Detection of complement-mediated antibody-dependent bactericidal activity in a fluorescence-based serum bactericidal assay for group B Neisseria meningitidis

Evaluating vaccine-elicited antibody activities against Neisseria gonorrhoeae: cross-protective responses elicited by the 4CMenB meningococcal vaccine

The bacterial pathogen Neisseria gonorrhoeae is an urgent global health problem due to increasing numbers of infections, coupled with rampant antibiotic resistance. Vaccines against gonorrhea are being prioritized to combat drug-resistant N. gonorrhoeae. Meningococcal serogroup B vaccines such as four-component meningococcal B vaccine (4CMenB) are predicted by epidemiology studies to cross-protect individuals from natural infection with N. gonorrhoeae and elicit antibodies that cross-react with N. gonorrhoeae. Evaluation of vaccine candidates for gonorrhea requires a suite of assays for predicting efficacy in vitro and in animal models of infection, including the role of antibodies elicited by immunization. Here, we present the development and optimization of assays to evaluate antibody functionality after immunization of mice: antibody binding to intact N. gonorrhoeae, serum bactericidal activity, and opsonophagocytic killing activity using primary human neutrophils [polymorphonuclear leukocytes (PMNs)]. These assays were developed with purified antibodies against N. gonorrhoeae and used to evaluate serum from mice that were vaccinated with 4CMenB or given alum as a negative control. Results from these assays will help prioritize gonorrhea vaccine candidates for advanced preclinical to early clinical studies and will contribute to identifying correlates and mechanisms of immune protection against N. gonorrhoeae.

Proof of concept for a single-dose Group B Streptococcus vaccine based on capsular polysaccharide conjugated to Qβ virus-like particles

A maternal vaccine to protect neonates against Group B Streptococcus invasive infection is an unmet medical need. Such a vaccine should ideally be offered during the third trimester of pregnancy and induce strong immune responses after a single dose to maximize the time for placental transfer of protective antibodies. A key target antigen is the capsular polysaccharide, an anti-phagocytic virulence factor that elicits protective antibodies when conjugated to carrier proteins. The most prevalent polysaccharide serotypes conjugated to tetanus or diphtheria toxoids have been tested in humans as monovalent and multivalent formulations, showing excellent safety profiles and immunogenicity. However, responses were suboptimal in unprimed individuals after a single shot, the ideal schedule for vaccination during the third trimester of pregnancy. In the present study, we obtained and optimized self-assembling virus-like particles conjugated to Group B Streptococcus capsular polysaccharides. The resulting glyco-nanoparticles elicited strong immune responses in mice already after one immunization, providing pre-clinical proof of concept for a single-dose vaccine.

High-throughput bactericidal assays for monoclonal antibody screening against antimicrobial resistant Neisseria gonorrhoeae

Neisseria gonorrhoeae (gonococcus) is an obligate human pathogen and the etiological agent of the sexually transmitted disease gonorrhea. The rapid rise in gonococcal resistance to all currently available antimicrobials has become a significant public health burden and the need to develop novel therapeutic and prophylactic tools is now a global priority. While high-throughput screening methods allowed rapid discovery of extremely potent monoclonal antibodies (mAbs) against viral pathogens, the field of bacteriology suffers from the lack of assays that allow efficient screening of large panels of samples. To address this point, we developed luminescence-based (L-ABA) and resazurin-based (R-ABA) antibody bactericidal assays that measure N. gonorrhoeae metabolic activity as a proxy of bacterial viability. Both L-ABA and R-ABA are applicable on the large scale for the rapid identification of bactericidal antibodies and were validated by conventional methods. Implementation of these approaches will be instrumental to the development of new medications and vaccines against N. gonorrhoeae and other bacterial pathogens to support the fight against antimicrobial resistance.

