Addition of Novobiocin in pre-enrichment step can improve Salmonella culture protocol of modified semisolid Rappaport–Vassiliadis

Verification of different methods used for isolating Salmonella enterica serovar Dublin from cattle feces

Salmonella enterica serovar Dublin is a cattle-adapted serovar, and some infected cattle can become asymptomatic carriers. Identification of carrier animals is important for preventing the spread of infection within a farm, but low diagnostic sensitivity of the fecal culture method is problematic. In this study, we investigated isolation methods of four S. enterica Dublin strains. Selective enrichment using the tetrathionate broth showed better performance than Rappaport-Vassiliadis R10 broth, but one of the strains was not detectable. Since isolation of such strains by selective enrichment can be difficult, we designed a method using immuno-plates that concentrates S. enterica Dublin by antigen-antibody reaction. Our method is able to detect approximately 200 clony-forming units of S. enterica Dublin in 0.1 g of cattle feces. If S. enterica Dublin was isolated from cattle with clinical signs, the method to identify carriers in the farm should be based on the growth kinetics of the target S. enterica Dublin strain.

Detection and Molecular Identification of Salmonella Pathogenic Islands and Virulence Plasmid Genes of Salmonella in Xuzhou Raw Meat Products

ABSTRACT

Virulence genes expressed in Salmonella are a primary contributing factor leading to the high morbidity and mortality of salmonellosis in humans. The pathogenicity of Salmonella is mainly determined by the specific virulence factors that it carries. These factors also confer greater virulence and play a role in infection of a host and transmission of disease, and most Salmonella enterica can cause cross-infections between humans and animals. In this study, 265 samples in total were collected from a farmer's market and two supermarkets in Xuzhou, Jiangsu province, China, including 205 pork samples and 60 chicken samples. The suspected Salmonella isolates were isolated and identified using microbiological and molecular methods, and the confirmed isolates were used for serovar analysis and antimicrobial susceptibility testing. The virulence genes of Salmonella pathogenic islands (SPIs) and Salmonella virulence plasmids (Spv) in Salmonella-positive isolates were subsequently detected. Salmonella was isolated from 29.0% of samples, and all isolates were confirmed by PCR targeting the stn gene. Among the Salmonella isolates, resistance was most frequently observed against ciprofloxacin (84.4%), followed by tetracycline (71.4%) and streptomycin (68.8%). Resistance to amoxicillin-clavulanic acid (6.3%) and aztreonam (5%) was less commonly detected. The presence of the following virulence genes was determined by specific PCRs: hilA (SPI-1), sifA (SPI-2), misL (SPI-3), siiE (SPI-4), sopB (SPI-5), and spvC. The detection rate for SPI-1 to SPI-5 was 93.5, 87.0, 97.4, 97.4, and 97.4%, respectively. In addition, the detection rate of the spvC gene was 96.1%. Except for sopB (94.7%), all isolates of the dominant serovar S. enterica subsp.. enterica serovar Enteritidis contained all virulence genes from SPI-1 to SPI-5. This study demonstrated the epidemiological status of Salmonella in raw meat products in Xuzhou, and the complex antibiotic resistance and high isolation rate of virulence genes observed reveal many potential risks of which the findings presented herein will provide orientation to improve public health safeguards.

HIGHLIGHTS

Virulence and antimicrobial resistance factors in Salmonella enterica serotypes isolated from pigs and chickens in central Chile

Salmonella enterica is a food-borne pathogen with a wide host-range that during decades has been of public health concern in developed and developing countries. In Chile, the poultry and pig industries represent the biggest contribution of meat consumption in the population, and sanitary regulations have been imposed for Salmonella control. The aim of this work was to determine and characterize Salmonella strains isolated from pigs and chicken raised on commercials farms in Chile. For this, isolates belonging to pigs (n = 46) and poultry (n = 57) were genotyped by two multiplex PCR reactions and virulotyped by the PCR detection of virulence-associated genes. In addition, isolates were serotyped and analyzed by the Kirby Bauer assay to determine their antimicrobial resistance phenotypes. From these analyses 52 genotypes, six serotypes and several multidrug resistance phenotypes and different combinations of virulence-associated genes were detected. These results suggest that S. enterica in pigs and poultry in central Chile should be monitored due to potential consequences in public and animal health.

