Phagocytic killing of encapsulated and microencapsulated Staphylococcus aureus by human polymorphonuclear leukocytes

Bacteriophage Resistance Alters Antibiotic-Mediated Intestinal Expansion of Enterococci

Enterococcus faecalis is a human intestinal pathobiont with intrinsic and acquired resistance to many antibiotics, including vancomycin. Nature provides a diverse and virtually untapped repertoire of bacterial viruses, or bacteriophages (phages), that could be harnessed to combat multidrug-resistant enterococcal infections. Bacterial phage resistance represents a potential barrier to the implementation of phage therapy, emphasizing the importance of investigating the molecular mechanisms underlying the emergence of phage resistance. Using a cohort of 19 environmental lytic phages with tropism against E. faecalis, we found that these phages require the enterococcal polysaccharide antigen (Epa) for productive infection. Epa is a surface-exposed heteroglycan synthesized by enzymes encoded by both conserved and strain-specific genes. We discovered that exposure to phage selective pressure favors mutation in nonconserved epa genes both in culture and in a mouse model of intestinal colonization. Despite gaining phage resistance, epa mutant strains exhibited a loss of resistance to cell wall-targeting antibiotics. Finally, we show that an E. faecalis epa mutant strain is deficient in intestinal colonization, cannot expand its population upon antibiotic-driven intestinal dysbiosis, and fails to be efficiently transmitted to juvenile mice following birth. This study demonstrates that phage therapy could be used in combination with antibiotics to target enterococci within a dysbiotic microbiota. Enterococci that evade phage therapy by developing resistance may be less fit at colonizing the intestine and sensitized to vancomycin, preventing their overgrowth during antibiotic treatment.

Epic Immune Battles of History: Neutrophils vs. Staphylococcus aureus

Neutrophils are the most abundant leukocytes in human blood and the first line of defense after bacteria have breached the epithelial barriers. After migration to a site of infection, neutrophils engage and expose invading microorganisms to antimicrobial peptides and proteins, as well as reactive oxygen species, as part of their bactericidal arsenal. Ideally, neutrophils ingest bacteria to prevent damage to surrounding cells and tissues, kill invading microorganisms with antimicrobial mechanisms, undergo programmed cell death to minimize inflammation, and are cleared away by macrophages. Staphylococcus aureus (S. aureus) is a prevalent Gram-positive bacterium that is a common commensal and causes a wide range of diseases from skin infections to endocarditis. Since its discovery, S. aureus has been a formidable neutrophil foe that has challenged the efficacy of this professional assassin. Indeed, proper clearance of S. aureus by neutrophils is essential to positive infection outcome, and S. aureus has developed mechanisms to evade neutrophil killing. Herein, we will review mechanisms used by S. aureus to modulate and evade neutrophil bactericidal mechanisms including priming, activation, chemotaxis, production of reactive oxygen species, and resolution of infection. We will also highlight how S. aureus uses sensory/regulatory systems to tailor production of virulence factors specifically to the triggering signal, e.g., neutrophils and defensins. To conclude, we will provide an overview of therapeutic approaches that may potentially enhance neutrophil antimicrobial functions.

A non-coding RNA promotes bacterial persistence and decreases virulence by regulating a regulator in Staphylococcus aureus

Staphylococcus aureus produces a high number of RNAs for which the functions are poorly understood. Several non-coding RNAs carry a C-rich sequence suggesting that they regulate mRNAs at the post-transcriptional level. We demonstrate that the Sigma B-dependent RsaA RNA represses the synthesis of the global transcriptional regulator MgrA by forming an imperfect duplex with the Shine and Dalgarno sequence and a loop-loop interaction within the coding region of the target mRNA. These two recognition sites are required for translation repression. Consequently, RsaA causes enhanced production of biofilm and a decreased synthesis of capsule formation in several strain backgrounds. These phenotypes led to a decreased protection of S. aureus against opsonophagocytic killing by polymorphonuclear leukocytes compared to the mutant strains lacking RsaA. Mice animal models showed that RsaA attenuates the severity of acute systemic infections and enhances chronic catheter infection. RsaA takes part in a regulatory network that contributes to the complex interactions of S. aureus with the host immune system to moderate invasiveness and favour chronic infections. It is the first example of a conserved small RNA in S. aureus functioning as a virulence suppressor of acute infections. Because S. aureus is essentially a human commensal, we propose that RsaA has been positively selected through evolution to support commensalism and saprophytic interactions with the host.

Staphylococcus aureus is a commensal and an opportunistic pathogen that causes a large range of community and hospital-acquired infections. The bacteria produce an array of virulence factors, the expression of which is regulated by a set of regulators including proteins and RNAs. In recent years, a large number of small non-coding RNAs encoded by the S. aureus genome have been identified but determination of their function is still lagging behind. This study shows that RsaA, a staphylococcal conserved non-coding RNA, operates at the post-transcriptional level by repressing the translation of the master regulatory protein MgrA. The repression is based on a direct interaction of RsaA with the ribosome binding site of mgrA mRNA. Through MgrA regulation, RsaA activates biofilm formation and inhibits capsule synthesis. Using appropriate animal models, we showed that RsaA acts as a suppressor of virulence because the deletion of its gene increases the invasiveness of S. aureus in the mice sepsis model. RsaA is thus part of complex regulatory network that modify the interactions of S. aureus with the eukaryotic immune system. These findings illustrate how small RNAs can have a major impact in bacterial biology.