Evaluating vaccine-elicited antibody activities against Neisseria gonorrhoeae: cross-protective responses elicited by the 4CMenB meningococcal vaccine

The bacterial pathogen Neisseria gonorrhoeae is an urgent global health problem due to increasing numbers of infections, coupled with rampant antibiotic resistance. Vaccines against gonorrhea are being prioritized to combat drug-resistant N. gonorrhoeae. Meningococcal serogroup B vaccines such as 4CMenB are predicted by epidemiology studies to cross-protect individuals from natural infection with N. gonorrhoeae and elicit antibodies that cross-react with N. gonorrhoeae. Evaluation of vaccine candidates for gonorrhea requires a suite of assays for predicting efficacy in vitro and in animal models of infection, including the role of antibodies elicited by immunization. Here we present assays to evaluate antibody functionality after immunization: antibody binding to intact N. gonorrhoeae, serum bactericidal activity, and opsonophagocytic killing activity using primary human neutrophils (polymorphonuclear leukocytes). These assays were developed with purified antibodies against N. gonorrhoeae and used to evaluate serum from mice that were vaccinated with 4CMenB or given alum as a negative control. Results from these assays will help prioritize gonorrhea vaccine candidates for advanced preclinical to early clinical study and will contribute to identifying correlates and mechanisms of immune protection against N. gonorrhoeae .

Development of a Gold Nanoparticle Vaccine against Enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic E. coli O157:H7 is a human pathogen and the causative agent of diarrhea and hemorrhagic colitis, which can progress to hemolytic uremic syndrome. These complications represent a serious global public health problem that requires laborious public health interventions and safety control measures to combat recurrent outbreaks worldwide. Today, there are no effective interventions for the control of EHEC infections, and, in fact, the use of antibiotics is counterindicated for EHEC disease. Therefore, a viable alternative for the prevention of human infections is the development of vaccines; however, no such vaccines are approved for human use. In this study, we developed a novel gold nanoparticle platform which acts as a scaffold for the delivery of various antigens, representing a nanovaccine technology which can be applied to several disease models.

Here we exploit the natural properties of a synthetic nanoparticle (NP) scaffold as a subunit vaccine against enterohemorrhagic Escherichiacoli (EHEC). Two EHEC-specific immunogenic antigens, namely, LomW and EscC, either alone or in combination, were covalently linked on the surface of gold nanoparticles (AuNPs) and used to immunize mice prior to challenge with EHEC O157:H7 strain 86-24. LomW is a putative outer membrane protein encoded in bacteriophage BP-933W, while EscC is a structural type III secretion system protein which forms a ring in the outer membrane. The resulting AuNP preparations, AuNP-LomW and AuNP-EscC, showed that the nanoparticles were able to incorporate the antigens, forming stable formulations that retained robust immunogenicity in vivo after subcutaneous immunization. When administered subcutaneously, AuNP-LomW or AuNP-EscC or a combination containing equivalent amounts of both candidates resulted in higher IgG titers in serum and secretory IgA titers in feces. The serum IgG titers correlated with a significant reduction in EHEC intestinal colonization after 3 days postinoculation. In addition, we showed that serum from antigen-coated AuNP-immunized mice resulted in a reduction of adherence to human intestinal epithelial cells for EHEC, as well as for two other E. coli pathotypes (enteropathogenic E. coli [EPEC], encoding EscC, and enteroaggregative E. coli [EAEC], encoding LomW). Further, the serum had antigen-specific bactericidal properties, engaging the classical complement pathway. Overall, our results demonstrate the immunogenicity and stability of a novel nanovaccine against EHEC. These results also strengthen the prospect of development of a synthetic nanoparticle vaccine conjugated to E. coli antigens as a promising platform against other enteric pathogens.

Immune Cross-Opsonization Within emm Clusters Following Group A Streptococcus Skin Infection: Broadening the Scope of Type-Specific Immunity

Background

Group AStreptococcus (GAS) skin infections are particularly prevalent in developing nations. The GAS M protein, by which strains are differentiated into >220 differentemm types, is immunogenic and elicits protective antibodies. A major obstacle for vaccine development has been the traditional understanding that immunity following infection is restricted to a singleemm type. However, recent evidence has led to the hypothesis of immune cross-reactivity betweenemm types.

Methods

We investigated the human serological response to GAS impetigo in Fijian schoolchildren, focusing on 3 majoremm clusters (E4, E6, and D4). Pre- and postinfection sera were assayed by enzyme-linked immunosorbent assay with N-terminal M peptides and bactericidal assays using the infecting-type strain,emm cluster–related strains, and nonrelated strains.