Selective pre-enrichment method to lessen time needed to recover Salmonella from commercial poultry processing samples

Conventional Salmonella detection is time consuming, often employing a 24-h pre-enrichment step in buffered peptone water (BPW), followed by a 24-h selective enrichment in either Rappaport Vassiliadis (RV) or tetrathionate (TT) broths before streaking onto selective indicator agar. To reduce this time, we sought to optimize pre-enrichment for Salmonella recovery by evaluating the addition of selective chemicals to BPW. Duplicate samples each representative of 500 carcasses were collected by catching processing water drip under moving carcass shackle lines immediately after feather removal in each of nine commercial processing plants. Carcass drip samples were cultured under selective pre-enrichment conditions in parallel with BPW pre-enrichment followed by RV and TT selective enrichment. Addition of bile salts (1 g/L) and novobiocin (0.015 g/L) resulted in Salmonella recovery from 89% samples when plated directly after pre-enrichment compared to 67% recovery in non-selective BPW alone. Salmonella serovar identities were determined using CRISPR-SeroSeq. Overall, serovars matched between selective pre-enrichment and traditional enrichment methods. These data suggest that increasing the selectivity of Salmonella pre-enrichment step may lessen the need for a separate selective enrichment step thereby reducing time required for Salmonella isolation by 24 h.

Persistent Salmonella enterica serovar Typhimurium Infection Increases the Susceptibility of Mice to Develop Intestinal Inflammation

Chronic intestinal inflammations are triggered by genetic and environmental components. However, it remains unclear how specific changes in the microbiota, host immunity, or pathogen exposure could promote the onset and exacerbation of these diseases. Here, we evaluated whether Salmonella enterica serovar Typhimurium (S. Typhimurium) infection increases the susceptibility to develop intestinal inflammation in mice. Two mouse models were used to evaluate the impact of S. Typhimurium infection: the chemical induction of colitis by dextran sulfate sodium (DSS) and interleukin (IL)-10^−/− mice, which develop spontaneous intestinal inflammation. We observed that S. Typhimurium infection makes DSS-treated and IL-10^−/− mice more susceptible to develop intestinal inflammation. Importantly, this increased susceptibility is associated to the ability of S. Typhimurium to persist in liver and spleen of infected mice, which depends on the virulence proteins secreted by Salmonella Pathogenicity Island 2-encoded type three secretion system (TTSS-2). Although immunization with a live attenuated vaccine resulted in a moderate reduction of the IL-10^−/− mice susceptibility to develop intestinal inflammation due to previous S. Typhimurium infection, it did not prevent bacterial persistence. Our results suggest that persistent S. Typhimurium infection may increase the susceptibility of mice to develop inflammation in the intestine, which could be associated with virulence proteins secreted by TTSS-2.

Detection of Salmonella enterica in Meat in Less than 5 Hours by a Low-Cost and Noncomplex Sample Preparation Method

Salmonella is recognized as one of the most important foodborne bacteria and has wide health and socioeconomic impacts worldwide. Fresh pork meat is one of the main sources of Salmonella, and efficient and fast methods for detection are therefore necessary. Current methods for Salmonella detection in fresh meat usually include >16 h of culture enrichment, in a few cases <12 h, thus requiring at least two working shifts. Here, we report a rapid (<5 h) and high-throughput method for screening of Salmonella in samples from fresh pork meat, consisting of a 3-h enrichment in standard buffered peptone water and a real-time PCR-compatible sample preparation method based on filtration, centrifugation, and enzymatic digestion, followed by fast-cycling real-time PCR detection. The method was validated in an unpaired comparative study against the Nordic Committee on Food Analysis (NMKL) reference culture method 187. Pork meat samples (n = 140) were either artificially contaminated with Salmonella at 0, 1 to 10, or 10 to 100 CFU/25 g of meat or naturally contaminated. Cohen's kappa for the degree of agreement between the rapid method and the reference was 0.64, and the relative accuracy, sensitivity, and specificity for the rapid method were 81.4, 95.1, and 97.9%, respectively. The 50% limit of detections (LOD50s) were 8.8 CFU/25 g for the rapid method and 7.7 CFU/25 g for the reference method. Implementation of this method will enable faster release of Salmonella low-risk meat, providing savings for meat producers, and it will help contribute to improved food safety.IMPORTANCE While the cost of analysis and hands-on time of the presented rapid method were comparable to those of reference culture methods, the fast product release by this method can provide the meat industry with a competitive advantage. Not only will the abattoirs save costs for work hours and cold storage, but consumers and retailers will also benefit from fresher meat with a longer shelf life. Furthermore, the presented sample preparation might be adjusted for application in the detection of other pathogenic bacteria in different sample types.