Capsular polysaccharides are an important immune evasion mechanism for Staphylococcus aureus

Staphylococcus aureus can cause severe life threatening invasive diseases. The principal immune effector mechanism by which humans are protected from Gram positive bacteria such as S. aureus is antigen specific antibody- and complement-dependent opsonophagocytosis. This process can be measured in vitro using the opsonophagocytic antibody assay (OPA), which is a complex assay composed of live S. aureus bacteria, a complement source, phagocytic effector cells such as differentiated HL-60 cells, and test serum. In this report, we investigated the impact on the OPA of S. aureus surface antigens capsular polysaccharides (CP) and protein A (SpA). We demonstrated that higher CP expression renders bacteria more resistant to non-specific opsonophagocytic killing than increased SpA expression, suggesting that the expression of capsular polysaccharides may be the more important immune evasion strategy for S. aureus. Bacteria that were not fully encapsulated were highly susceptible to non-specific killing in the assay in the absence of immune serum. This non-specific killing was prevented by growing the bacteria under conditions that increased capsular polysaccharide levels on the surface of the bacteria. In contrast, the level of SpA expression had no detectable effect on non-specific killing in OPA. Using anti-CP antibodies we demonstrated type-specific killing in OPA of both MRSA and MSSA clinical isolates. SpA expression on the cell surface did not interfere with OPA activity, providing evidence that despite the role of SpA in sequestering antibodies by their Fc region, killing is easily accomplished in the presence of high titered anti-capsular polysaccharide antibodies. This highlights the role of CP as an important immune evasion mechanism and supports the inclusion of capsular polysaccharide antigens in the formulation of multi-component prophylactic vaccines against S. aureus.

Detection of virulence-associated genes in Staphylococcus aureus isolated from bovine clinical mastitis milk samples in Guangxi

Staphylococcus aureus is recognized worldwide as a pathogen causing many serious diseases in humans and animals and is one of the most common etiological agents of clinical and subclinical bovine mastitis. The purpose of this study was to determine the presence of genes encoding clfA, fnbA, fnbB, cap5, cap8, hla, hlb, nuc, sea, and tst of S. aureus strains (n = 39) isolated from bovine clinical mastitis in Guangxi by polymerase chain reaction amplification. The results of the present study indicated that all isolates were found to contain one or more virulence-associated genes. The most frequently encountered genes were fnbA (97 %) and nuc (90 %), followed by hla (85 %) and hlb (82 %), respectively. None of the investigated S. aureus strains harbored fnbB and sea genes. The data in the present study showed a relatively wide distribution of the genes fnbA and nuc among the investigated isolates, indicating that they play an important role on bovine mastitis pathogenesis. The study provides a valuable insight into the virulence-associated genes of this important pathogen.

Characterization of the structure and biological functions of a capsular polysaccharide produced by Staphylococcus saprophyticus

Staphylococcus saprophyticus is a common cause of uncomplicated urinary tract infections in women. S. saprophyticus strain ATCC 15305 carries two staphylococcal cassette chromosome genetic elements, SCC(15305RM) and SCC(15305cap). The SCC(15305cap) element carries 13 open reading frames (ORFs) involved in capsular polysaccharide (CP) biosynthesis, and its G+C content (26.7%) is lower than the average G+C content (33.2%) for the whole genome. S. saprophyticus strain ATCC 15305 capD, capL, and capK (capD(Ssp), capL(Ssp), and capK(Ssp)) are homologous to genes encoding UDP-FucNAc biosynthesis, and gtaB and capI(Ssp) show homology to genes involved in UDP-glucuronic acid synthesis. S. saprophyticus ATCC 15305 CP, visualized by immunoelectron microscopy, was extracted and purified using anionic-exchange and size exclusion chromatography. Analysis of the purified CP by (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy and gas-liquid chromatography revealed two types of branched tetrasaccharide repeating units composed of the following: -4)-beta-Glc-(1-3)-Sug-(1-4)-beta-GlcA-(1- | beta-GlcNAc-(1-2) Sug represents two stereoisomers of 2-acetamido-2,6-dideoxy-hexos-4-ulose residues, one of which has an arabino configuration. The encapsulated ATCC 15305 strain was resistant to complement-mediated opsonophagocytic killing by human neutrophils, whereas the acapsular mutant C1 was susceptible. None of 14 clinical isolates reacted with antibodies to the ATCC 15305 CP. However, 11 of the 14 S. saprophyticus isolates were phenotypically encapsulated based on their resistance to complement-mediated opsonophagocytic killing and their failure to hemagglutinate when cultivated aerobically. Ten of the 14 clinical strains carried homologues of the conserved staphylococcal capD gene or the S. saprophyticus gtaB gene, or both. Our results suggest that some strains of S. saprophyticus are encapsulated and that more than one capsular serotype exists.

