Comparison of chemical assay, bioassay, enzyme-linked immunosorbent assay, and dot blot hybridization for detection of aerobactin in members of the family Enterobacteriaceae

Siderophore vaccine conjugates protect against uropathogenic Escherichia coli urinary tract infection

Uropathogenic Escherichia coli (UPEC) is the primary cause of uncomplicated urinary tract infections (UTIs). Whereas most infections are isolated cases, 1 in 40 women experience recurrent UTIs. The rise in antibiotic resistance has complicated the management of chronic UTIs and necessitates new preventative strategies. Currently, no UTI vaccines are approved for use in the United States, and the development of a highly effective vaccine remains elusive. Here, we have pursued a strategy for eliciting protective immunity by vaccinating with small molecules required for pathogenesis, rather than proteins or peptides. Small iron-chelating molecules called siderophores were selected as antigens to vaccinate against UTI for this vaccine strategy. These pathogen-associated stealth siderophores evade host immune defenses and enhance bacterial virulence. Previous animal studies revealed that vaccination with siderophore receptor proteins protects against UTI. The poor solubility of these integral outer-membrane proteins in aqueous solutions limits their practical utility. Because their cognate siderophores are water soluble, we hypothesized that these bacterial-derived small molecules are prime vaccine candidates. To test this hypothesis, we immunized mice with siderophores conjugated to an immunogenic carrier protein. The siderophore-protein conjugates elicited an adaptive immune response that targeted bacterial stealth siderophores and protected against UTI. Our study has identified additional antigens suitable for a multicomponent UTI vaccine and highlights the potential use of bacterial-derived small molecules as antigens in vaccine therapies.

Rapid detection of specific polyclonal and monoclonal antibodies against bovine leukemia virus

ELISA and Western blot have been used for detecting specific antibodies or antigens for routine diagnostic laboratory tests and experimental protocols, as well as for screening hybridomas secreting antibodies. Although these techniques are sensitive, some slow growing hybridomas are identified as positive only when they are grown slowly long time. We standardized the dot-ELISA, a more sensitive technique, for the detection of antibodies against BLV. The main advantages of the dot-ELISA described in this study are (a) its sensitivity, detecting hybridomas which would otherwise be considered negative and discarded from the results of indirect ELISA and/or Western blot; and (b) the possibility of economizing reagents using as little as 1 microl of the antigen and 0.5 microl of antibody and conjugate. Different BLV-antigen preparations were bound to nitrocellulose membranes (NC), including cells lysed chemically (LYS) or by sonication (SOC), semi-purified virus (PV), and supernatant from infected cultures, either without treatment (SUP) or sonicated (SOS). The antigen preparations most adequate for detecting monoclonal antibodies against BLV and polyclonal antibodies in cattle sera were undiluted cell lysates (LYS) and semi-purified BLV (PV). When testing bovine sera, the supernatant (SUP) and sonicated supernatant (SOS) antigens gave a high background due to the presence of FCS which reacted with the anti-bovine labeled antibodies. In this study, 59 BLV specific antibody secreting hybridomas were identified using the dot-ELISA, compared to only 20 detected using iELISA, and doubtful reactions due to nonspecific binding to fetal calf serum (FCS) and cellular components were measured. The results of the improved dot-ELISA described may be stored at room temperature for future reference. Results were consistently reproducible in coated nitrocellulose membranes kept at different storage temperatures (-20 degrees C, 4 degrees C, and 25-30 degrees C) 48 h, 1 week and 5 months.

Potential virulence determinants of Salmonella serovars from poultry and human sources in Spain

A total of 173 Salmonella strains of different serovars isolated during 1992 from poultry and human sources in Zaragoza (NE Spain) were investigated for potential virulence factors. Parameters studied included production of aerobactin, enterobactin, colicin (including colicin V) and hemolysin, serum resistance against serum from man, sheep, cattle and chicken, binding of Congo red and crystal violet, auto-agglutination and calcium dependency at 37 degrees C and double colony morphology. Preliminary studies showed tests with completely negative results (colicins and hemolysin production, double colony morphology, auto-agglutination and calcium dependency at 37 degrees C) and tests with completely positive results (enterobactin production and binding of Congo red). The tests with variability of results were production of aerobactin, serum resistance and crystal violet binding. Aerobactin production was detected in 80% of Salmonella strains of clinical human origin and in 30% of Salmonella strains isolated from healthy slaughtered chickens. Sixty-five per cent of patient isolates were human serum resistant, 73% were ovine serum resistant, 85% were bovine serum resistant and 98% were chicken serum resistant. The percentages of poultry isolates serum resistant were 61, 86, 60 and 89% in human, ovine, bovine and chicken serum, respectively. Crystal violet binding was detected in 22.5% of Salmonella isolates from human origin and in 32.3% of Salmonella strains from poultry origin. This study can form a marker for the prevalence of strains with various characteristics (production of aerobactin, serum resistance and crystal violet binding) for comparison in future epidemiological studies. Furthermore, the data of this work suggested that strains causing enteric salmonellosis in man are partially identical to strains isolated from carrier broilers.