A Single-Step Enrichment Medium for Nonchromogenic Isolation of Healthy and Cold-Injured Salmonella spp. from Fresh Vegetables

Culture-based detection of nontyphoidal Salmonella spp. in foods requires at least four working days; therefore, new detection methods that shorten the test time are needed. In this study, we developed a novel single-step Salmonella enrichment broth, SSE-1, and compared its detection capability with that of commercial single-step ONE broth-Salmonella (OBS) medium and a conventional two-step enrichment method using buffered peptone water and Rappaport-Vassiliadis soy broth (BPW-RVS). Minimally processed lettuce samples were artificially inoculated with low levels of healthy and cold-injured Salmonella Enteritidis (100 or 101 colony-forming unit/25 g), incubated in OBS, BPW-RVS, and SSE-1 broths, and streaked on xylose lysine deoxycholate (XLD) agar. Salmonella recoverability was significantly higher in BPW-RVS (79.2%) and SSE-1 (83.3%) compared to OBS (39.3%) (p < 0.05). Our data suggest that the SSE-1 single-step enrichment broth could completely replace two-step enrichment with reduced enrichment time from 48 to 24 h, performing better than commercial single-step enrichment medium in the conventional nonchromogenic Salmonella detection, thus saving time, labor, and cost.

Growth of healthy and sanitizer-injured Salmonella cells on mung bean sprouts in different commercial enrichment broths

The ability of nine commercial broths to enrich healthy and 90% sanitizer-injured Salmonella Typhimurium and Salmonella cocktail on mung bean sprouts was evaluated to select an optimum broth for detection. Results showed that S. Typhimurium multiplied faster and reached a higher population in buffered peptone water (BPW), Salmonella AD media (AD) and ONE broth-Salmonella (OB), compared with other broths. Healthy and 90% sanitizer-injured Salmonella at low concentrations increased by 4.0 log CFU/ml in these three broths. However, no Salmonella growth was observed in lactose broth (LB). Further investigation showed that during incubation, pH of LB dropped from 6.7 to 4.2, due to production of lactic (66 mM) and acetic acids (62 mM) by lactic acid bacteria that were identified as dominant microbiota in bean sprouts. Though no cell membrane damage was detected by propidium monoazide combined with real-time PCR, it was found that LB inhibited Salmonella growth, especially from low inoculum levels. This study suggests that in consideration of effectiveness and cost, BPW would be a suitable enrichment broth to use for isolating and detecting Salmonella on mung bean sprouts, while using LB might cause false negative results in Salmonella detection by either PCR or standard cultural method.

Detection of Salmonella enterica in Magellanic penguins (Spheniscus magellanicus) of Chilean Patagonia: evidences of inter-species transmission

Patagonia in southern South America is among the few world regions where direct human impact is still limited but progressively increasing, mainly represented by tourism, farming, fishing and mining activities. The sanitary condition of Patagonian wildlife is unknown, in spite of being critical for the assessment of anthropogenic effects there. The aim of this study was the characterization of Salmonella enterica strains isolated from wild colonies of Magellanic penguins (Spheniscus magellanicus) located in Magdalena Island and Otway Sound, in Chilean Patagonia. Eight isolates of Salmonella were found, belonging to Agona and Enteritidis serotypes, with an infection rate of 0·38%. Resistance to ampicillin, cefotaxime, ceftiofur and tetracycline antimicrobials were detected, and some of these strains showed genotypic similarity with Salmonella strains isolated from humans and gulls, suggesting inter-species transmission cycles and strengthening the role of penguins as sanitary sentinels in the Patagonian ecosystem.