Further characterization of the epa gene cluster and Epa polysaccharides of Enterococcus faecalis

We previously identified a gene cluster, epa (for enterocococcal polysaccharide antigen), involved in polysaccharide biosynthesis of Enterococcus faecalis and showed that disruption of epaB and epaE resulted in attenuation in translocation, biofilm formation, resistance to polymorphonuclear leukocyte (PMN) killing, and virulence in a mouse peritonitis model. Using five additional mutant disruptions in the 26-kb region between orfde2 and OG1RF_0163, we defined the epa locus as the area from epaA to epaR. Disruption of epaA, epaM, and epaN, like prior disruption of epaB and epaE, resulted in alteration in Epa polysaccharide content, more round cells versus oval cells with OG1RF, decreased biofilm formation, attenuation in a mouse peritonitis model, and resistance to lysis by the phage NPV-1 (known to lyse OG1RF), while mutants disrupted in orfde2 and OG1RF_163 (the epa locus flanking genes) behaved like OG1RF in those assays. Analysis of the purified Epa polysaccharide from OG1RF revealed the presence of rhamnose, glucose, galactose, GalNAc, and GlcNAc in this polysaccharide, while carbohydrate preparation from the epaB mutant did not contain rhamnose, suggesting that one or more of the glycosyl transferases encoded by the epaBCD operon are necessary to transfer rhamnose to the polysaccharide. In conclusion, the epa genes, uniformly present in E. faecalis strains and involved in biosynthesis of polysaccharide in OG1RF, are also important for OG1RF shape determination, biofilm formation, and NPV-1 replication/lysis, as well as for E. faecalis virulence in a mouse peritonitis model.

Staphylococcus aureus infection after splenectomy and splenic autotransplantation in BALB/c mice

Splenectomy results in an increased risk of sepsis. The autogenous transplant of the spleen is an option for preserving splenic functions after total splenectomy. In this study, the capacity of animals undergoing autogenous spleen transplantation to respond to Staphylococcus aureus infection was investigated. BALB/c mice were divided into three groups: splenectomy followed by autotransplantation in the retroperitonium (AT), splenectomized only (SP) and operated non-splenectomized sham control (CT). Thirty days after surgery the mice were infected intravenously with S. aureus. Splenectomized mice had a higher number of colony-forming units (CFU) of S. aureus in liver and lungs in comparison with either AT or with CT mice (P < 0.05). Higher CFU numbers in lung of SP mice correlated with elevated production of interleukin-10 associated with a lower production of interferon-gamma and tumour necrosis factor-alpha. However, systemically, the level of tumour necrosis factor-alpha was higher in the SP group than in CT or AT. Lower titres of specific anti-S. aureus immunoglobulin (Ig)M and IgG1 were observed 6 days after infection in SP mice in comparison either with the AT or CT groups. Thus, splenectomy is detrimental to the immune response of BALB/c mice against infection by S. aureus which can be re-established by autogenous implantation of the spleen.

Structural and biological characterization of a capsular polysaccharide produced by Staphylococcus haemolyticus

The DNA sequence of the genome of Staphylococcus haemolyticus JCSC1435 revealed a putative capsule operon composed of 13 genes in tandem. The first seven genes (capABCDEFG(Sh)) showed > or = 57% similarity with the Staphylococcus aureus cap5 or cap8 locus. However, the capHIJKLM(Sh) genes are unique to S. haemolyticus and include genes encoding a putative flippase, an aminotransferase, two glycosyltransferases, and a transcriptional regulator. Capsule-like material was readily apparent by immunoelectron microscopy on bacteria harvested in the postexponential phase of growth. Electron micrographs of a JCSC1435 mutant with a deleted cap region lacked the capsule-like material. Both strains produced small amounts of surface-associated material that reacted with antibodies to polyglutamic acid. S. haemolyticus cap genes were amplified from four of seven clinical isolates of S. haemolyticus from humans, and three of these strains produced a serologically cross-reactive capsular polysaccharide. In vitro assays demonstrated that the acapsular mutant strain showed greater biofilm formation but was more susceptible to complement-mediated opsonophagocytic killing than the parent strain. Structural characterization of capsule purified from S. haemolyticus strain JCSC1435 showed a trisaccharide repeating unit: -3-alpha-L-FucNAc-3-(2-NAc-4-N-Asp-2,4,6-trideoxy-beta-D-Glc)-4-alpha-D-GlcNAc-. This structure is unique among staphylococcal polysaccharides in that its composition includes a trideoxy sugar residue with aspartic acid as an N-acyl substituent.

Staphylococcus aureus strains that express serotype 5 or serotype 8 capsular polysaccharides differ in virulence

Most isolates of Staphylococcus aureus produce a serotype 5 (CP5) or 8 (CP8) capsular polysaccharide. To investigate whether CP5 and CP8 differ in their biological properties, we created isogenic mutants of S. aureus Reynolds that expressed CP5, CP8, or no capsule. Biochemical analyses of CP5 and CP8 purified from the isogenic S. aureus strains were consistent with published structures. The degree of O acetylation of each polysaccharide was similar, but CP5 showed a greater degree of N acetylation. Mice challenged with the CP5(+) strain showed a significantly higher bacteremia level than mice challenged with the CP8(+) strain. Similarly, the CP5(+) strain survived preferentially in the bloodstream and kidneys of infected mice challenged with a mixed inoculum containing both strains. The enhanced virulence of the CP5(+) strain in vivo correlated with its greater resistance to in vitro killing in whole mouse blood. Likewise, in vitro opsonophagocytic killing assays with human neutrophils and sera revealed greater survival of the Reynolds (CP5) strain, even though the kinetics of opsonization by C3b and iC3b was similar for both the CP5(+) and CP8(+) strains. Electron micrographs demonstrated C3 molecules on the cell wall beneath the capsule layer for both serotype 5 and 8 strains. Purified CP5 and CP8 stimulated a modest oxidative burst in human neutrophils but failed to activate the alternative complement pathway. These results indicate that CP5 and CP8 differ in a number of biological properties, and these differences likely contribute to the relative virulence of serotype 5 and 8 S. aureus in vivo.

