Evaluation of new culture media for rapid detection and isolation of salmonellae in foods

Developing a multiplex real-time PCR with a new pre-enrichment to simultaneously detect four foodborne bacteria in milk

Aim: The aim of this study is to formulate a new single nonselective pre-enrichment medium (ELSS) that can support the concurrent growth of four major foodborne pathogens containing E. coli O157: H7, L. monocytogenes, S. aureus and S. enterica serovar Entertidis to develop a multiplex TaqMan Real-time PCR (mRT-PCR). Methods: The mRT-PCR with a new pre-enrichment was carried out for simultaneous detection and quantification of these foodborne bacteria. Results: By using mRT-PCR after 16 h pre-enrichment in ELSS, the detection limit of each pathogen was 1 CFU/25 ml contaminated milk, as well as inclusivity and exclusivity reached 100%. Conclusion: The mRT-PCR assay with pre-enrichment step is a fast and reliable technique for detecting single or multiple pathogens in food products.

Lactic Acid Bacteria as Biological Control of Staphylococcus aureus in Coalho Goat Cheese

The aim of this study is to investigate the bacterial population in coalho goat cheese produced in the semi-arid northeast region of Brazil, to analyse the antibiotic resistance profiles of the identified pathogenic bacteria, to detect the staphylococcal enterotoxin genes and to evaluate the addition of autochthonous lactic acid bacteria (LAB) with technofunctional properties for the control of Staphylococcus aureus growth. In the analysed samples, strains of Escherichia coli (N=11), Salmonella spp. (N=18), Listeria spp. (N=6) and S. aureus (N=9) were classified as multidrug resistant (MDR). The most commonly isolated pathogen from the studied coalho goat cheese was S. aureus. Its isolates were positive for the genes encoding enterotoxins A (sea), B (seb), C (sec) and D (sed). The autochthonous LAB with the potential to inhibit S. aureus were identified as Enterococcus faecium. These strains were selected for in vitro tests of protective, safety, technological and functional properties. In the coalho goat cheese food matrix, these selected autochthonous LAB were able to reduce the enterotoxigenic MDR S. aureus load by approx. 3 log units.

Feed withdrawal and transportation effects on Salmonella enterica levels in market-weight pigs

Feed withdrawal and transport commonly occur together in pigs. Objectives of this study were to determine if these preslaughter stressors, feed withdrawal and transportation, affect the levels of , stress hormone concentrations, and immune functions in infected market pigs. A 2 × 2 factorial analysis of a randomized complete block design with feed withdrawal and transport as fixed effects was used. Sixty market-weight pigs were individually inoculated with serovar Typhimurium. The experiment was replicated 3 times (blocking factor) with 20 pigs per replicate. Three days after inoculation, the pigs were randomly assigned to 1 of 4 treatments (5 pigs per treatment in each/replicate), including 1) control (Control; or no stress), 2) feed withdrawal for 12 h (FW), 3) transportation for 2 h (T), and 4) feed withdrawal for 12 h followed by transportation for 2 h (FWT). Feed withdrawal by itself or followed by transportation caused an increase of levels in ileal contents ( < 0.05), whereas only FWT caused an increase of levels in cecal contents ( < 0.05). Rectal contents (feces) consistently contained very low levels of , with no difference among treatments ( > 0.10). Cortisol increased in pigs from all 3 stress treatments ( < 0.001), with T and FWT pigs having greater concentrations than Control pigs ( < 0.05), although total white blood cell counts were lower for FWT pigs compared with Controls ( > 0.03). Each granulocyte percentage (neutrophil, eosinophils, and basophils) increased ( < 0.05) following transport but was attenuated ( > 0.05) by feed withdrawal with transport. Lymphocytes were suppressed ( < 0.05) by all stressors, and the greatest suppression occurred when pigs were transported (T and FWT). However, monocytes were suppressed ( < 0.05) compared with Controls only by FWT. Expression of IL-1 (produced by monocytes/macrophages) from the spleen cells increased ( < 0.05) with FW compared with Controls, whereas its receptor antagonist was suppressed by FWT ( < 0.05). It is concluded that some typical preslaughter practices, such as feed withdrawal and transportation, lead to greater intestinal levels and gut-associated lymphoid tissue markers of inflammation in market pigs and, consequently, to an increased food safety risk.