Synthesis of optically pure chrysobactin and immunoassay development

Chrysobactin (alpha-N-(2,3-dihydroxybenzoyl)-D-lysyl-L-serine), a siderophore that is essential for systemic virulence by plant pathogenic Erwinia chrysanthemi, was synthesized with high diastereomeric purity. Chrysobactin was prepared by coupling the N-hydroxysuccinimide ester of alpha-N-(2,3-dibenzyloxybenzoyl)-epsilon-N-Cbz-D-lysine with L-serine benzyl ester followed by deprotection via hydrogenolysis. Optically pure chrysobactin was obtained with 98% overall yield. A monoclonal antibody to ferric chrysobactin was developed and characterized as IgM. The antibody reacts with chrysobactin, ferric chrysobactin and less strongly with ferric dihydroxybenzoic acid. The antibody reacts weakly with the siderophores ferrichrome, A, ferric pseudobactin and ferric rhodotorulic acid. This antibody was used in a competitive immunoassay to detect ferric chrysobactin at 10(-8) to 10(-10) mol. This immunoassay may provide a useful method for the detection of chrysobactin in plant samples.

Results of passive and active immunization directed against ferric aerobactin in experimental enterobacterial infections in mice and chickens

Production of aerobactin has been reported to be a virulence factor in members of the family Enterobacteriaceae. To investigate the protection afforded by humoral immunity directed towards aerobactin in infectious diseases caused by aerobactin-producing strains, we tested the efficacy of mAbAERO1, a murine monoclonal antibody directed to ferric aerobactin, which, in vitro, was found to impair the growth of aerobactin-dependent strains of Enterobacteriaceae under iron-limited conditions. The mortality of mice experimentally infected with the aerobactin-producing strains Escherichia coli V2019 (LD50 = 3.5 x 10(5) CFU/mice) or Klebsiella pneumoniae Caroli (LD50 = 1.3 CFU/mice) was not reduced when 1 mg of mAbAERO1 was injected intravenously 1 h before or 1 h after bacterial challenge. Nor was mortality reduced after challenge with either E. coli V2019 or K. pneumoniae Caroli, even though the active immunization of mice with purified FeAero (ferric aerobactin) conjugated with thyroglobulin as followed by a rise in systemic anti-FeAero antibodies. Lastly, chicks born of hens immunized with FeAero showed evidence of antibody transmission towards FeAero, but were not protected when challenged with E. coli MT78, an aerobactin-producing strain highly virulent for chickens. Therefore, under the experimental conditions tested, humoral immunity against aerobactin appeared to play only a minor role in protection against infections caused by aerobactin-producing members of the family Enterobacteriaceae. However, other experimental models should be tested to confirm these observations.

Immunological variants of the aerobactin-cloacin DF13 outer membrane protein receptor IutA among enteric bacteria

Mouse monoclonal antibodies (MAbs) were generated against a 76-kDa IutA receptor of pathogenic avian Escherichia coli 15972. Six of the eight IutA-specific MAbs isolated (AB1 to AB6) were shown to be directed toward membrane-exposed conformational epitopes, although they did not interfere with the uptake of ferric aerobactin and cloacin DF13 as assessed by competition experiments with purified ligands. The two remaining IutA MAbs (AB9 and AB10) recognized linear epitopes buried in the IutA molecule. The panel of IutA MAbs was used to characterize IutA variants occurring in strains of E. coli, Klebsiella pneumoniae, Enterobacter spp., and Shigella spp., resulting in the identification of four immunological groups of IutAs. MAb AB9 defined an epitope conserved in all IutA variants. In addition, the panel of IutA MAbs served to identify the presence of IutA in wild-type bacteria grown in the presence of diphenylamine to reduce the expression of O-specific polysaccharide.