Innate defences against methicillin-resistantStaphylococcus aureus (MRSA) infection

The innate immune system is the primary defence against bacterial infection. Among the factors involved in innate defence, anti-microbial peptides produced by humans have recently attracted attention due to their relevance to some diseases and also to the development of new chemotherapeutic agents. Staphylococcus aureus is one of the major human pathogens, causing a variety of infections from suppurative disease to food poisoning. Methicillin-resistant S. aureus (MRSA) is a clinical problem and with the recent emergence of a vancomycin-resistant strain, this will pose serious problems in the near future. In investigating the molecular biology of S. aureus infections to develop new chemotherapeutic agents against MRSA infections, knowledge of the interaction of innate anti-microbial peptides with S. aureus is important. In vitro and in vivo experiments demonstrate that exposure of S. aureus to host cells can induce the anti-microbial peptides beta-defensin-2 (hBD2), hBD3, and LL37/CAP18. The induction level of these peptides differs among strains, as does the susceptibility of the strains, with MRSA strains exhibiting lower susceptibility. In summary, the susceptibility of S. aureus strains, including MRSA strains, to components of the innate immune system varies, with the MRSA strains showing more resistance to both innate immune factors and chemotherapeutic agents.

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.

Availability of complement bound to Staphylococcus aureus to interact with membrane complement receptors influences efficiency of phagocytosis

Complement-mediated opsonization of encapsulated Staphylococcus aureus (CP+) of the predominant capsule types, 5 and 8, remains poorly understood. Our previous work showed that complement is important for mouse survival of CP+ type 5 bacteremia and that the type 5 capsule inhibits the binding of opsonic C3 fragments to the organism. The importance of complement-mediated opsonization of CP+ was tested by neutrophil phagocytosis assays. Complement-mediated opsonization of CP+ increased phagocytosis by 57% compared to opsonization in complement-inhibited serum. Agar-grown CP+, enhancing capsule expression, was phagocytosed only one-tenth as well as the capsule-negative organisms (CP-), supporting the belief that staphylococcal polysaccharide capsules impair phagocytosis. Despite relatively poor phagocytosis of CP+ compared to CP-, complement activation increased the phagocytosis of CP+ by 103%. Thus, complement in normal human serum may have an important role in opsonizing CP+, even when capsule expression is strong. The ability of bound C3 fragments to interact with complement receptor 1 (CD35) on the membrane of human erythrocytes was tested in an immune adherence assay. S. aureus capsule was able to mask C3 fragments on the organism from binding to complement receptor 1. The inhibition of C3 binding to CP+ and the masking of deposited C3 fragments caused by the presence of capsule was associated with markedly decreased phagocytosis. The addition of anti-capsule antibodies to normal human serum was found to markedly improve the recognition of deposited C3 fragments by complement receptor 1 even when the absolute number of C3 molecules bound to S. aureus was not increased.

Capsule production and growth phase influence binding of complement to Staphylococcus aureus

Complement-mediated opsonization of bacteria by C3 binding is an important component of the host innate immune system. Little information is available concerning the interaction between complement proteins and capsule type 5 and 8 Staphylococcus aureus strains, even though these isolates are responsible for approximately 70% of human staphylococcal infections. To investigate the importance of an intact complement pathway in an experimental staphylococcal infection, control and C3-depleted mice were challenged intravenously with 10(7) CFU of a serotype 5 S. aureus isolate. Whereas only 8% of the control mice succumbed to the infection, 64% of the complemented-depleted animals died. In vitro parameters of C3 binding to two heavily encapsulated (CP++) strains, three encapsulated (CP+) strains, and an isogenic capsule-negative (CP-) mutant were examined. The alternative pathway contributed 90% of C3 binding in 20% serum at 30 min, whereas it accounted for only 13% of C3 binding in 2% serum. Stationary-phase organisms bound only 10% as much C3 as mid-log-phase organisms; this was only in part due to capsule. When the S. aureus strains were cultivated on solid medium, the CP++ isolates bound 50% less C3 than CP+ strains; a CP+ strain bound 42% less C3 than the CP- mutant. Both C3b and iC3b fragments of C3 bound to S. aureus cells, and about one-third of the bound C3 was shed from the staphylococcal surface as iC3b, regardless of the CP phenotype of the strain. Thus, the phase of growth and presence of capsule are critical to opsonization.