Results

Twenty of the 53 paired sera demonstrated a ≥4-fold increase in antibody titer against the infecting type. When tested against all cluster-related M peptides, we found that 9 of 17 (53%) paired sera had a ≥4-fold increase in antibody titer to cluster-related strains as well. When grouped by cluster, the mean change to cluster-relatedemm types in E4 and E6 was >4-fold (5.9-fold and 19.5-fold, respectively) but for D4 was 3.8-fold. The 17 paired sera were tested in bactericidal assays against selected cluster-related and nonrelated strains. While the responses were highly variable, numerous instances of cross-reactive killing were observed.

Conclusions

These data demonstrate that M type–specific and cross-reactive immune responses occur following skin infection. The cross-reactive immune responses frequently align withemm clusters, raising new opportunities to design multivalent vaccines with broad coverage.

Development of a high-throughput method to evaluate serum bactericidal activity using bacterial ATP measurement as survival readout

Serum Bactericidal Activity (SBA) assay is the method of choice to evaluate the complement-mediated functional activity of both infection- and vaccine-induced antibodies. To perform a typical SBA assay, serial dilutions of sera are incubated with target bacterial strains and complement. The conventional SBA assay is based on plating on agar the SBA reaction mix and counting the surviving bacterial colony forming units (CFU) at each serum dilution. Even with automated colony counting, it is labor-intensive, time-consuming and not amenable for large-scale studies. Here, we have developed a luminescence-based SBA (L-SBA) method able to detect surviving bacteria by measuring their ATP. At the end of the SBA reaction, a single commercially available reagent is added to each well of the SBA plate, and the resulting luminescence signal is measured in a microplate reader. The signal obtained is proportional to the ATP present, which is directly proportional to the number of viable bacteria. Bactericidal activity is subsequently calculated. We demonstrated the applicability of L-SBA with multiple bacterial serovars, from 5 species: Citrobacter freundii, Salmonella enterica serovars Typhimurium and Enteritidis, Shigella flexneri serovars 2a and 3a, Shigella sonnei and Neisseria meningitidis. Serum bactericidal titers obtained by the luminescence readout method strongly correlate with the data obtained by the conventional agar plate-based assay, and the new assay is highly reproducible. L-SBA considerably shortens assay time, facilitates data acquisition and analysis and reduces the operator dependency, avoiding the plating and counting of CFUs. Our results demonstrate that L-SBA is a useful high-throughput bactericidal assay.

The Contribution of High-Order Metabolic Interactions to the Global Activity of a Four-Species Microbial Community

The activity of a biological community is the outcome of complex processes involving interactions between community members. It is often unclear how to accurately incorporate these interactions into predictive models. Previous work has shown a range of positive and negative metabolic pairwise interactions between species. Here we examine the ability of a modified general Lotka-Volterra model with cell-cell interaction coefficients to predict the overall metabolic rate of a well-mixed microbial community comprised of four heterotrophic natural isolates, experimentally quantifying the strengths of two, three, and four-species interactions. Within this community, interactions between any pair of microbial species were positive, while higher-order interactions, between 3 or more microbial species, slightly modulated community metabolism. For this simple community, the metabolic rate of can be well predicted only with taking into account pairwise interactions. Simulations using the experimentally determined interaction parameters revealed that spatial heterogeneity in the distribution of cells increased the importance of multispecies interactions in dictating function at both the local and global scales.

Many wild microbial ecosystems contain hundreds to thousands of species, suggesting that interactions between species likely play an important role in regulating the behavior of such complex cellular networks. Predicting how these interactions impact the overall activity of microbial communities remains a challenge. Here we quantify the contribution of interactions between more than two species to the overall metabolic rate of a mixture of four freshwater bacteria. We systematically measure interactions between these species and use theoretical models to examine the influence cell-cell interactions on spatially non-uniform microbial populations. Our results demonstrate that although interactions between species are key regulators of system behavior, only considering interactions between pairs of species is sufficient to predict ecosystem activity. Simulations demonstrate that activity at both the single-cell and population level would be strongly influenced by how microbes are distributed in space. These findings improve our understanding of how best to examine groups of microbes that coexist in environments such as soil, water, and the human body.