Selection of autochthonous lactic acid bacteria from goat dairies and their addition to evaluate the inhibition of Salmonella typhi in artisanal cheese

This study aimed to select autochthonous lactic acid bacteria (LAB) with probiotic and functional properties from goat dairies and test their addition to artisanal cheese for the inhibition of Salmonella typhi. In vitro tests, including survival in the gastrointestinal tract (GIT), auto- and co-aggregation, the hemolytic test, DNase activity, antimicrobial susceptibility, antibacterial activity, tolerance to NaCl and exopolysaccharide (EPS), gas and diacetyl production were conducted for sixty isolates. Based on these tests, four LAB isolates (UNIVASF CAP 16, 45, 84 and 279) were selected and identified. Additional tests, such as production of lactic and citric acids by UNIVASF CAP isolates were performed in addition to assays of bile salt hydrolase (BSH), β-galactosidase and decarboxylase activity. The four selected LAB produced high lactic acid (>17 g/L) and low citric acid (0.2 g/L) concentrations. All selected strains showed BSH and β-galactosidase activity and none showed decarboxylase activity. Three goat cheeses (1, 2 and control) were produced and evaluated for the inhibitory action of selected LAB against Salmonella typhi. The cheese inoculated with LAB (cheese 2) decreased 0.38 log10 CFU/g of S. Typhy population while in the cheese without LAB inoculation (cheese 1) the pathogen population increased by 0.29 log units. Further, the pH value increased linearly over time, by 0.004 units per day in cheese 1. In the cheese 2, the pH value decreased linearly over time, by 0.066 units per day. The cocktail containing selected Lactobacillus strains with potential probiotic and technological properties showed antibacterial activity against S. typhi in vitro and in artisanal goat cheese. Thus, goat milk is important source of potential probiotic LAB which may be used to inhibit the growth of Salmonella population in cheese goat, contributing to safety and functional value of the product.

Bacterial microbiota present in the gallbladder of cattle and antimicrobial resistance of Staphylococcus isolates

Pathogenic microorganisms can reside transiently or permanently in the gallbladder of cattle. Thus, during slaughter, more attention should be given to the gastrointestinal tract, especially to the accessory organ, the gallbladder. The main aim of this study was to characterize the bacterial microbiota present in bile and gallbladder epithelium of cattle slaughtered in a slaughtering plant under sanitary conditions and to evaluate the antimicrobial resistance in strains of the genus Staphylococcus. Thirty intact gallbladders were collected and the in bile and epithelium were researched for the presence of Aerobic Mesophilic Heterotrophic Bacteria (AMHB), Staphylococcusspp., total Enterobacteriaceae, Enterococcus spp. and Salmonella spp. The frequency of isolation of the microorganism mentioned above were, respectively: 23.02%, 14.39%, 13.67%, 24.46%, 0% and 24.46%. Concerning both gallbladder environments, the frequency of isolation of the microorganisms in the epithelium was 64.03%, and in the bile 35.97%, with no statistical difference, but with significant difference between the population averages. In antimicrobial susceptibility testing, strains of Staphylococcusfrom both bile and gallbladder epithelium showed sensitivity to the antimicrobials: penicillin G, ceftriaxone, chloramphenicol and gentamicin. The observation that the gallbladder supports a high frequency of microorganisms brings us to the possible fact that cattle might be a persistent carrier of pathogens of great importance to public health.

Comparison of 3 culture methods and PCR assays for Salmonella gallinarum and Salmonella pullorum detection in poultry feed

To detect Salmonella gallinarum or Salmonella pullorum in artificially contaminated poultry feed, 9 culture combinations were compared, including 3 preenrichment/enrichment methods (tryptic soy broth plus ferrous sulfate/tetrathionate Hajna, tryptic soy broth plus ferrous sulfate/selenite cystine broth, and Salmosyst) in combination with 3 selective agars (xylose lysine desoxicholate agar added with tergitol 4, EF-18, and Önöz), respectively. Additionally, a single PCR technique was applied combined with 2 different preenrichment media (tryptic soy broth plus ferrous sulfate and Salmosyst). The specificity and positive predictive value were 1 for all methods. There were some differences among Salmonella strains for sensitivity and accuracy in the culture and Salmosyst-PCR methods. The sensitivity and accuracy values were less than 0.60 and 0.64, respectively, whereas the negative predictive values were between 0.12 and 0.23. Two PCR methods did not show any difference in the parameters of performance evaluated. Kappa coefficients showed good agreement between both methods. None of the culture combinations was able to detect S. gallinarum or S. pullorum when the inoculum was less than 3 × 10² cfu/25 g, except the Salmosyst broth method, which could recover S. gallinarum from 3 × 10¹ cfu/25 g onward. Overall, there were differences in the detection limits among the strains and methods used. In general, the 3 selective plating media did not show any significant difference in the parameters of performance studied for each strain. On the other hand, the agreements were slight to fair when culture methods were compared among them and with both PCR methods. The differences in the detection levels that were obtained using these methods and the difficulty in detecting S. gallinarum or S. pullorum in feed represent a potential problem when a poultry feed sample is considered to be negative. It is highly recommended to use at least 2 methods to increase the chances of detecting S. gallinarum or S. pullorum in poultry feed.

