Semisolid media for isolation of Salmonella spp. from coastal waters

Specific discrimination of three pathogenic Salmonella enterica subsp. enterica serotypes by carB-based oligonucleotide microarray

It is important to rapidly and selectively detect and analyze pathogenic Salmonella enterica subsp. enterica in contaminated food to reduce the morbidity and mortality of Salmonella infection and to guarantee food safety. In the present work, we developed an oligonucleotide microarray containing duplicate specific capture probes based on the carB gene, which encodes the carbamoyl phosphate synthetase large subunit, as a competent biomarker evaluated by genetic analysis to selectively and efficiently detect and discriminate three S. enterica subsp. enterica serotypes: Choleraesuis, Enteritidis, and Typhimurium. Using the developed microarray system, three serotype targets were successfully analyzed in a range as low as 1.6 to 3.1 nM and were specifically discriminated from each other without nonspecific signals. In addition, the constructed microarray did not have cross-reactivity with other common pathogenic bacteria and even enabled the clear discrimination of the target Salmonella serotype from a bacterial mixture. Therefore, these results demonstrated that our novel carB-based oligonucleotide microarray can be used as an effective and specific detection system for S. enterica subsp. enterica serotypes.

Detection ofSalmonellaspp. Using Microsphere-Based, Fiber-Optic DNA Microarrays

Salmonella spp. are one of the most problematic food pathogens in public health, as they are responsible for food poisoning associated with contamination of meat, poultry, and eggs. Thus, rapid and sensitive detection of Salmonella spp. is required to ensure food safety. In this study, a fiber-optic DNA microarray using microsphere-immobilized oligonucleotide probes specific for the Salmonella invA and spvB genes was developed for detection of Salmonella spp. Microarrays were prepared by randomly distributing DNA probe-functionalized microspheres (3.1-microm diameter) into microwells created by etching optical fiber bundles. Hybridization of the probe-functionalized microspheres to target DNA from Salmonella was performed and visualized using Cy3-labeled secondary probes in a sandwich-type assay format. In this study, 10(3)-10(4) cfu/mL of the target organism could be detected after 1-h hybridization without any additional amplification. The DNA microarray showed no cross-reactivity with other common food pathogens, including E. coli and Y. enterocolitica, and could even detect Salmonella spp. from cocktails of bacterial strains with only moderate loss of sensitivity due to nonspecific binding. This work suggests that fiber-optic DNA microarrays can be used for rapid and sensitive detection of Salmonella spp. Since fiber-optic microarrays can be prepared with different probes, this approach could also enable the simultaneous detection of multiple food pathogens.

Influence of enrichment media and application of a PCR based method to detect Salmonella in poultry industry products and clinical samples

To attempt the rapid detection of Salmonella enterica, we have coupled a culture procedure with PCR amplification of the genus-specific invE/invA genes. The method was applied to different kinds of samples from the poultry industry and evaluated by using hydrolyzed feather meal, meat meal, litter and viscera, all experimentally inoculated with a known number of Salmonella followed by cultivation in selenite--cystine broth prior to the PCR reaction. The expected 457bp specific DNA fragment could be amplified from dilutions containing as few as 5.7CFU, indicating that the PCR technique can be successfully coupled with culture in an enrichment broth to distinguish Salmonella species from other enteric bacteria present in samples from the poultry industry. Tetrathionate broth proved to be a much better enrichment media compared to selenite-cystine when the presence of Salmonella was evaluated by PCR in 1-day-old chicks experimentally infected with known numbers of Salmonella. Samples included cecal tonsils and viscera, collected at 48h and 7 days postinfection. The PCR technique was more sensitive in detecting infected animals than the standard microbiological procedure, which detected only 47% of all PCR positive samples.