Development of an automated, high-throughput bactericidal assay that measures cellular respiration as a survival readout for Neisseria meningitidis

Complement-mediated bactericidal activity has long been regarded as the serological correlate of protective immunity against Neisseria meningitidis. This was affirmed in 2005 at a WHO-sponsored meningococcal serology standardization workshop. The assay currently employed by most laboratories involves determining surviving bacterial colony counts on agar as a readout which is labor-intensive, time-consuming, and not amendable to rapid data analysis for clinical trials. Consequently, there is an acute need to develop a sensitive, high-throughput bactericidal assay to enable a rapid and robust assessment of the effectiveness of vaccine candidates. To this end, we have developed an automated, kinetic assay based on the fluorescent respiration product of resazurin which reduces assay volume, shortens assay time, and facilitates automation of data analysis. We demonstrate proof of concept for applicability of this high-throughput system with multiple meningococcal strains and utilizing different lots of human complement. The assay is robust and highly reproducible. Titers obtained by the fluorescence readout method are strongly correlated with the data obtained using the conventional, agar plate-based assay. These results demonstrate that the detection of bacteria that have survived the bactericidal reaction by measuring metabolic activity using a fluorescent dye as an alternative readout is a promising approach for the development of a high-throughput bactericidal assay.

Detecting bacteria and determining their susceptibility to antibiotics by stochastic confinement in nanoliter droplets using plug-based microfluidics

This article describes plug-based microfluidic technology that enables rapid detection and drug susceptibility screening of bacteria in samples, including complex biological matrices, without pre-incubation. Unlike conventional bacterial culture and detection methods, which rely on incubation of a sample to increase the concentration of bacteria to detectable levels, this method confines individual bacteria into droplets nanoliters in volume. When single cells are confined into plugs of small volume such that the loading is less than one bacterium per plug, the detection time is proportional to plug volume. Confinement increases cell density and allows released molecules to accumulate around the cell, eliminating the pre-incubation step and reducing the time required to detect the bacteria. We refer to this approach as 'stochastic confinement'. Using the microfluidic hybrid method, this technology was used to determine the antibiogram - or chart of antibiotic sensitivity - of methicillin-resistant Staphylococcus aureus (MRSA) to many antibiotics in a single experiment and to measure the minimal inhibitory concentration (MIC) of the drug cefoxitin (CFX) against this strain. In addition, this technology was used to distinguish between sensitive and resistant strains of S. aureus in samples of human blood plasma. High-throughput microfluidic techniques combined with single-cell measurements also enable multiple tests to be performed simultaneously on a single sample containing bacteria. This technology may provide a method of rapid and effective patient-specific treatment of bacterial infections and could be extended to a variety of applications that require multiple functional tests of bacterial samples on reduced timescales.

L-lactic acid secreted from gastric mucosal cells enhances growth of Helicobacter pylori

BACKGROUND

Helicobacter pylori mainly inhabit the mucus layer in the gastric mucosa. However, mechanisms involving H. pylori colonization and proliferation in gastric mucosa are not well established. This study focuses on elucidating the role of gastric mucosal cells on growth of H. pylori.

MATERIALS AND METHODS

H. pylori was co-cultured with the murine gastric surface mucosal cells (GSM06), and the growth of H. pylori on the cells was assessed by enumerating the colony-forming units (CFU). The H. pylori growth factor in the culture media conditioned by GSM06 cell was purified by HPLC, and the chemical structure of the growth factor was identified by analyses of (1)H- and (13)C-NMR spectra.

RESULTS

A marked increase in the number of CFU of H. pylori was observed in the GSM06 cells. The enhanced H. pylori growth was also observed when indirectly incubated with GSM06 cells through semi-permeable membrane. In addition, culture media conditioned by GSM06 cell stimulated H. pylori growth approximately one thousand-fold. By bioassay-guided purification, the H. pylori growth factor was isolated from the conditioned medium of GSM06 cells and identified as L-lactic acid. The H. pylori growth-enhancing activity under microaerobic condition was well correlated with L-lactic acid concentrations in the conditioned media.

CONCLUSIONS

This study demonstrates that L-lactic acid secreted by gastric mucosal cells enhances the growth of H. pylori, and this L-lactic acid-dependent growth of H. pylori may be important to the long-term colonization of H. pylori in the stomach.