Staphylococcus aureus capsular polysaccharide type 5 conjugate and whole cell vaccines stimulate antibody responses in cattle

Dairy heifers were immunized subcutaneously with one of four different vaccines which contained preparations of Staphylococcus aureus capsular polysaccharide type 5 (CP5) and a mineral oil adjuvant, or received a placebo containing saline and adjuvant. The vaccine containing a CP5-human serum albumin conjugate (CP5-HSA) and the vaccine with formaldehyde inactivated whole cells expressing CP5, both elicited strong anti-CP5 antibody responses. After two injections three weeks apart and a third injection 10 months later, the mean level and duration of the anti-CP5 antibody response was significantly higher in the whole cell group. No differences were found between the two groups with regard to the relative proportion of IgG subclasses, and the antibody responses to the polysaccharide were composed of both the IgG1 and IgG2. Vaccines containing only free CP5 or CP5 mixed with HSA produced weak and transient humoral immune responses. Only animals vaccinated with the whole cell vaccine or the conjugate vaccine showed responses to CP5 in a lymphocyte proliferation assay conducted one year after the third vaccination. This study indicates that CP5 expressed on the surface of formaldehyde inactivated whole cells, emulsified in an oil adjuvant, gives a strong and long lasting immune response in cattle. The use of conjugation technology, although effective, might not be necessary in order to achieve an immune response against S. aureus CP5 in cattle.

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.

Survival of Staphylococcus aureus inside neutrophils contributes to infection

Neutrophils have long been regarded as essential for host defense against Staphylococcus aureus infection. However, survival of the pathogen inside various cells, including phagocytes, has been proposed as a mechanism for persistence of this microorganism in certain infections. Therefore, we investigated whether survival of the pathogen inside polymorphonuclear neutrophils (PMN) contributes to the pathogenesis of S. aureus infection. Our data demonstrate that PMN isolated from the site of infection contain viable intracellular organisms and that these infected PMN are sufficient to establish infection in a naive animal. In addition, we show that limiting, but not ablating, PMN migration into the site of infection enhances host defense and that repletion of PMN, as well as promoting PMN influx by CXC chemokine administration, leads to decreased survival of the mice and an increased bacterial burden. Moreover, a global regulator mutant of S. aureus (sar-) that lacks the expression of several virulence factors is less able to survive and/or avoid clearance in the presence of PMN. These data suggest that the ability of S. aureus to exploit the inflammatory response of the host by surviving inside PMN is a virulence mechanism for this pathogen and that modulation of the inflammatory response is sufficient to significantly alter morbidity and mortality induced by S. aureus infection.

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.

Staphylococcus aureus serotype 5 capsular polysaccharide is antiphagocytic and enhances bacterial virulence in a murine bacteremia model

Controversy persists over the role that the capsular polysaccharide plays in the pathogenesis of Staphylococcus aureus infections. To address this issue, we compared the mouse virulence of S. aureus Reynolds and capsule-defective mutant strains cultivated under conditions of high or low capsule expression. Strain Reynolds cells cultivated on Columbia salt agar plates expressed approximately 100-fold more type 5 capsular polysaccharide than did cells cultivated in Columbia salt broth. The relative virulence of strain Reynolds and its capsule-defective mutants after growth on either solid or liquid medium was examined in mice challenged intraperitoneally or intravenously. The results indicated that agar-grown Reynolds cells were cleared from the bloodstream of mice less readily than broth-grown Reynolds cells. When the parental and mutant strains were cultivated on solid medium, strain Reynolds sustained a higher level of bacteremia than did the capsular mutants. We performed in vitro opsonophagocytic killing assays to determine whether staphylococcal virulence for mice correlated with resistance to phagocytosis. S. aureus Reynolds cultivated on solid medium was susceptible to phagocytic killing only in the presence of specific capsular antibodies and complement. Strain Reynolds grown in broth showed opsonic requirements for phagocytic killing that were similar to those of the capsular mutants (grown in broth or on agar); i.e., the bacteria were opsonized for phagocytosis by nonimmune serum with complement activity. These studies indicate that optimal expression of capsule enhances bacterial virulence in the mouse model of bacteremia, probably by rendering the organisms resistant to opsonophagocytic killing by leukocytes.

Identification of a gene essential for O-acetylation of the Staphylococcus aureus type 5 capsular polysaccharide

The Staphylococcus aureus serotype 5 capsular polysaccharide (CP5) has a trisaccharide repeating unit of (-->4)-3-O-Ac-beta-D-ManNAcAp-(1-->4)-alpha-L-FucNAcp-(1-->3 )-beta-D-FucNAcp-(1-->). Tn918 mutagenesis of strain Reynolds yielded a mutant that produced wild-type levels of O-deacetylated CP5. The site and orientation of the single transposon insertion in mutant JL232 were determined by analysis of Southern blots and amplification of DNA flanking the transposon. DNA sequencing revealed that Tn918 was inserted within an open reading frame of 627 bp. The predicted amino acid sequence encodes a protein of approximately 26 kDa with homology to members of the NodL-LacA-CysE family of bacterial acetyltransferases. Southern blot analysis showed that genes similar to cap5H were present only in strains of S. aureus belonging to capsular serotypes 2, 4 and 5. In an in vitro assay, the parental strain was more resistant to opsonophagocytic killing than the mutant strain. In a mouse model of staphylococcal infection, the parental strain was able to seed the bloodstream from the peritoneal cavity and colonize the kidneys more efficiently than the O-deacetylated mutant. When cap5H was provided to the mutant in trans, it fully restored CP5 O-acetylation. The virulence of the complemented mutant strain closely approximated that of the parental strain.