Phage typing combined with pulsed-field gel electrophoresis and random amplified polymorphic DNA increases discrimination in the epidemiological analysis of Salmonella enteritidis strains

Phage typing (PT) combined with pulsed-field gel electrophoresis (PFGE) and a random amplified polymorphic DNA (RAPD) fingerprinting method was used to characterize Salmonella enteritidis strains. Twenty-four epidemiologically unrelated isolates, sampled from diverse ecological niches and fifteen isolates from four well-defined outbreaks of foodborne gastroenteritis, were studied. Seven phage types, with a predominance of PT 4 (63% of isolates), were observed when analysing the epidemiologically unrelated group. PT 4 was detected in all of the ecological niches studied, including food and fecally polluted river and beach water. The discriminatory power for phage typing, the average probability that the typing system will assign a different type to two unrelated strains randomly sampled in the microbial population, was 0.62. Ten PFGE pattern types were obtained with Xba I restriction endonuclease enzyme among the unrelated isolates; thirteen isolates belonged to PFGE pattern type 1 and the rest of the PFGE types were assigned to one or two isolates. The Dice coefficient clustered the similarities of the PFGE patterns between 80-100%. PFGE showed a discriminatory power of 0.72. Five clearly distinct RAPD patterns were observed with the OPS-19 oligonucleotide, but the discrimination obtained was low (0.46). The combination of the three typing methods increased the number of types to seventeen, giving high discrimination (0.92). Seven of the isolates recovered from various ecological niches belonged to the combination PT 4/PFGE 1/RAPD A and other combinations were unique or included only two strains. The four epidemiologically well-defined foodborne outbreaks were associated with the PT 4 phage type. In two of the outbreaks, other phage types (PT 7a and RDNC) were also observed in two isolates. Most of the isolates belonging to the foodborne outbreaks had an identical PFGE pattern (PFGE pattern type 1), but a difference in a restriction band was observed in an isolate belonging to an outbreak. Two RAPD patterns were observed in the outbreaks; RAPD pattern type A was detected in three of the four outbreaks. When the combined typing method was applied to the study, high concordance was observed and most of the outbreak strains belonged to the combination PT 4/PFGE 1/RAPD A. It is concluded that the combination of phage type with PFGE and RAPD provides a powerful discriminatory tool for the epidemiological analysis of unrelated and related strains of S. enteritidis.

Molecular analysis of environmental and human isolates of Salmonella typhi

Molecular characterization of a total of 54 isolates of Salmonella typhi from Santiago, Chile, was performed by pulsed-field gel electrophoresis (PFGE) after digestion of chromosomal DNA with three restriction endonucleases: XbaI (5'-TCTAGA-3'), AvrII (5'-CCTAGG-3'), and SpeI (5'-ACTAGT-3'). Thirteen of the 54 isolates were obtained from environmental sources (sewage and river water), and the rest were isolates from clinical cases of typhoid fever. Considerable genetic diversity was detected among the human isolates obtained in 1994, as evidenced by the presence of 14 to 19 different PFGE patterns among 20 human isolates, with F (coefficient of similarity) values ranging from 0.69 to 1.0 (XbaI), 0.61 to 1.0 (AvrII), and 0.70 to 1.0 (SpeI). A total of eight phage types were detected among these 20 isolates, with 50% possessing the E1 or 46 phage type. There was no correlation between PFGE pattern and phage types. Similar diversity was seen among 21 isolates obtained in 1983, with 17 to 19 PFGE patterns detected and F values of 0.56 to 1.0 (XbaI), 0.55 to 1.0 (AvrII), and 0.67 to 1.0 (SpeI). Comparison of these two groups of human isolates obtained 11 years apart indicated that certain molecular types of S. typhi are shared and are able to persist for considerable periods. A similar degree of genetic diversity was also detected among the environmental isolates of S. typhi, for which 10 to 12 different PFGE patterns were detected among the 13 isolates analyzed, with F values ranging from 0.56 to 1.0 (XbaI), 0.52 to 1.0 (AvrII), and 0.69 to 1.0 (SpeI). Certain molecular types present among the environmental isolates of S. typhi were also found among the human isolates from the same time period, providing evidence for the epidemiological link between environmental reservoirs and human infection.

Evaluation of a rapid cultural method for identification of salmonellas in naturally contaminated veterinary samples

Three thousand and forty-two samples of post-mortem tissues, environmental swabs, animal feed ingredients and eggs were examined by conventional methods and by a simple rapid culture method. The rapid method gave reliable presumptive identification of salmonella contamination within 48 h of the start of culture. The rapid method also showed enhanced sensitivity for detection of salmonellas, in most types of sample, particularly those taken in poultry houses and hatcheries where high levels of competitive faecal bacteria are present.