Comparison of enrichment broths for the recovery of healthy and heat-injured Salmonella typhimurium on raw duck wings

This study was performed to optimiz eSalmonella Typhimurium recovery from raw duck wings with five nonselective broths (buffered peptone water, tryptic soy broth, lactose broth, universal preenrichment broth, nutrient broth) and four selective broths (selenite broth, BAX System MP media [MP], Salmonella AD media [AD], ONE broth-Salmonella [OB]). Healthy or heat-injured (50 and 85% injury) cells were inoculated at a level of 10(2), 10(1), or 10(0) CFU/25 g on raw duck wings. Growth was modeled using DMfit with four growth parameters: lag-phase duration, maximum growth rate, doubling time, and maximum population density. Most enrichments were able to recover Salmonella Typhimurium to greater than 6 log CFU/ml. AD, MP, and OB had significantly (P < 0.05) higher maximum growth rate (0.9 to 1.0/h) and lower doubling time (0.7 to 0.8 h). Buffered peptone water, AD, MP, and OB recovered healthy and 50%-injured cells at low inoculum levels to more than 6.0 log CFU/ml; OB achieved the greatest recovery (7.6 and 7.9 log CFU/ml), following 24 h of incubation. The 85%-injured cells at 10(0) and 10(1) CFU/25 g, however, were only recovered in OB, reaching 7.3 and 7.5 log CFU/ml, respectively. These results suggest that OB may be an appropriate enrichment broth for the recovery of Salmonella Typhimurium from raw duck wings in standard diagnostic tests or other rapid detection methods, to avoid false-negative results.

Comparison of Five Culture Methods for Salmonella Isolation from Swine Fecal Samples of Known Infection Status

The current study was conducted to evaluate 5 bacteriologic culture methods (methods 1, 2, 3, 4, and 5) for recovery of Salmonella enterica from swine feces, both for sensitivity of detection (ability to recover Salmonella from a positive sample) and for specificity (not to inadvertently identify an organism as a Salmonella species in a negative sample). Fifty-six negative samples and 46 positive samples were processed using each of the 5 methods, which differed primarily in the combinations of enrichment media used. All negative samples were negative for Salmonella when cultured by all 5 methods (100% specificity). Two of the methods (methods 1 and 4) resulted in the recovery of significantly less ( P < 0.05) Salmonella when compared with the remaining 3 methods (methods 2, 3, and 5). No one method was successful in recovering Salmonella from all positive samples, although recovery with method 2 was statistically similar to the total number of positive samples analyzed (42 vs. 46 Salmonella-positive samples, P > 0.05). This study shows that culture methods differ significantly in their performance regarding the isolation of Salmonella from swine fecal samples.

Growth inhibitory factors in bovine faeces impairs detection of Salmonella Dublin by conventional culture procedure

AIMS

To analyse the relative importance of different biological and technical factors on the analytical sensitivity of conventional culture methods for detection of Salmonella Dublin in cattle faeces.

METHODS AND RESULTS

Faeces samples collected from six adult bovines from different salmonella-negative herds were split into subpools and spiked with three strains of S. Dublin at a concentration level of c. 10 CFU g(-1) faeces. Each of the 18 strain-pools was divided into two sets of triplicates of four volumes of faecal matter (1, 5, 10 and 25 g). The two sets were pre-enriched with and without novobiocin, followed by combinations of culture media (three types) and selective media (two types). The sensitivity of each combination and sources of variation in detection were determined by a generalized linear mixed model using a split-plot design.

CONCLUSIONS

Biological factors, such as faecal origin and S. Dublin strain influenced the sensitivity more than technical factors. Overall, the modified semi-solid Rappaport Vassiliadis (MSRV)-culture medium had the most reliable detection capability, whereas detection with selenite cystine broth and Mueller Kauffman tetrathionate broth combinations varied more in sensitivity and rarely reached the same level of detection as MSRV in this experiment.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study showed that for MSRV-culture medium and xylose lysine decarboxylase agar as the indicative medium, the sensitivity of the faecal culture method may be improved by focusing on the strain variations and the ecology of the faecal sample. Detailed investigation of the faecal flora (pathogens and normal flora) and the interaction with chemical factors may result in developing an improved method for detection of S. Dublin.