Immunological, physiological, and behavioral effects of Salmonella enterica carriage and shedding in experimentally infected finishing pigs

Finishing pigs infected with Salmonella pose significant food safety risks by carrying the pathogen into abattoirs. This study was conducted to determine the dynamics of Salmonella infection in finishing pigs, and associated immunological, physiological, and behavioral alterations, by longitudinally comparing infected to noninfected pigs during 6 weeks postinfection (p.i.). Bacteriological data revealed that all inoculated pigs started shedding Salmonella within 2 h p.i., and persistently shed the bacteria up to the end of the study. Ileal and cecal contents, as well as mesenteric lymph node samples, were all positive throughout the study, containing 3-4 log(10) cfu/g of Salmonella at 24 h p.i., and 4-5 log(10) cfu/g of Salmonella up to 4 weeks p.i. Levels of Salmonella dropped markedly (p < 0.05) in all samples at 5 weeks p.i. There was no difference between groups for blood cell counts. Tumor necrosis factor-α was greater (p < 0.05) in infected pigs: (1) in the mesenteric lymph nodes by 48 h p.i.; (2) at 24 h and 3 weeks p.i. in the ileum; and (3) in the cecum and spleen at 3 weeks p.i. Interleukin-12, interleukin-1 and its antagonist, and a porcine-specific antimicrobial peptide RNA expression in tissues changed over time, but were not different between groups. Infected pigs spent more time in ventral recumbency, standing, and sitting than controls (p < 0.01). Infected pigs were also more active (p < 0.01), and approached a novel object more quickly than control pigs (p < 0.05). No treatment differences were detected for rectal temperature or plasma cortisol (p > 0.10). This study shows that finishing pigs can carry high levels of Salmonella for up to 4 weeks p.i. in the gastrointestinal contents and mesenteric lymph nodes, shedding high levels of the bacteria without developing clinical symptoms, but developing an immune response throughout the intestinal tract. Moreover, subtle behavioral changes measured as postures were detected, and therefore warrant additional investigation.

A multipathogen selective enrichment broth for simultaneous growth ofSalmonella entericaserovar Enteritidis,Staphylococcus aureus, andListeria monocytogenes

A selective enrichment broth (SSL) was formulated to allow concurrent growth of 3 prominent food-borne pathogens: Salmonella enterica serovar Enteritidis, Staphylococcus aureus , and Listeria monocytogenes . Nalidixic acid, lithium chloride, and potassium tellurite were added as the selective agents, while sodium pyruvate and mannitol were employed as the supplemented elements. In the individual growth trial, the target pathogens were capable of growing in SSL to as high as 7–8 log10colony-forming units (CFU)/mL after 24 h incubation at 37 °C when being inoculated at 50–100 CFU/mL. In the simultaneous growth trial, the 3 combined target pathogens showed similar growth rates. The results show that SSL could support the successful simultaneous enrichment of 3 pathogens; however, SSL inhibited the growth of nontarget bacteria. In the artificial contaminated raw beef and ready-to-eat chicken, a high recovery of these 3 target pathogens was obtained in SSL. Finally, Salmonella Enteritidis, Staphylococcus aureus, and L. monocytogenes were detected from 710 suspicious food samples by SSL with real-time PCR, and no false-positive or -negative results were reported. In summary, SSL has been shown to be a suitable broth for the simultaneous detection of the 3 prominent food-borne pathogens by multipathogen detection on a single-assay platform.