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.

Beyond vancomycin: new therapies to meet the challenge of glycopeptide resistance

The incidence of infections caused by resistant Gram-positive pathogens is increasing, while emergence of vancomycin resistance is reducing the number of therapeutic options. New agents are being rapidly evaluated as candidates to replace vancomycin; some of the most promising include semisynthetic glycopeptides, quinupristin-dalfopristin, oxazolidinones and everninomycins. Alternative strategies, including immunization and therapeutic vaccines, may also have a role.

Heterogeneity of cell-associated CP5 expression on Staphylococcus aureus strains demonstrated by flow cytometry

It was reported previously that two capsular polysaccharides, types 5 and 8 (CP5 and CP8), account for 70 to 80% of Staphylococcus aureus strains isolated from human and animal sources. The capsular material has been shown to play a part in virulence and in resistance to phagocytosis. With a view to investigating the role that CP plays in pathogenicity or protection, relative measurement of cell-associated CP is desirable. Flow cytometry, which permits the analysis of individual bacteria, was used to that end. Thirty isolates expressing CP5, of human (n = 7) and animal (cow, n = 11; goat, n = 3; swine, n = 3; hen, n = 3; and rabbit, n = 3) origin, were cultivated on either brain heart infusion agar (BHI) or modified medium 110 (mod 110) agar. Staphylococci were incubated with a mouse anti-CP5 monoclonal antibody (an immunoglobulin M, which does not react with staphylococcal protein A) and then stained with a fluorescein-labeled anti-murine antibody. The bacteria were washed, sonicated, and analyzed by flow cytometry. Except for three isolates, the expression of cell-bound CP5 was higher when bacteria were cultivated on mod 110 than when they were cultivated on BHI. We found a wide intraisolate phenotypic heterogeneity in surface-exposed CP5 in many strains, which appeared as mixtures of stained and unstained bacteria. Four main patterns could be distinguished on the basis of the distribution of the fluorescence of individual bacteria within the strain population as a function of growth medium. Great variations in both percentages of stained bacteria and fluorescence intensity were recorded among strains regardless of their origin. Flow cytometry analysis provided information on both the relative amounts and the distribution patterns of the surface expression of CP. This information is potentially useful for the evaluation of the part played by the capsule in the interaction of bacteria with host cells or for the study of the activities of antibodies to this target antigen, such as opsonization or prevention of adherence.

Molecular characterization and transcriptional analysis of type 8 capsule genes in Staphylococcus aureus

A 20.5-kb contiguous DNA fragment from Staphylococcus aureus Becker affecting type 8 capsule (CP8) biosynthesis was previously cloned. Sequencing analysis indicated that 16 open reading frames (ORFs) encoded within this fragment might be involved in CP8 synthesis. Using various plasmids containing DNA inserts derived from the 20.5-kb region, we showed by complementation of chemical mutants that 8 of the 16 ORFs were required for CP8 synthesis. To determine the involvement of the remaining eight ORFs, nonpolar gene-specific chromosomal mutations located in each of these ORFs were constructed. We found that three additional ORFs were also involved in the CP8 synthesis. Thus, 11 of the 16 ORFs were shown to affect CP8 synthesis. Complementation analyses of these 11 type 8 capsule (cap8) genes affecting CP8 production showed several promoters within the cap8 gene cluster. However, by Northern hybridization using either the entire cap8 gene cluster or the internal fragments of individual ORFs as probes, one 17-kb cap8-specific transcript was detected. Using xylE as the reporter gene, we found that the promoter at the beginning of the cap8 operon was much stronger than any of the internal promoters. These results suggest that the cap8 genes are transcribed mainly as a single large transcript. In addition, Southern hybridization analyses showed that cap8H, cap8I, cap8J, and cap8K, located in the central region of the cap8 gene cluster, were CP8 specific.

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.

Association of staphylococcal type-1 capsule-encoding genes with a discrete genetic element

Previous studies have shown that 13 genes located in a 14.6-kb region of the chromosome of Staphylococcus aureus (Sa) M are required for type-1 capsular polysaccharide (CP1) biosynthesis. In this report, a total of 17 Sa strains producing different CP serotypes were analyzed by Southern hybridization using DNA probes from the cap1 coding region and the flanking sequences. The results showed that the sequence encoding cap1 genes was specific to CP1-producing strains. In addition, DNA regions of at least 18 kb flanking the cap1 genes were absent in most of the non-type-1 strains. These data suggest that the cap1 genes are associated with a chromosomally located discrete genetic element. One end of the element, referred to as the cap1 element, is located in a 1.7-kb fragment about 11.1 kb upstream from the first gene of the cap1 locus and the other end is located in a 0.8-kb region about 7.6 kb downstream from the last gene of the cap1 locus. Thus, the size of the cap1 element is between 33.3 and 35.8 kb.