Semiquantitative assessment of the distribution of Salmonella in the environment of caged layer flocks

AIMS

To evaluate a semiquantitative technique for the enumeration of Salmonella in the environment of layer flocks and to compare findings with those of a standard qualitative technique.

METHODS AND RESULTS

Samples were taken from faeces, floor dust, dust on cages, feeders and egg belts. After mixing with buffered peptone water, serial dilutions were prepared and culture was performed using pre-enrichment, then plating on semisolid selective and solid isolation media. Comparison with a qualitative pre-enrichment technique indicated a similar sensitivity for both methods despite smaller sample sizes. The numbers of Salmonella detected for a site or sample type did not correlate closely with the prevalence of positive samples.

CONCLUSIONS

The sensitive detection and quantification of Salmonella in the flock environment is practicable with the technique described. Quantitative data in many cases do not correlate with qualitative findings.

SIGNIFICANCE AND IMPACT OF THE STUDY

The significance of certain environmental factors and interventions in the maintenance and dissemination of Salmonella in poultry houses may be over- or under-represented by prevalence data alone. The technique described allows the issue of poultry house contamination to be examined from a new perspective.

Optimization of a 12-hour TaqMan PCR-based method for detection of Salmonella bacteria in meat

We developed a 12-h Salmonella detection method, based on 8 h of preenrichment, followed by automated DNA extraction and a sensitive real-time PCR. The method was optimized to obtain the highest possible yield of cells and DNA. The growth of different Salmonella strains in various preenrichment media and the effects of adding growth-promoting and selective reagents were explored, taking into account their PCR compatibility. The effects of (i) analyzing larger volumes (1 to 5 ml) from preenriched samples and introducing wash steps prior to DNA extraction, (ii) regulating the amount of paramagnetic particles (increasing it from 60 to 90 microl) in the DNA extraction, (iii) eluting the DNA in reduced volumes (25 or 50 microl rather than 100 microl), and (iv) increasing the PCR template volume (from 5 to 20 microl) were investigated. After 8 h of preenrichment, buffered peptone water yielded the highest number of salmonellae. When analyzing minced meat samples, positive effects of increasing the initial sampling volume from 1 to 5 ml and increasing the amount of paramagnetic particles to 90 microl were observed. However, washing the pellet and eluting the DNA in reduced volumes (25 and 50 microl) had no positive effects and resulted in decreased reproducibility. Increasing the amount of PCR template DNA from 5 to 20 mul improved the threshold cycle value by approximately 2. The improved 12-h PCR method was successfully compared to a reference culture method with 100 minced meat and poultry samples, with a relative accuracy of 99%, a relative sensitivity of 98%, and a relative specificity of 100%.

Survival and transmission of Salmonella enterica serovar typhimurium in an outdoor organic pig farming environment

It was investigated how organic rearing conditions influence the Salmonella enterica infection dynamics in pigs and whether Salmonella persists in the paddock environment. Pigs inoculated with S. enterica serovar Typhimurium were grouped with Salmonella-negative tracer pigs. Bacteriological and serological testing indicated that organic pigs were susceptible to Salmonella infections, as 26 of 46 (56%) tracer pigs turned culture positive. An intermittent and mainly low-level excretion of Salmonella (<100 cells g-1) partly explains why the bacteriological prevalence appeared lower than the seroprevalence. Salmonella persisted in the paddock environment, as Salmonella was isolated from 46% of soil and water samples (n=294). After removal of pigs, Salmonella was found in soil samples for up to 5 weeks and in shelter huts during the entire test period (7 weeks). Subsequent introduction of Salmonella-negative pigs into four naturally Salmonella-contaminated paddocks caused Salmonella infections of pigs in two paddocks. In one of these paddocks, all tracer pigs (n=10) became infected, coinciding with a previous high Salmonella infection rate and high Salmonella excretion level. Our results showed that pigs reared under organic conditions were susceptible to Salmonella infections (just like conventional pigs) and that Salmonella persisting in the paddock environment could pose an infection risk. A driving force for these infections seemed to be pigs with a high Salmonella excretion level, which caused substantial contamination of the environment. This suggests that isolation of animals as soon as a Salmonella infection is indicated by clinical symptoms of diarrhea could be a means of reducing and controlling the spread and persistence of Salmonella in outdoor organic pig production environments.