Quantification of Salmonella spp. in composted biosolids using a TaqMan qPCR assay

Composting is increasingly used to transform biosolids, obtained following wastewater treatment, into a more stable organic product that can be released in the environment. The process must however be closely monitored to assure that the end product meets the regulations set by environmental agencies with regards to the amount of pathogenic microorganisms present. In this study, a TaqMan qPCR approach targeting the invA gene was developed to monitor the presence of Salmonella spp. in composted biosolids. A validation step was first performed to evaluate the effect of compost age on the quantification of various concentrations of seeded Salmonella typhimurium. Secondly, qPCR was used to investigate the effect of composting time, varying from 1 month to 24 months, on the presence of Salmonella spp. naturally present in biosolids samples. Culture media were used in parallel to corroborate the results obtained by qPCR. The detection limit of the invA gene obtained experimentally from composts seeded with S. typhimurium was 5.8 copies or the equivalent of 5.8 CFU per qPCR reaction. Although the results indicated that compost age had a marginal effect on the detection of seeded S. typhimurium, the TaqMan qPCR approach was efficient at detecting and quantifying the amount of Salmonella spp. present in naturally contaminated composted biosolids of different ages. Results showed that there was a significant decrease in the amount of Salmonella DNA present in composted biosolids over time, which was also corroborated by the CFU counts obtained on the BSA culture medium. However, qPCR was more specific, robust and rapid to execute than performing counts on culture media. qPCR shows promise for routine examination of composted biosolids to ascertain that pathogenic microorganisms, including Salmonella spp., are decreased below acceptable limits before their application in the environment.

Isolation of Salmonella strains from reptile faeces and comparison of different culture media

AIMS

To provide information on epidemiology and isolation of Salmonella strains from reptiles.

METHODS AND RESULTS

Ninety-one samples collected from reptiles of the zoo of Rome or belonging to private owners were analysed using a standard protocol for isolation of Salmonella from food. Salmonella strains were tested for susceptibility to 15 antimicrobics by a disc-agar diffusion method. Forty-six samples (50.5%) were positive for Salmonella. Of the 22 strains serotyped, 17 belonged to Salmonella enterica subsp. I, four to the subsp. IIIa and one strain resulted untypeable. Rappaport-Vassiliadis broth (RVB) allowed to recover more Salmonella strains when bacterial growth in buffered peptone water (BPW) was scarce, while selenite cystine broth (SCB) was more efficient, whereas growth in BPW was abundant. The maximum isolation score was obtained by plating onto xylose lysine desoxycholate agar (XLD). The strains exhibited resistance at high percentages to colistin sulphate (58.7%), sulphamethoxazole (55.5%), streptomycin (32.6%), tetracycline (19.6%), ampicillin (17.4%) and nalidixic acid (13.1%).

CONCLUSIONS

A high prevalence of Salmonella in reptiles was observed. For isolation, the choice of the enrichment broth depending on the degree of growth in BPW followed by plating onto XLD may be suggested.

SIGNIFICANCE AND IMPACT OF THE STUDY

This paper provides epidemiological data on the prevalence of Salmonella and laboratory protocols useful for isolation of Salmonella from faeces of reptiles.

Optimization of a rapid and sensitive identification system for Salmonella enteritidis by capillary electrophoresis with laser-induced fluorescence

A system coupling capillary electrophoresis (CE) with a laser-induced fluorescence (LIF) system for the identification of Salmonella enteritidis was developed. Addition of an appropriate amount of sodium alginate and NaCl to the running buffer made it possible to obtain a reproducible sharp peak. Two fluorescent staining methods using a cell-permeable nucleic acid stain and a salmonellae-specific polyclonal antibody were adapted to the system. The CE-LIF successfully detected as few as three cells per injection from a pure culture of S. enteritidis. The CE-LIF system can be conveniently used for rapid and highly sensitive identification of S. enteritidis.

Comparison of TaqMan Salmonella amplification/detection kit with standard culture procedure for detection of Salmonella in meat samples

We evaluated the TaqMan PCR Salmonella amplification/detection kit (PE Applied Biosystems) for rapid detection of Salmonella from a variety of meat samples. This system uses the 5' nuclease activity of Taq DNA polymerase, which digests an internal fluorogenic probe to monitor the amplification of the target gene. The detection sensitivity of the kit, using 2 kinds of DNA extraction protocols, was compared with that obtained with 4 protocols of official culture methods. A total of 98 meat samples (16 raw beef, 31 pork and 51 chicken) were tested. The results of the TaqMan PCR method and the combined results of the 4 cultural protocols showed excellent agreement. However, no single culture protocol showed optimal recovery of Salmonella comparable to the PCR method. These results suggest that the TaqMan PCR method is a reliable and rapid method useful for detecting Salmonella in meat products.

