Comparison of fluorescent-antibody methods and enrichment serology for the detection of Salmonella

Early Recovery of Salmonella from Food Using a 6-Hour Non-selective Pre-enrichment and Reformulation of Tetrathionate Broth

Culture based methods are commonly employed to detect pathogens in food and environmental samples. These methods are time consuming and complex, requiring multiple non-selective and selective enrichment broths, and usually take at least 1 week to recover and identify pathogens. Improving pathogen detection in foods is a primary goal for regulatory agencies and industry. Salmonella detection in food relies on a series of culture steps in broth formulations optimized to resuscitate Salmonella and reduce the abundance of competitive bacteria. Examples of non-selective pre-enrichment broths used to isolate Salmonella from food include Lactose, Universal Pre-enrichment, BPW, and Trypticase Soy broths. Tetrathionate (TT) and Rappaport-Vassiliadis (RV) broths are employed after a 24-h non-selective enrichment to select for Salmonella and hamper the growth of competitive bacteria. In this study, we tested a new formulation of TT broth that lacks brilliant green dye and has lower levels of TT . We employed this TT broth formulation in conjunction with a 6-h non-selective pre-enrichment period and determined that Salmonella recovery was possible one day earlier than standard food culture methods. We tested the shortened culture method in different non-selective enrichment broths, enumerated Salmonella in the non-selective enrichments, and used 16S rRNA gene sequencing to determine the proportional abundances of Salmonella in the TT and RV selective enrichments. Together these data revealed that a 6-h non-selective pre-enrichment reduces the levels of competitive bacteria inoculated into the selective TT and RV broths, enabling the recovery of Salmonella 1 day earlier than standard culture enrichment methods.

Detection of group D salmonellae in blood culture broth and of soluble antigen by tube agglutination using an O-9 monoclonal antibody latex conjugate

Latex particles sensitized with a Salmonella O-9 monoclonal antibody were used to detect group D salmonellae in blood culture broths. The system was found to be 100% accurate when used on 104 clinical specimens that included 13 S. typhi isolates. Enteric fever could thus be diagnosed as early as 1 day after the initiation of culture in some (18%) cases. The latex particles were also used in an experimental situation to detect soluble S. typhi lipopolysaccharide by a tube agglutination method. This was found to be more than four times as sensitive as the conventional slide agglutination method because at least 50 ng of antigen per ml in either buffer or urine was detected, albeit with less efficiency when serum was used.

A murine monoclonal antibody specific for the outer core oligosaccharide of Salmonella lipopolysaccharide

Immunoglobulin G3 murine monoclonal antibody T6 specific for the lipopolysaccharide of Salmonella O serogroups A to E was established. By using R mutants of Salmonella spp., Escherichia coli, and Shigella spp., the major reactive epitope with T6 was tentatively identified as the terminal disaccharide, N-acetylglucosamine 1.2----alpha glucose, of the core oligosaccharide. T6 was reactive with 10 clinical isolates of each of the Salmonella O serogroups A to E but not with 58 isolates of other gram-negative bacteria. Its selective reactivity against Salmonella spp. renders T6 a potentially more useful reagent than the conventional polyvalent serum for the identification of Salmonella spp. It may also serve as a useful molecular tool for the study of the outer core structure of all Salmonella and related species.

Rapid detection of Salmonella spp. by using Felix-O1 bacteriophage and high-performance liquid chromatography

A method is described whereby the presence of Salmonella spp. can be detected within 8 to 24 h of sample collection. The method depends upon the interaction of Salmonella spp. with the Salmonella-specific Felix-O1 bacteriophage. This interaction results in an increase in concentration of the bacteriophage which is detected by high-performance liquid chromatographic techniques.

Rapid detection of Salmonella spp. in food by use of the ISO-GRID hydrophobic grid membrane filter

A rapid hydrophobic grid-membrane filter (HGMF) method was developed and compared with the Health Protection Branch cultural method for the detection of Salmonella spp. in 798 spiked samples and 265 naturally contaminated samples of food. With the HGMF method, Salmonella spp. were isolated from 618 of the spiked samples and 190 of the naturally contaminated samples. The conventional method recovered Salmonella spp. from 622 spiked samples and 204 unspiked samples. The isolation rates from Salmonella-positive samples for the two methods were not significantly different (94.6% overall for the HGMF method and 96.7% for the conventional approach), but the HGMF results were available in only 2 to 3 days after sample receipt compared with 3 to 4 days by the conventional method.