Intranasal immunization of mice with recombinant lipidated P2086 protein reduces nasal colonization of group B Neisseria meningitidis

Neisseria meningitidis is a major cause of bacterial meningitis in the human population, especially among young children. There is a need to develop a non-capsular vaccine to prevent meningococcal B infections due to the inadequate immune response elicited against the capsular polysaccharide of these strains. Previously, we developed a Swiss Webster adult mouse intranasal challenge model for group B N. meningitidis and evaluated several potential vaccine candidates including a meningococcal outer membrane protein, P2086, through parenteral immunization. Since N. meningitidis is a respiratory pathogen, a mucosal immune response may play an important role in the defense against meningococcal infections. Thus, intranasal immunization may be more effective than traditional parenteral immunization. In this study, mice were immunized intranasally with purified recombinant lipidated P2086 protein (rLP2086) adjuvanted with either CT-E29H, a genetically modified cholera toxin that is significantly reduced in enzymatic activity and toxicity or RC529-AF, a synthetic immunostimulant molecule in aqueous formulation. rLP2086-specific serum and mucosal IgG and IgA antibodies were induced. IgG antibodies reacted with whole cells of multiple strains of group B N.meningitidis. The antibodies have functional activity against N. meningitidis as demonstrated by bactericidal assays. Moreover, immunized mice exhibited reduced nasal colonization of group B meningococcal strains in the intranasal challenge model. These results demonstrate that an intranasal immunization with rLP2086 protein formulated with a detoxified cholera toxin or RC529-AF could prevent the initial colonization of group B meningococcus and become an effective immunization strategy against group B N. meningitidis.

Fluorescent multivalent opsonophagocytic assay for measurement of functional antibodies to Streptococcus pneumoniae

We developed fluorescent mono- and multivalent opsonophagocytic assays (fOPA and fmOPA, respectively) specific for seven Streptococcus pneumoniae serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F). Bacterial survival was quantitated with alamar blue, a fluorescent metabolic indicator. Both fOPA and fmOPA allow for determination of viability endpoints for up to seven serotypes with high levels of agreement to the reference method. The fmOPA eliminates colony counting, reduces serum volume, and produces results in 1 day.

Evaluation of recombinant lipidated P2086 protein as a vaccine candidate for group B Neisseria meningitidis in a murine nasal challenge model

Neisseria meningitidis is a major causative agent of bacterial meningitis in human beings, especially among young children (</=2 years of age). Prevention of group B meningococcal disease represents a particularly difficult challenge in vaccine development, due to the inadequate immune response elicited against type B capsular polysaccharide. We have established an adult mouse intranasal challenge model for group B N. meningitidis to evaluate potential vaccine candidates through active immunization. Swiss Webster mice were inoculated intranasally with meningococci, and bacteria were recovered from the noses for at least 3 days postchallenge. Iron dextran was required in the bacterial inoculum to ensure sufficient meningococcal recovery from nasal tissue postchallenge. This model has been utilized to evaluate the potential of a recombinant lipidated group B meningococcal outer membrane protein P2086 (rLP2086) as a vaccine candidate. In this study, mice were immunized subcutaneously with purified rLP2086 formulated with or without an attenuated cholera toxin as an adjuvant. The mice were then challenged intranasally with N. meningitidis strain H355 or M982, and the colonization of nasal tissue was determined by quantitative culture 24 h postchallenge. We demonstrated that immunization with rLP2086 significantly reduced nasal colonization of mice challenged with the two different strains of group B N. meningitidis. Mice immunized with rLP2086 produced a strong systemic immunoglobulin G response, and the serum antibodies were cross-reactive with heterologous strains of group B N. meningitidis. The antibodies have functional activity against heterologous N. meningitidis strain, as demonstrated via bactericidal and infant rat protection assays. These results suggest that rLP2086 is a potential vaccine candidate for group B N. meningitidis.

High-throughput imaging of bacterial colonies grown on filter plates with application to serum bactericidal assays

The ability to accurately enumerate viable bacteria has applications in antibiotic screening assays, toxicology testing, and serological assays for functional antibodies. An impediment to high-throughput bacterial assays is the requirement to grow bacteria as individual colonies on semisolid media containing agar. We have now developed a method for growth, staining, and counting of bacterial colonies in 96-well filter plates. A unique feature of the method is that colony size is inversely proportional to the number of colonies in each well, presumably due to nutrient depletion. As a result, as many as 300 colony-forming units (cfu) can be detected as discrete colonies within a single assay well. The resulting colonies can be counted automatically using an imaging system originally developed for ELISPOT assays. The method has been applied to the measurement of serum bactericidal activity (SBA) in human sera.