New developments in chromogenic and fluorogenic culture media

This review describes some recent developments in chromogenic and fluorogenic culture media in microbiological diagnostic. The detection of beta-D-glucuronidase (GUD) activity for enumeration of Escherichia coli is well known. E. coli O157:H7 strains are usually GUD-negative and do not ferment sorbitol. These characteristics are used in selective media for these organisms and new chromogenic media are available. Some of the new chromogenic media make the Salmonella diagnostic easier and faster. The use of chromogenic and fluorogenic substrates for detection of beta-D-glucosidase (beta-GLU) activity to differentiate enterococci has received considerable attention and new media are described. Rapid detection of Clostridium perfringens, Listeria monocytogenes, Bacillus cereus and Staphylococcus aureus are other application of enzyme detection methods in food and water microbiology.

Update on media for isolation of Enterobacteriaceae from foods

Testing for 'total' Enterobacteriaceae, coliforms and Escherichia coli as marker organisms in foods and detection of specific pathogens of the family Enterobacteriaceae, including pathogenic E. coli, Salmonella, Shigella and Yersinia spp. is widely applied in many food control laboratories. This review describes some recent developments in culture media for these organisms. Methods for enumeration of E. coli include the standard MPN technique, a membrane-filter method and the use of media containing chromogenic and fluorogenic indicators for beta-D-glucuronidase (GUD) activity. Shiga toxin-producing E. coli O157 strains usually do not ferment sorbitol and are GUD-negative. These characteristics are used in selective media for these organisms, such as cefixime tellurite sorbitol MacConkey agar. For the detection of salmonellae, motility enrichment in Modified Semisolid Rappaport-Vassiliadis (MSRV) medium shows equal or better results than the use of standard Rappaport-Vassiliadis broth. Addition of nitrofurantoin to diagnostic semisolid salmonella agar and to xylose lysine desoxycholate agar favours the isolation of S. enteritidis. Recently developed salmonella media use different selective and diagnostic properties, such as acid formation from propylene glycol, glucuronate fermentation, fermentation of glycerol and addition of Tergitol 4 as selective agent. The isolation of Shigella spp. from foods is rather difficult and further evaluation of suggested isolation systems and the development of more effective methods for the isolation of this pathogen are needed. Yersinia enterocolitica includes both pathogenic and nonpathogenic biotypes and serogroups. As no single procedure will recover all pathogenic strains of Y. enterocolitica, the use of two isolation procedures in parallel is recommended.

Sensitive detection of viable Listeria monocytogenes by reverse transcription-PCR

Detection of pathogens in contaminated food products by PCR can result in false-positive data due to the amplification of DNA from nonviable cells. A new method based on reverse transcription-PCR (RT-PCR) amplification of mRNA for the specific detection of viable Listeria monocytogenes was developed. The expression of three L. monocytogenes genes, iap, hly, and prfA, was examined to determine a suitable target for amplification of RT-PCR. Total RNA from L. monocytogenes was isolated, and following DNase treatment, the RNA was amplified by both RT-PCR and PCR with primers specific for the three genes. Amplicon detection was accomplished by Southern hybridization to digoxigenin-labeled gene probes. The levels of expression of these three genes differed markedly, and the results indicated that the iap gene would provide a good target for development of a specific method for detection of viable L. monocytogenes based on RT-PCR amplification. After a 1-h enrichment, the 371-bp iap-specific product was detected with a sensitivity of ca. 10 to 15 CFU/ml from pure culture. Detection of the 713-bp hly-specific amplicon was ca. 4,000 times less sensitive after 1 h, whereas detection of the 508-bp prfA product showed the lowest level of sensitivity, with detection not observed until after a 5-h enrichment period. The amplification of the iap mRNA was specific for L. monocytogenes. Overall, the assay could be completed in ca. 54 h. The use of RT-PCR amplification for the detection of viable L. monocytogenes was validated in artificially contaminated cooked ground beef. Following a 2-h enrichment incubation, the iap-specific amplification product could be detected in a cooked meat sample that was originally inoculated with ca. 3 CFU/g. These results support the usefulness of RT-PCR amplification of mRNA as a sensitive method for the specific detection of viable L. monocytogenes and indicate that this method may prove useful in the detection of this pathogen in ready-to-eat, refrigerated meat products.