Genetically engineered microorganisms to rescue plants from frost injury

Ice nucleation active bacteria belonging to genera Pseudomonas, Xanthomonas and Erwinia contribute to frost damage to plants by initiating the formation of ice in plants that would otherwise supercool and avoid the damaging ice formation. The biological control of frost injury can be achieved by the application of non-ice nucleation active bacteria to the plant surfaces before they become colonized by Ice+ species. ice genes have been cloned from Pseudomonas and isogenic Ice- derivatives constructed via genetic manipulations. These genetically engineered microorganisms (GEMs) have been released into the environment to control the frost damage. The incidence of frost injury to the plants has, thereby, been reduced by 50-85% during natural frosts. These GEMs do not survive in soil and show no aerial dispersal in the environment.

Detection of Salmonella enteritidis in environmental samples by monoclonal antibody-based ELISA

We have developed a enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (ASCII) for the detection of Salmonella enteritidis in environmental samples. ELISA was used to test for sensitivity and specificity of ASCII. 38 other species of bacteria, including 31 Salmonella species were included in cross-reactivity testing with ELISA. ASCII showed no reactivity with any other species tested. ASCII was found to be an IgG1 specific for S. enteritidis lipopolysaccharide (LPS). The lower limits for S. enteritidis detection was 10(5) cells/ml for pure cultures and in 10% sludge (w/v). Environmental samples (raw wastewater, wastewater effluents, mixed liquor and aerobically digested sludge) were obtained twice from five sites and ELISA tested for the presence of S. enteritidis. ELISA results compared to the American Public Health Association (APHA) method of Salmonella detection were not significantly different (P greater than 0.05). The ELISA took 24 h for completion compared to 96-120 h for the APHA procedure. Results demonstrate the reliability of the ELISA and, more importantly, provides a rapid means of detection of S. enteritidis in environmental samples.

Comparison between semisolid Rappaport and modified semisolid Rappaport-Vassiliadis media for the isolation of Salmonella spp. from foods and feeds

Two semisolid media poured in petri dishes, semisolid Rappaport and modified semisolid Rappaport Vassiliadis were compared for the recovery of Salmonella spp. from 154 high- and low-moisture foods and feeds. Semisolid media were inoculated after pre-enrichment in buffered peptone water and after enrichment in Müller-Kauffmann tetrathionate broth and selenite cystine broth. Also a conventional procedure was used, where the enrichment broths were streaked on Brilliant Green and bismuth sulphite agars. Of 62 samples found positive for Salmonella by one or more methods, semisolid Rappaport detected all of them and 60 when inoculated directly from the pre-enrichment broth. Modified semisolid Rappaport-Vassiliadis gave only 20 positive samples. The difference in productivity between semisolid Rappaport and the other media poured in plates (modified semisolid Rappaport-Vassiliadis, Brilliant Green and bismuth sulphite agar) was statistically significant (P less than 0.001). The incubation temperature recommended for modified semisolid Rappaport-Vassiliadis (42 degrees C) was found to be too high. By comparison to the other media, semisolid Rappaport gave a 500% increase in the detection of the enteritidis serotype of Salmonella.

Enzyme-linked immunoassays for the detection of Salmonella spp.: a comparison with other methods

The first enzyme immunoassay for Salmonella was reported in 1977 and since that time several enzyme-linked immuno assays (ELISAs) have been developed, using either polyclonal or monoclonal antibodies that will detect most Salmonella serotypes. Two of these kits have been declared official first status by the Association of Official Analytical Chemists (AOAC). In comparison with a culture method used in collaborative studies, the total assay time is reduced by 2 days and statistical analysis of the data indicated no significant difference. The main problem related to all methods other than traditional culture methods is the occurrence of false-positive and/or false-negative results. False-positive ELISA results can be eliminated by using (combinations of) highly specific monoclonal antibodies. Good enrichment procedures are very important to be sure that the detection limit of approx. 10(5) cells/ml will be reached. In the future even better limits of detection may be achieved by using enzyme amplification or chemiluminescence to decrease the number of false-negative results.