Effects of low dose chlorhexidine mouthrinses on oral bacteria and salivary microflora including those producing hydrogen sulfide

BACKGROUND/AIMS

Clinical studies have demonstrated the considerable effects of chlorhexidine on dental plaque and oral microbiota as well as improvements in indices of oral health. This investigation examined the efficacy of lower concentrations of chlorhexidine.

METHODS

Mouthrinses with 0.03%, 0.06%, 0.12% chlorhexidine and a control rinse without chlorhexidine were examined. Alamar blue, an oxidation-reduction dye with fluorescent end-points proportional to bacterial viability, was used to determine bacterial viability. Further clinical studies examined the effects of these rinses on salivary bacteria and on bacteria producing hydrogen sulfide (H(2)S) and implicated in halitosis.

RESULTS

In laboratory tests, a significant dose-dependent effect was observed with Actinomyces viscosus as a model system using the Alamar blue procedure (P < 0.05). Clinical studies examined the effects 1.5 h and 3 h post-treatment on salivary bacteria and bacteria producing H(2)S. The first study compared the control rinse with the 0.03% and 0.06% chlorhexidine rinses; a second study compared the effects of the control rinse and the 0.06% and 0.12% chlorhexidine mouthrinses. In both studies, chlorhexidine rinses demonstrated significant dose-dependent effects post-treatment on salivary bacteria vs. the control rinse (P < 0.05). Significant decreases in H(2)S-producing bacteria were noted with these chlorhexidine rinses vs. the control rinse (P < 0.05).

CONCLUSION

The results highlight the dose-dependent relationships noted in laboratory and clinical tests which have potential implications for the use of lower doses of chlorhexidine to inhibit oral bacteria, including those implicated in halitosis.

Measurement of serum bactericidal activity specific for Haemophilus influenzae type b by using a chromogenic and fluorescent metabolic indicator

We evaluated alamarBlue as a metabolic indicator in a standardized assay for the measurement of serum bactericidal activity (SBA) to Haemophilus influenzae type b (Hib) using sera containing natural and vaccine-induced anticapsular (polyribosylribitol phosphate) antibodies. SBA assays with a colorimetric and a fluorometric end point in the presence of alamarBlue were developed and compared to a standard SBA assay, where colony counts are performed to determine the titer (12). A colorimetric end point required a spectrophotometer, whereas a fluorometric end point required a fluorometer. Prevaccination sera (n = 27) and postvaccination sera (n = 13) were tested by all three methodologies, and the SBA titers obtained in the presence of alamarBlue were compared to those from the standard method. Both the colorimetric and the fluorometric SBA titers were significantly correlated (r = 0.87 and r = 0.95, respectively) with those of the standard assay (>/= 50% killing as the SBA titer end point), and titers were not significantly different when compared to those of the standard assay (P > 0.68). However, the fluorometric end point had superior performance and ease of titer determination compared to the colorimetric end point (95 versus 87% of SBA titers were within 2 dilutions of the standard titer). Hib SBA assays with alamarBlue are reproducible, faster (same-day assay), and easier to perform than the standardized assay, which requires manual or automated colony counts. These semiautomated methodologies result in increased sample throughput and collection of data in digital formats that can be exported to data analysis programs for determination of SBA titers.

Vaccine potential of the Neisseria meningitidis 2086 lipoprotein

A novel antigen that induces cross-reactive bactericidal antibodies against a number of Neisseria meningitidis strains is described. This antigen, a approximately 28-kDa lipoprotein called LP2086, was first observed within a complex mixture of soluble outer membrane proteins (sOMPs) following a series of fractionation, protein purification, and proteomics steps. Approximately 95 different neisserial isolates tested positive by Western blotting and PCR screening methods for the presence of the protein and the gene encoding LP2086. The strains tested included isolates of N. meningitidis serogroups A, B, C, W135, and Y, Neisseria gonorrhoeae, and Neisseria lactamica. To better understand the microheterogeneity of this protein, the 2086 genes from 63 neisserial isolates were sequenced. Two different subfamilies of LP2086 were identified based on deduced amino acid sequence homology. A high degree of amino acid sequence similarity exists within each 2086 subfamily. The highest degree of genetic diversity was seen between the two subfamilies which share approximately 60 to 75% homology at the nucleic acid level. Flow cytometry (fluorescence-activated cell sorting) analyses and electron microscopy indicated that the LP2086 is localized on the outer surface of N. meningitidis. Antiserum produced against a single protein variant was capable of eliciting bactericidal activity against strains expressing different serosubtype antigens. Combining one recombinant lipidated 2086 (rLP2086) variant from each subfamily with two rPorA variants elicited bactericidal activity against all strains tested. The rLP2086 family of antigens are candidates worthy of further vaccine development.