Effects of culture conditions on production of type 5 capsular polysaccharide by human and bovine Staphylococcus aureus strains

Two Staphylococcus aureus strains, the prototype human Reynolds strain and a bovine isolate, were grown in different complex media and in a synthetic medium (D. Taylor and K. T. Holland, J. Appl. Bacteriol. 66:319-329, 1989) and compared for their ability to produce type 5 capsular polysaccharide. Cell-bound and cell-free type 5 capsular polysaccharide were measured by a new one-step competition enzyme-linked immunosorbent assay. The total production and the proportion of cell-bound type 5 capsular polysaccharide were dependent on the nature of the medium, the duration of the culture, and the strain. Both strains produced more type 5 capsular polysaccharide when cultivated in the synthetic medium than when cultivated in complex media. The best yield of type 5 capsular polysaccharide, about 300 micrograms/ml of medium, was obtained with strain Reynolds grown for 48 h with shaking in the synthetic broth containing glucose as a carbon source.

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

The protective efficacy of antibodies to the Staphylococcus aureus capsular polysaccharide was examined in a rat model of catheter-induced endocarditis. Capsular antibodies were induced either by active immunization with killed S. aureus or by passive immunization with hyperimmune rabbit antiserum to S. aureus. Control rats were injected with phosphate-buffered saline or passively immunized with normal rabbit serum or rabbit antiserum to a nonencapsulated strain. Animals with indwelling catheters were challenged intravenously with 5 x 10(4) to 4 x 10(6) CFU of the homologous S. aureus strain (capsular serotype 5 strain Reynolds or serotype 1 strain SA1 mucoid). Both immunized and control rats developed S. aureus endocarditis. The numbers of S. aureus cells recovered from the blood and aortic valve vegetations of immunized rats were similar to those of control rats, indicating that capsule-specific antibodies were not protective. To determine whether the presence of an indwelling catheter interfered with antibody-mediated protection against S. aureus endocarditis, catheters were removed 2 h after insertion in additional groups of rats. An inoculum of 10(8) CFU of strain Reynolds was needed to provoke endocarditis in rats catheterized for 2 h, compared with 5 x 10(4) CFU for rats with indwelling catheters. Passively transferred capsular antibodies were not protective since both immunized and nonimmunized animals developed endocarditis, and quantitative cultures of blood and valvular vegetations revealed no differences between immunized and control animals. The findings of this study indicate that antibodies to the capsular polysaccharide are not protective in the rat model of experimental S. aureus endocarditis.

Sequence analysis and molecular characterization of genes required for the biosynthesis of type 1 capsular polysaccharide in Staphylococcus aureus

We previously cloned a 19.4-kb DNA region containing a cluster of genes affecting type 1 capsule production from Staphylococcus aureus M. Subcloning experiments showed that these capsule (cap) genes are localized in a 14.6-kb region. Sequencing analysis of the 14.6-kb fragment revealed 13 open reading frames (ORFs). Using complementation tests, we have mapped a collection of Cap- mutations in 10 of the 13 ORFs, indicating that these 10 genes are involved in capsule biosynthesis. The requirement for the remaining three ORFs in the synthesis of the capsule was demonstrated by constructing site-specific mutations corresponding to each of the three ORFs. Using an Escherichia coli S30 in vitro transcription-translation system, we clearly identified 7 of the 13 proteins predicted from the ORFs. Homology search between the predicted proteins and those in the data bank showed very high homology (52.3% identity) between capL and vipA, moderate homology (29% identity) between capI and vipB, and limited homology (21.8% identity) between capM and vipC. The vipA, vipB, and vipC genes have been shown to be involved in the biosynthesis of Salmonella typhi Vi antigen, a homopolymer polysaccharide consisting of N-acetylgalactosamino uronic acid, which is also one of the components of the staphylococcal type 1 capsule. The homology between these sets of genes therefore suggests that capL, capI, and capM may be involved in the biosynthesis of amino sugar, N-acetylgalactosamino uronic acid. In addition, the search showed that CapG aligned well with the consensus sequence of a family of acetyltransferases from various prokaryotic organisms, suggesting that CapG may be an acetyltransferase. Using the isogenic Cap- and Cap+ strains constructed in this study, we have confirmed that type 1 capsule is an important virulence factor in a mouse lethality test.

Genetic analysis of type 5 capsular polysaccharide expression by Staphylococcus aureus

Capsules are produced by over 90% of Staphylococcus aureus strains, and approximately 25% of clinical isolates express type 5 capsular polysaccharide (CP5). We mutagenized the type 5 strain Reynolds with Tn918 to target genes involved in CP5 expression. From a capsule-deficient mutant, we cloned into a cosmid vector an approximately 26-kb EcoRI fragment containing the transposon insertion. In the absence of tetracycline selection, Tn918 was spontaneously excised, thereby resulting in a plasmid containing 9.4 kb of S. aureus DNA flanking the Tn918 insertion site. The 9.4-kb DNA fragment was used to screen a cosmid library prepared from the wild-type strain. Positive colonies were identified by colony hybridization, and a restriction map of one clone (pJCL19 with an approximately 34-kb insert) carrying the putative capsule gene region was constructed. Fragments of pJCL19 were used to probe genomic DNA digests from S. aureus strains of different capsular serotypes. Fragments on the ends of the cloned DNA hybridized to fragments of similar sizes in most of the strains examined. Blots hybridized to two fragments flanking the central region of the cloned DNA showed restriction fragment length polymorphism. A centrally located DNA fragment hybridized only to DNA from capsular types 2, 4, and 5. DNA from pJCL19 was subcloned to a shuttle vector for complementation studies. A 6.2-kb EcoRI-ClaI fragment complemented CP5 expression in a capsule-negative mutant derived by mutagenesis with ethyl methanesulfonate. These experiments provide the necessary groundwork for identifying genes involved in CP5 expression by S. aureus.

