches to address this significant unmet medical need.

Staphylococcus aureus capsular polysaccharides

Serotype 5 and 8 capsular polysaccharides predominate among clinical isolates of Staphylococcus aureus. The results of experiments in animal models of infection have revealed that staphylococcal capsules are important in the pathogenesis of S. aureus infections. The capsule enhances staphylococcal virulence by impeding phagocytosis, resulting in bacterial persistence in the bloodstream of infected hosts. S. aureus capsules also promote abscess formation in rats. Although the capsule has been shown to modulate S. aureus adherence to endothelial surfaces in vitro, animal studies suggest that it also promotes bacterial colonization and persistence on mucosal surfaces. S. aureus capsular antigens are surface associated, limited in antigenic specificity, and highly conserved among clinical isolates. With the emergence of vancomycin-resistant S. aureus in the United States in 2002, new strategies are needed to combat staphylococcal infections. Purified serotype 5 and 8 capsular polysaccharides offer promise as target antigens for a vaccine to prevent staphylococcal infections, although the inclusion of other antigens is likely to be essential in the development of an effective S. aureus vaccine. The genetics and mechanisms of capsule biosynthesis are complex, and much work remains to enhance our understanding of capsule biosynthesis and its regulation.

Prevalence of capsular polysaccharide (CP) types of Staphylococcus aureus isolated from bovine mastitic milk and protection of S. aureus infection in mice with CP vaccine

To determine the prevalence of capsular polysaccharide (CP) types of Staphylococcus aureus isolated from bovine mastitic milk in Korea, the protective effect of the conjugates, composed of microencapsulated S. aureus clinical isolate type 8 CP bound to Pseudomonas aeruginosa exotoxin A (ETA) was evaluated in mice. Of 107 S. aureus isolates, serotype 5 and 8 accounted for only 26 or 24.2%. When serotype 336 antiserum was employed, fifty of the remaining 81 isolates were typed as 336, 26 reacted with two serotypes, and 5 were nontypeable. Mice challenged with the same strain used for immunization had fewer S. aureus cells in their kidneys than mice challenged with the heterologous strain. But the magnitudes of difference on bacterial clearance were similar in both groups, indicating that the significance of this result remains to be determined. Mice immunized with the conjugate elicited an antibody response 3 days post injection, which persisted for 13 days of the observation period after second injection in some mice. The mice immunized with the CP8-ETA conjugates developed antibodies significantly higher than those immunized with CP-Freund's adjuvant or PBS. In in vivo bacterial challenge experiment, the survival rate of mice immunized with CPS-ETA conjugate was significantly higher than that of mice immunized with PBS. It was suggested that CP8-ETA vaccine had a potential to protect mice against experimental S. aureus bacteremia.

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.

Capsule expression by bovine isolates of Staphylococcus aureus from Argentina: genetic and epidemiologic analyses

Staphylococcus aureus is an important cause of bovine mastitis worldwide, and effective preventive or therapeutic modalities are lacking. Although most human S. aureus isolates produce capsular polysaccharides (CPs), few reports have described the prevalence of capsules on bovine isolates. This information is important for the rational design of a vaccine for the prevention of staphylococcal mastitis. We serotyped 195 S. aureus strains isolated between 1989 and 1997 from the milk of mastitic cows in Argentina. Only 14 (7.1%) of the strains were serotype 5, and all were recovered between 1989 and 1992. Thirteen serotype 8 strains were identified, and 12 of these were isolated between 1991 and 1994. The remaining 168 isolates were nonreactive (NR) with CP serotype 5 (CP5)- or CP8-specific antibodies. Hybridization studies performed with genomic DNA from eight NR strains revealed that only three of them carried the capsule genes. Pulsed-field gel electrophoresis (PFGE) performed with 127 of the 195 S. aureus isolates revealed that most (86%) strains belonged to one of four major PFGE groups. Although 8 of 14 CP5 isolates showed a common PFGE pattern (arbitrarily defined as A1), 31 other A1 isolates from the same time period (1989 to 1992) were not CP5 positive. In contrast, only nine PFGE type B3 isolates were recovered between 1990 and 1994, and eight of these were positive for CP8 (P < 0.0003). The results of this study underscore the variability in capsule expression by S. aureus strains isolated from different geographical regions and cast doubt on the roles of CP5 and CP8 in the pathogenesis and immunoprophylaxis of bovine mastitis in Argentina.

Immunization for Prevention of Infective Endocarditis

Immunization has been used for many years to prevent certain infectious diseases. Often it is targeted to populations at increased risk of a particular infection. Patients at increased risk of infective endocarditis can be identified and would be eligible candidates for immunization if vaccines were available to prevent common bacterial causes of infective endocarditis. The idea of using preventive therapy among patients at increased risk of infective endocarditis is not novel, and recommendations for use of antibiotics prior to performing certain invasive procedures have been in place for years. Findings from immunization experiments using animal models of experimental endocarditis support the notion that vaccine development is appropriate for study in humans, and these findings are reviewed in this paper.