A rapid procedure to ascertain the antimicrobial efficacy of oral care formulations

A rapid method examining the antimicrobial efficacy of oral care formulations with alamar blue, an oxidation-reduction dye with fluorescent end-points, is described. Significant correlations between increasing viable plate counts of the oral bacteria Actinomyces viscosus, Streptococcus sanguis, Streptococcus mutans and Actinobacillus actinomycetemcomitans and increased alamar fluorescence were noted. Metabolically active bacteria reduced alamar with the reduced dye found in the cell-free filtrate. Insignificant alamar reductions were noted in the absence of bacteria or by spent culture supernatants. The efficacy of mouthrinses with clinically proven antiplaque agents such as chlorhexidine or cetylpyridinium chloride were determined by alamar blue. In a model system with A. viscosus, triclosan dentifrices demonstrate a dose-dependent effect on bacteria. Human salivary bacteria demonstrate increasing alamar fluorescence with increasing plate counts. A clinical study examined the effects of rinsing with chlorhexidine or cetylpyridinium chloride mouthrinses in comparison with a placebo mouthrinse and water on salivary bacteria. Rinsing with chlorhexidine resulted in the least number of bacteria by alamar and plate count methods. In summary, the current study demonstrates the utility of alamar blue to examine the antimicrobial effects of oral care formulations in laboratory and clinical studies.

Validation of colorimetric assay to detect complement-mediated antibody-dependent bactericidal activity against serogroups B and C Neisseria meningitidis

Colorimetric serum bactericidal assay (cSBA), based on the addition of glucose and a pH indicator to the culture medium after the bactericidal reaction, was validated. The precision measured as repeatability, intermediated precision, and reproducibility was determined as a percentage in titer coincidence between replicas >/=50. Moreover the use of the freeze-dried complement was evaluated in comparison to the traditionally stored by freezing. The results were the following: precision: titer +/-1 two-fold dilution (except for the highly positive serum against serogroup B, where there was titer +/-2 dilutions) percentage in titer coincidence: >/=50. The known titer or +/-1 two-fold dilution was found in the sera titrated with the complement either frozen or freeze-dried. Concluding, cSBA showed to be highly precise, allowing also the use of freeze-dried complement which is another important advantage for this kind of assay.

A glycoconjugate vaccine for Neisseria meningitidis induces antibodies in human infants that afford protection against meningococcal bacteremia in a neonate rat challenge model

The functional activities of serum samples from human infants immunized with a glycoconjugate vaccine for Neisseria meningitidis serogroup C were assessed in a complement-mediated antibody-dependent serum bactericidal assay (SBA) and in a neonate rat model of protection from bacteremia. Selective serum samples from individual human infants were combined to make a panel of 11 serum pools to obtain a sufficient volume for testing. Each pool was assayed (i) for the anti-N. meningitidis serogroup C capsular polysaccharide (PS) immunoglobulin G (IgG) concentration as determined by reactivity in a direct-binding enzyme-linked immunosorbent assay, (ii) for bactericidal activity against N. meningitidis serogroup C strain C11, and (iii) for the ability to reduce bacteremia after passive transfer into a neonate rat model. Representative serum samples from infants who were not previously immunized with any N. meningitidis serogroup C vaccine served as a negative control. The prepared serum pools ranged in antibody concentration from 0.18 to 17.31 micro g of IgG specific for N. meningitidis serogroup C PS per ml. For this serum panel, a direct relationship between concentrations of anti-N. meningitidis serogroup C PS-specific IgG and serum SBA titers (r = 0.9960) was observed. Passive transfer to neonate rats demonstrated the ability of postimmunization serum samples to significantly reduce (> or =2-log(10) reduction compared to control animals) the level of bacteremia following a challenge. Of 79 neonate rats that received > or =0.031 micro g of human infant anti-N. meningitidis serogroup C PS IgG, 75 (94.9%) had a > or =2-log(10) reduction in bacteremia, whereas of the animals that received <0.031 micro g of antigen-specific IgG, 10.3% (4 of 39 rats) showed a > or =2-log(10) reduction in bacteremia. It was concluded that the anti-N. meningitidis serogroup C PS IgG antibody induced by this glycoconjugate vaccine had in vitro functional activity (as determined by a SBA) and also afforded protection against meningococcal bacteremia in an animal model.