Functional activity of antibodies to the group B polysaccharide of group B streptococci elicited by a polysaccharide-protein conjugate vaccine

Group B streptococci (GBS) are a major cause of sepsis and meningitis in infants. While antibodies directed to the type-specific GBS capsule have been shown to be protective, it is less clear whether antibodies to the group B polysaccharide, a noncapsular, cell wall-associated antigen, may play a role in immunity. To investigate the functional activity of group B polysaccharide-specific antibodies, we tested sera from rabbits vaccinated with group B polysaccharide coupled to tetanus toxoid (B-TT). Anti-B-TT was weakly opsonic in vitro for a highly encapsulated type III strain, while antiserum elicited by vaccination with type III capsular polysaccharide linked to tetanus toxoid (III-TT) was a very effective opsonin. In contrast to anti-III-TT, anti-B-TT given before or after bacterial challenge was only marginally effective in protecting newborn mice against lethal infection with type III GBS. The number of C3 molecules bound to type III GBS was augmented by anti-III-TT but not by high antibody concentrations of anti-B-TT. These results suggest that the difference in opsonic activity between anti-B-TT and anti-III-TT may be due to a difference in their ability to deposit C3. In addition, the maximum number of antibody molecules bound to the bacterial surface was greater for anti-III-TT than for anti-B-TT. That anti-B-TT binds to fewer sites than anti-III-TT may explain the differences in complement activation and in opsonic and protective efficacy of antibodies to group B polysaccharide compared with antibodies to the type-specific capsular polysaccharide.

Effects of in vitro and in vivo growth conditions on expression of type 8 capsular polysaccharide by Staphylococcus aureus

Type 8 capsular polysaccharide (CP8) is widely prevalent among clinical isolates of Staphylococcus aureus, but the role that the capsule plays in the pathogenesis of staphylococcal infections is unclear. This study was performed to identify growth conditions that would optimize the production of CP8 and to determine whether enhanced CP8 expression would influence staphylococcal virulence. S. aureus Becker grown in a chemically defined broth medium with < 1 microM ferric nitrate produced up to eightfold more CP8 per milligram of biomass than did bacteria cultivated in the same medium containing 20 microM ferric nitrate. The bacteria produced > 350-fold more cell-associated CP8 per milligram of biomass when grown on the surface of Columbia agar than when grown in Columbia broth. Most of the CP8 produced by broth-grown cells was secreted into the culture medium. S. aureus cultivated on the surface of nitrocellulose membranes floating on Columbia broth produced levels of CP8 similar to those produced by cells grown on Columbia agar. Similarly, bacteria harvested from endocardial vegetations of rabbits infected with S. aureus produced high levels of CP8. These results indicate that staphylococci grown on surfaces, both in vitro and in vivo, produce larger quantities of cell-associated CP8 than those grown in liquid cultures. However, no differences were observed in the 50% lethal dose for mice of strain Becker grown on solid medium (high levels of capsule expression) or in liquid medium (low levels of capsule expression).

Laboratory and clinical evaluation of conjugate vaccines composed of Staphylococcus aureus type 5 and type 8 capsular polysaccharides bound to Pseudomonas aeruginosa recombinant exoprotein A

The synthesis, standardization, and immunogenicity in young outbred mice and clinical evaluation in adult volunteers of investigational vaccines designed to induce serum antibodies to the type 5 and type 8 capsular polysaccharides (CPs) of Staphylococcus aureus are described. Conjugates composed of the type 5 CP and a sonicated preparation of a high-molecular-weight type 8 CP bound to a nontoxic recombinant protein derived from Pseudomonas aeruginosa exotoxin A (rEPA) were synthesized. The conjugates were nontoxic and elicited serum CP antibodies after two subcutaneous injections into young outbred mice; a third injection elicited a booster response. The lower-molecular-weight type 8 CP was not immunogenic in the mice, and the high-molecular-weight type 8 CP elicited low levels of antibodies without a booster effect. In the volunteers, neither the conjugates nor the type 8 CP alone caused significant local reactions or fever. The conjugates elicited type-specific antibodies of both the immunoglobulin M (IgM) and IgG classes after the first injection; a second injection 6 weeks later did not stimulate a booster effect. The high-molecular-weight type 8 CP alone, injected once only, elicited levels of IgG and IgM type-specific antibodies similar to those of the conjugate. The vaccine-induced CP antibodies were mostly of the IgG1 and IgG2 subclasses and had opsonophagocytic activity. The conjugates elicited IgG antibodies to the native exotoxin A with neutralizing activity. In summary, the type 5 and type 8 conjugates were safe and elicited biologically active antibodies to both the CP and rEPA components.