Promoter analysis of the cap8 operon, involved in type 8 capsular polysaccharide production in Staphylococcus aureus

The production of type 8 capsular polysaccharide (CP8) in Staphylococcus aureus is regulated in response to a variety of environmental factors. The cap8 genes required for the CP8 production in strain Becker are transcribed as a single large transcript by a primary promoter located within a 0.45-kb region upstream of the first gene of the cap8 gene cluster. In this study, we analyzed the primary cap8 promoter region in detail. We determined the transcription initiation site of the primary transcript by primer extension and identified the potential promoter sequences. We found several inverted and direct repeats upstream of the promoter. Deletion analysis and site-directed mutagenesis showed that a 10-bp inverted repeat of one of the repeats was required for promoter activity. We showed that the distance but not the specific sequences between the inverted repeat and the promoter was critical to the promoter activity. However, insertion of a DNA sequence with two or four helix turns in this intervening region had a slight effect on promoter activity. To demonstrate the biological significance of the 10-bp inverted repeat, we constructed a strain with a mutation in the repeat in the S. aureus Becker chromosome and showed that the repeat affected CP8 production mostly at the transcriptional level. By gel mobility shift assay, we demonstrated that strain Becker produced at least one protein capable of specific binding to the 10-bp inverted repeat, indicating that the repeat serves as a positive regulatory protein binding site. In addition, reporter gene fusion analysis showed that the cap8 promoter activity was influenced by various growth media and affected most by yeast extract. Our results suggest that yeast extract may exert its profound inhibitory effect on cap8 gene expression through the 10-bp inverted repeat element.

Protective efficacy of antibodies to the Staphylococcus aureus type 5 capsular polysaccharide in a modified model of endocarditis in rats

The protective efficacy of antibodies to the Staphylococcus aureus type 5 capsular polysaccharide (CP5) was examined in a modified model of catheter-induced endocarditis. Rats were catheterized by surgically passing a polyethylene catheter through the right carotid artery and aortic valve into the left ventricle. The following day, the rats were injected by the intraperitoneal (i.p.) route with immunoglobulin G (IgG) purified from nonimmunized rabbits or from rabbits immunized with a conjugate vaccine composed of CP5 and CP8 linked covalently to recombinant Pseudomonas aeruginosa exotoxoid A. One day after passive immunization, the animals were challenged i.p. with one of three serotype 5 S. aureus isolates (strain Reynolds, Lowenstein, or VP) or nontypeable strain 521. Protection was evaluated by comparing quantitative cultures of blood, endocardial vegetations, and kidneys from control and immune animals. For experiments performed with S. aureus Reynolds and Lowenstein, rats given capsular antibodies (645 microg of CP5-specific IgG) showed a significantly (P < 0.05) lower prevalence of endocarditis than rats injected with nonimmune IgG. Similarly, quantitative cultures of the blood, kidneys, and aortic valve vegetations revealed that fewer S. aureus cells were recovered from rats given capsule-specific IgG than from rats administered nonimmune IgG. Rats challenged with strain VP were protected with 1.145 mg of CP5-specific IgG. Capsular antibodies did not protect against infection elicited by a nontypeable strain. These results demonstrate that capsular antibodies elicited by immunization with a polysaccharide-protein conjugate vaccine protect experimental animals against serotype 5 S. aureus infection in a modified model of endocarditis.

The role of staphylococcal polysaccharide microcapsule expression in septicemia and septic arthritis

Staphylococcus aureus arthritis is a rapidly progressive and highly erosive disease of the joints in which both host and bacterial factors are of pathogenic importance. One potential bacterial virulence factor is the ability to express a polysaccharide capsule (CP). Among 11 reported capsular serotypes, CP type 5 (CP5) and CP8 comprise 80 to 85% of all clinical blood isolates. The aim of this study was to assess the role of CP5 as a virulence factor in staphylococcal septicemia and septic arthritis with a recently established murine model of hematogenously spread S. aureus arthritis. NMRI mice were inoculated intravenously with S. aureus strains isogenic for expression of CP5, and clinical, bacteriological, serological, and histopathological progression of disease was studied. Inoculation of 7 x 10(6) CFU of S. aureus per mouse induced 55% mortality in the group inoculated with the CP-expressing bacteria, compared to 18% in the group inoculated with CP- mutants. A lower dose of inoculum (3 x 10[6] per mouse) did not give rise to mortality in mice inoculated with CP mutant strains, whereas 18% of the mice inoculated with the CP5-expressing S. aureus died. Importantly, mice inoculated with S. aureus expressing CP5 had a significantly higher frequency of arthritis and a more severe form of the disease. In vitro assays suggested that macrophages were not able to phagocytize CP5+ staphylococci as efficiently as they were CP5- strains. In addition, once phagocytized, CP5+ bacteria were less efficiently killed than CP- mutants. In summary, CP5 leads to a higher frequency of arthritis and a more severe course of the disease. This seems to be related to the effects of the downregulatory properties of CP on the ingestion and intracellular killing capacity of phagocytes. Our results clearly indicate that the expression of CP5 is a determinant of the virulence of S. aureus in arthritis and septicemia.