Modified enrichment-serology procedure for detection of salmonellae in soy products

A modified enrichment-serology (MES) procedure was used to reduce the time necessary for salmonella analysis. Naturally contaminated samples of soy products were preenriched in 1% proteose peptone for 6 h at 37 degrees C followed by inoculation into tetrathionate broth for 18 h at 37 degrees C. Two drops of the tetrathionate sample were inoculated into M broth. After incubation at 37 degrees C for 6 h, 0.85 ml of the mixture was formolized, and 0.1 ml of polyvalent H antiserum was added. After incubation in water bath at 50 degrees C for 1 h, the appearance of a typical floccular flagellar precipitate was observed in tubes positive for salmonellae. Over 3,000 samples were subjected to standard biochemical and serological procedures, and the results were compared with those of the MES method with a 96.7% correlation. Eleven of the samples (0.3%) were false-negative with the MES procedure, and 3% were false-negative with the U.S. Food and Drug Administration Bacteriological Analytical Manual procedure. The 3% negative samples by this latter procedure were subsequently found to be positive by the MES procedure. The MES procedure reduced the time required for salmonella analysis from 4 days to 32 h.

Synthetic disaccharide-protein antigens for production of specific 04 and 09 antisera for immunofluorescence diagnosis of salmonella

The synthetic disaccharides abequose 1 leads to a 3 mannose and tyvelose 1 leads to a 3 mannose, representative of Salmonella O-antigen 4 and 9 respectively, were covalently linked to bovine serum albumin (BSA) . Antisera from rabbits immunized with these immunogens were used in indirect immunofluorescence assay for the identification of group B (O-antigen 4) and D (O-antigen 9) Salmonella. A total of 1030 enteric bacterial strains were tested, including 207 group B and 55 group D Salmonella. The anti-abequose-mannose-BSA serum correctly identified all Salmonella group B strains tested. The anti-tyvelose-mannose-BSA serum correctly indentified all Salmonella group D bacteria examined with the exception of 11 of 18 Vi-positive S. typhi strains which did not not stain until the Vi-antigen was removed by boiling. Among the 768 strains representing Salmonella other than groups B and D, E. coli, Shigella, Citrobacter, Klebsiella, Enterobacter, Serratia, Proteus, Vibrio, Pseudomonas, Aeromonas, Yersinia, Bacteroides and Fusobacterium only 5 positive reactions were found. These were observed with Y. pseudotuberculosis strains which have the same disaccharide antigenic determinants as Salmonella O-antigen 4 and 9 respectively. The high specificity of the antisera elicited by the synthetic disaccharide-BSA immunogens make them suitable for a specific and rapid identification of Salmonella bacteria belonging to serogroups B and D.

Use of enzyme-labeled antibodies to detect Salmonella in foods

An indirect enzyme-labeled antibody technique (ELAT), in which Salmonella typhimurium was used as a model, was developed as a method to detect Salmonella in food samples. A cellulose-acetate membrane filter, the matrix for detection, was placed on a membrane-filter base and overlaid with a multiwelled lucite template. Mixed broth enrichment cultures were dispensed in the template wells, and cells were spotted onto the membrane via suction. After fixation, the membranes were immersed in rabbit anti-S. typhimurium flagella antibody, washed, immersed in goat anti-rabbit antibody conjugated to peroxidase, and washed. Exposure of membranes to the substrates 3,3'-diaminobenzidine or benzidine resulted in development of brown or blue macroscopic reaction products, respectively, on spots containing S. typhimurium. ELAT results agreed with those of enrichment serology and cultural procedures on three food products containing known levels of S. typhimurium. Because of the magnification effect of the enzyme-substrate reaction, fewer cells were needed for detection than with enrichment serology, thereby reducing the total analysis time. The ability to test 14 or more samples simultaneously on a 47-mm membrane filter would facilitate screening large number of samples. Pending the development of a pure H antisera pool for the common Salmonella serotypes free from O antibodies, the ELAT demonstrated potential as a Salmonella detection methodology.

Timed-Release Capsule Method for the Detection of Salmonellae in Foods and Feeds

A new method was developed for the detection of injured and uninjured salmonellae in foods and feeds. The steps of pre-enrichment in a nonselective broth and selective enrichment in a selective medium were combined into a single procedure. This was achieved by the gradual release of selective agents from wax-coated gelatin capsules added at the time of inoculation of nonselective basal broths. Pre-enrichment in lactose broth was combined with selective enrichment in tetrathionate or selenite-cystine broth by using timed-release capsules containing iodine or selenite. Five different categories of foods and feeds, naturally contaminated with salmonellae, were examined to compare the efficiencies of the capsule methods with conventional procedures. Combination of the separate steps of pre-enrichment and selective enrichment into a single procedure was feasible and resulted in substantial savings of labor and materials.