Neisseria meningitidis serogroup B: laboratory correlates of protection

Meningococcal disease in the Western countries is frequently caused by Neisseria meningitidis serogroup B. Major efforts have been made to develop a safe and efficacious vaccine against this serogroup which is suitable for use in infants and young children. To assess the quality of the immune response after vaccination with candidate vaccines, laboratory correlates of protection are needed. For serogroups A and C, serum bactericidal activity (SBA) is a well established predictor for protection, but for serogroup B other mechanisms besides SBA may also be involved in conferring protection from disease. Several laboratory methods for identification and evaluation of the immunogenicity of possible vaccine antigens are described in this review.

Opsonophagocytosis of fluorescent polystyrene beads coupled to Neisseria meningitidis serogroup A, C, Y, or W135 polysaccharide correlates with serum bactericidal activity

We developed a polysaccharide-specific flow cytometric opsonophagocytic assay (OPA) for the simultaneous measurement of functional antibody to Neisseria meningitidis serogroups A, C, Y, and W135. OPA titers significantly correlated with serum bactericidal assay titers for all serogroups tested (mean r = 0.96; P < 0.001). OPA could be used in meningococcal vaccine evaluation.

Standardization of Neisseria meningitidis serogroup B colorimetric serum bactericidal assay

The correlate of protection for serogroup B meningococci is not currently known, but for serogroup C it is believed to be the serum bactericidal assay (SBA). The current SBAs are labor intensive and the variations in protocols among different laboratories make interpretation of results difficult. A colorimetric SBA (cSBA), based on the ability of Neisseria meningitidis serogroup B to consume glucose, leading to acid production, was standardized by using group B strain Cu385-83 as the target. The cSBA results were compared to those obtained for a traditional colony-counting microassay (mSBA). Glucose and bromocresol purple pH indicator were added to the medium in order to estimate growth of cSBA target cell survivors through color change. Different variants of the assay parameters were optimized: growth of target cells (Mueller Hinton agar plates), target cell number (100 CFU/per well), and human complement source used at a final concentration of 25%. After the optimization, three other group B strains (H44/76, 490/91, and 511/91) were used as targets for the cSBA. The selection of the assay parameters and the standardization of cSBA were done with 13 sera from vaccinated volunteers. The titers were determined as the higher serum dilution that totally inhibited the bacterial growth marked by the color invariability of the pH indicator. This was detected visually as well as spectrophotometrically and was closely related to a significant difference in the growth of target cell survivors determined using Student's t test. Intralaboratory reproducibility was +/-1 dilution. The correlation between bactericidal median titers and specific immunoglobulin G serum concentration by enzyme immunoassay was high (r = 0.910, P < 0.01). The bactericidal titers generated by the cSBA and the mSBA were nearly identical, and there was a high correlation between the two assays (r = 0.974, P < 0.01). The standardized cSBA allows easy, fast, and efficient evaluation of samples.

Passive immunization with Neisseria meningitidis PorA specific immune sera reduces nasopharyngeal colonization of group B meningococcus in an infant rat nasal challenge model

To examine the protective efficacy of specific immune sera generated by meningococcal vaccine candidates against nasopharyngeal colonization, we developed an infant rat nasal colonization model for group B meningococcus. In this model, Sprague-Dawley infant rats were challenged intranasally in with host adapted, piliated Neisseria meningitidis group B strains H355 or H44/76 administered concurrently with iron dextran. Colonization was assessed by quantitative culture of nasal homogenates and expressed as log(10) colony forming units (c.f.u.) per nose. Three to five log(10) c.f.u. of N. meningitidis were routinely recovered from the nasal tissue up to 4 days post-challenge. Passive immunization (i.p.) of the infant rats with either PorA or whole cell antisera 24 h prior to homologous challenge resulted in a significant reduction of N. meningitidis colonization in the nasal tissues of these animals. These results demonstrate that this model can be utilized to evaluate the role of antibody to prevent the initial nasopharyngeal colonization by group B meningococcus.