The Staphylococcus aureus allelic genetic loci for serotype 5 and 8 capsule expression contain the type-specific genes flanked by common genes

The nucleotide sequences of two gene clusters, cap5 and cap8, involved in the synthesis of Staphylococcus aureus type 5 and type 8 capsular polysaccharides (CPs), respectively were determined. Each gene cluster contained 16 ORFs, which were named cap5A through cap5P for type 5 CP and cap8A through cap8P for type 8 CP. The cap5 and cap8 loci were allelic and were mapped to the SmaI-G fragment in the standard SmaI map of Staph. aureus strain NCTC 8325. The predicted gene products of cap5A through cap5G and cap5L through cap5P are essentially identical to those of cap8A through cap8G and cap8L through cap8P, respectively, with very few amino acid substitutions. Four ORFs located in the central region of each locus are type-specific. A comparison of the predicted amino acid sequences of cap5 and cap8 with sequences found in the databases allowed tentative assignment of functions to 15 of the 16 ORFs. The majority of the capsule genes are likely to be involved in amino sugar synthesis; the remainder are likely to be involved in sugar transfer, capsule chain-length regulation, polymerization and transport.

Staphylococcus aureus vaccination for dialysis patients--an update

Staphylococcus aureus infections are a major cause in both hemodialysis and peritoneal dialysis patients. The availability of a safe and effective protective vaccine would be of great benefit to these patients, but attempts at using vaccines consisting of inactivated whole cells have been unsuccessful. This article discusses an alternate approach to S. aureus vaccine design using a capsular polysaccharide conjugate and preliminary results in hemodialysis and peritoneal patients.

A Staphylococcus aureus capsular polysaccharide (CP) vaccine and CP-specific antibodies protect mice against bacterial challenge

The efficacy of capsular polysaccharide (CP)-specific antibodies elicited by active immunization with vaccines composed of Staphylococcus aureus types 5 and 8 CP linked to Pseudomonas aeruginosa exoprotein A or with immune immunoglobulin G (I-IgG) obtained from vaccinated plasma donors was tested in lethal and sublethal bacterial mouse challenge models. A dose of 2 x 10(5) CFU of S. aureus type 5 CP per mouse administered intraperitoneally (i.p.) with 5% hog mucin was found to cause 80 to 100% mortality in BALB/c mice within 2 to 5 days. Mice passively immunized i.p. 24 h earlier or subcutaneously 48 h earlier with 0.5 ml of I-IgG showed significantly higher average survival rates than animals receiving standard IgG or saline (P < 0.01) following the bacterial challenge. Animals actively immunized with the monovalent type 5 CP-P. aeruginosa exoprotein A conjugate showed a survival rate of 73% compared with 13% in phosphate-buffered saline-immunized animals. The prechallenge geometric mean titer of type 5 CP antibodies in animals that died was significantly (P < 0.05) lower than that of animals which survived the challenge (95.7 versus 223.6 micrograms/ml, respectively). The IgG was further evaluated in mice challenged i.p. with a sublethal dose of 5 x 10(4) CFU per mouse. Serial blood counts were performed on surviving animals at 6, 12, 24, and 48 h. Surviving animals were sacrificed at 72 h, and bacterial counts were performed on their kidneys, livers, and peritoneal lavage fluids. Animals receiving I-IgG had lower bacterial counts in blood samples and lower bacterial densities in kidneys, livers, and peritoneal lavage samples than mice immunized with standard IgG (P < 0.05). These data suggest that S. aureus type 5 CP antibodies induced by active immunization or administered by passive immunization confer protection against S. aureus infections.

Cloning of type 8 capsule genes and analysis of gene clusters for the production of different capsular polysaccharides in Staphylococcus aureus

Eleven serotypes of capsular polysaccharide from Staphylococcus aureus have been reported. We have previously cloned a cluster of type 1 capsule (cap1) genes responsible for type 1 capsular polysaccharide biosynthesis in S. aureus M. To clone the type 8 capsule (cap8) genes, a plasmid library of type 8 strain Becker was screened with a labelled DNA fragment containing the cap1 genes under low-stringency conditions. One recombinant plasmid containing a 14-kb insert was chosen for further study and found to complement 14 of the 18 type 8 capsule-negative (Cap8-) mutants used in the study. Additional library screening, subcloning, and complementation experiments showed that all of the 18 Cap8- mutants were complemented by DNA fragments derived from a 20.5-kb contiguous region of the Becker chromosome. The mutants were mapped into six complementation groups, indicating that the cap8 genes are clustered. By Southern hybridization analyses under high-stringency conditions, we found that DNA fragments containing the cap8 gene cluster show extensive homology with all 17 strains tested, including type 1 strains. By further Southern analyses and cloning of the cap8-related homolog from strain M, we show that strain M carries an additional capsule gene cluster different from the cap1 gene cluster. In addition, by using DNA fragments containing different regions of the cap8 gene cluster as probes to hybridize DNA from different strains, we found that the central region of the cap8 gene cluster hybridizes only to DNAs from certain strains tested whereas the flanking regions hybridize to DNAs of all strains tested. Thus, the cap8 gene clusters and its closely related homologs are likely to have organizations similar to those of the encapsulation genes of other bacterial systems.